Lecture 4 Cells and DNA


Hi everybody. This is introduction to, biological
anthropology, anthropology 225, the online version, and I’m Dr Sharon
Gursky-Doyen, I’m your instructor. Today’s lecture is Cells and DNA. [BLANK_AUDIO]. Some of you may be wondering why we’re
talking about cells and DNA, in a course on biological anthropology, and I’d
like to remind everybody that this course is about
evolution. And evolution is grounded in the basic
mechanisms of life. These basic mechanisms of life include things like cells, cell replication,
replication and decoding of genetic information, and transmission of this genetic information
between generations. To understand evolution, we need to
examine how life is organized at cellular and
molecular levels. And in order to do that, the first step is
to understand cells and cellular function. [BLANK_AUDIO]. Cytology is the branch of science that
specializes in cell biology. Cytology. Cyto means cell, ology means the study of. Cytologenetics is the study of the
heredity mechanisms within the cell. And the cell are the basic units of life
in all organisms. They are also the smallest entities that
are capable of self-reproduction. Prokaryotes are single celled organisms
that are represented by bacteria and blue green
algae. In these organisms, a single cell
constitutes the entire organism. These cells are enclosed by a plasma membrane, but they differ from multi
cellular organisms, in that they do not possess a membrane around their nucleus, or around their
genetic material. They also differ from eukaryotic,
multicellular organisms, in that they do not possess membranous
organelles. Things like the mitochondria, the Golgi
apparatus, the endoplasmic reticulum. All of these organelles are not present in
prokaryotic cells. So what we see with prokaryotic cells is
that the genetic material intermingles with the cytoplasm and is found scattered,
in, in effect, throughout the cell. [BLANK_AUDIO]. These cells, as i mentioned, first appear
approximately 3.7 billion years ago. This is an example of a prokaryotic cell,
and you can see that this genetic material is just free floating within the cell and
it’s not segregated into a certain area. This is very distinct from later cells. Eukaryotic cells are much more complex
cells, multicellular organisms. They include things like fungi, plants,
animals, including humans. All components of humans are comprised of
multicellular features. These types of cells first appear 1.2
billion years ago. So about 2.5 billion years after
prokaryotic cells first appear on Earth. One of the main things that we see in
these eukaryotic cells is that they contain a
large number of organelles. These organelles are held within the cell
by a surrounding membrane known as the plasma membranes, so much like the
prokaryotic cells, eukaryotic cells have a plasma
membrane. But eukaryotic cells also contains
organelles. This plasma membrane that surrounds the
cell in both the prokaryotic and eukaryotic cells
is very important. It is an active component of cellular
function, because it literally controls the exchange of materials of that
cell with the external environment. It determines what, from the external
environment can pass into the cell and what from the cell can pass out
of the cell. Okay. [BLANK_AUDIO]. This is an example of a eukaryotic cell. This purple is the plasma membrane. This blue is the endoplasmic reticulum. The little blue ones are called ribosomes. This is the nucleus and this is the
nuclear membrane. See how it’s all separate and everything
has their own little compartment. We have the Golgi apparatus, we have the
mitochondria, and we have lysosomes. [BLANK_AUDIO]. Most animal cells have a covering over
their plasma membrane called a cell coat. It consists of glycoproteins. Proteins and polysaccharides, and it
functions for providing the cell with its biochemical
identity. As an example, there are various antigenic
determinants, like the AB and MN antigens that are found of the surfaces of
our, MN, of our red blood cells. These are things that provide our red
blood cells with its biochemical identity. [BLANK_AUDIO]. Probably the most important organelle
within a eukaryotic cell is the nucleus. Within the nucleus are molecules that
contain the genetic information that controls all of
the cell’s functions. The molecules and the nucleus are comprised of DNA, commonly called
deoxyribonucleic acid. It is commonly called chromosomal DNA. [BLANK_AUDIO]. Surrounding the nucleus is cytoplasm and
the cytoplasm contains a variety of other organelles and each is involved in very
specific celu, cellular function. So organelles effectively divide the cell
into functional compartments. [BLANK_AUDIO]. One of the, one of the important
cytoplasmic organelles is the endoplasmic reticulum, the,
commonly just called ER. It is a network of membranish channels
located in the cytoplasm. It basically looks like folded sheets of
paper, and it is the sight of protein synthesis. It was where proteins are literally
synthesized. The outer surface of the endoplasmic
reticulum you studied with ribosomes. Ribosomes are spherical in shape and are
the most common cytoplasmic organelle. They’re comprised of RNA, ribonucleic
acid. And as I said, they’re studied throughout the outer part of the endoplasmic
reticulum. [BLANK_AUDIO]. The Golgi apparatus is another type of
organelle within the cell. It constitutes clusters of flattened me,
membranous sacs and it’s primarily found in animal
cells. The purpose of the Golgi Apparatus is to
modify and distribute proteins that have been
synthesized by the Endoplasmic Reticulum. So it creates proteins, for it it modifies and distributes proteins to synthesize by
the endoplasmic reticulum. Just as importantly, the Golgi apparatus
also serves as a source of membranes for other
organelles. So it literally creates membranes for other organelles such as the nuclear
membrane. [BLANK_AUDIO]. The mitochondria is also an extremely important organelle within the, within
eukaryotic cells. It is primarily used for producing energy
by converting energy in food into what is known as adenosine triphosphate, ATP,
which is the main fuel of the cells. Mitochondria are generally capsule-shaped. Okay, so they’re kind of oval in shape. There are thousands of them per cell. And as I said, they’re responsible for
producing energy that the cells can use to do the things they need
to do. The membrane around the mitochondria are
also produced from proteins and they’re proteins produced by the DNA
in the nucleus. [BLANK_AUDIO]. Another very important type of cytoplasmic
organelle that is not mentioned in your textbook are things
called centrioles. Centrioles are very important for the
organization of the spindle fibers that function in
cellular division. They are generally found in specialized
regions of the cytoplasm known as the centromere. [BLANK_AUDIO]. Now, there are two primary types of
eukaryotic cells. We have somatic cells and sex cells. Somatic cells make up the structural
composition of your body. That is, they are the cellular. Cellular components of bodily tissues and
organs. Things like muscle, bone, skin, heart. They also constitute sex cells, or
commonly called gametes. These are the cells that are specifically
involved in reproduction. They’re not as important as the structural
components of the body, but they’re certainly important in
terms of creating new individuals. [BLANK_AUDIO]. Gametes. A gamete functions to unite with a sex
cell from another individual to form a zygote and a zygote is effectively
a fer, a fertilized egg. They transmit information from one parent
to the offspring. In addition, there are two main types of,
of gametes. There are ova or eggs which are found in females and then there’re sperm which are
found in males. [BLANK_AUDIO] [INAUDIBLE] Now, many cells in the body
alternate between states of cellular division and
non division. Cell Division is the process that results
in the production of new cells. The sequence of events from one division
to another is called the cell cycle. The period that is located between cell
divisions is commonly called inter-phase. During interphase, the cell is involved in
other basic metabolic processes, and most of the cell’s existence is actually spent in interphase and not in cellular
division. Interphase has 3 primary phases. The first is known as G1 or gap 1 stage. It begins directly after a cell division. During G1, the cell’s DNA exist as
chromatin. Chromatin constitutes long, uncoiled
strands of DNA. The synthetic activity in G1 effectively
doubles the size of the cell and replaces cellular organelles
that have been lost during cell division. It has been estimated that there is more
than 6 feet of DNA in the nucleus of every one of your
somatic cells. [BLANK_AUDIO]. The second phase of interphase is known as
the S Phase. The S phase is the time in which a duplicate copy of the DNA or the chromatin
is created. Okay. The G2 phase is a period of growth. This is when, and it concludes directly
before cell division begins. When the cell reaches the end of the G2 phase, the cell undergoes cellular
division. [BLANK_AUDIO]. Now there are two main types of cell
division. We have sex and somatic cell division, sex, sexual cell division and somatic cell
division. Mitosis is commonly referred to as somatic
cell division. Meiosis is commonly called sex cell
division. And I’d like to go through each of these
types of cell division separately. Somatic cell division is commonly called
simple cell division. This is because a somatic cell divides
once to produce two daughter cells which are genetically identical to each
other and genetically identical to the original
cell. In mitosis the original pro, possesses 46 chromosomes and each new daughter cell
inherits. It’s exact copies of all 46 chromosomes. How is this possible? Well this is possible due to the ability
of DNA molecule to replicate itself. And when does that occur? During inner phase, during the s phase. With somatic cell division we, each cell
that is created has a diploid complement. That is the full complement of chromosomes
in humans, 46. The, there are 4 primary phases of mitosis, prophase, metaphase, anaphase,
and telophase. [BLANK_AUDIO]. In mitosis, during prophase, the
chromosomes coil. So instead of becoming long strands, they
start to condense and coil into actual structural chromosomes, and they become
visible as chromosomes instead of just long thin
strands. The duplicated chromosomes become joined
at a location on the chromosome known as the
centromere. The centromere is like a constricted area
of the chromosome. Also during mitosis prophase we see that
the nuclear membranes begin to break down and the centrioles migrate
to opposite sides of the cell. During metaphase, this begins when the
nuclear membrane fully disappear. Well you’ve seen here that the chromosome
aligning along the equator of this cell. The spindle fibers are attaching to the
centromeres with this. Remember that’s the constricted area of
the chromosomes. And they’re separating. In them, the next phase in mitosis is anaphase. The spindle fibers contract, pulling apart
the probe, the duplicating chromosomes causing them to
split at the centromere. So identical chromosomes are being pulled
to opposite sides of the cell. At the end of Anaphase, what you see are
complete sets of chromosomes. Chromosome, at each pole of the cell. And then we have mitosis. During mitosis, the spindle fibers are
breaking down, the endoplasmic reticulum has formed two new nuclear
membranes, one around each nucleus. The chromosomes uncoil and become
chromatin again, you don’t see. Specific strands of chromosomes. And the cell has formerly divided into two
genetically identical daughter cells. The result of this division, is the
production, of two daughter cells, each identical to the
mother cell. Each daughter cell has the exact number of
chromosomes as the mother, 46 chromosomes in humans,
or 23 pairs. This differs, substantially from mitosis,
commonly called sex cell division. While my note [INAUDIBLE] while mitosis
produces new cells. Meiosis leads to the development of new individuals, because meiosis produces
reproductive cells or gametes. Meiosis only occurs in the testes of
males, and in the ovaries of females. During meiosis. It is characterized by two divisions, that
result in four daughter cells. Each of those cells, have 23 chromosomes,
one half the original number. They have a haploid complement of
chromosomes. That is, they only have one of each. Okay, so each cell receives one, just one
of each chromosome. From each parent. Okay. So, meiosis, you’ll remember, I said has
two actual divisions. The first occurs after the first
telophase. And the second occurs after the second
telophase. Notice that you have two prophases, two
metaphases, two anaphase, two telophase. But what you are missing, during here, is
that there’s no interphase. No time for growth or duplication of additional genetic material, before the
second division. So, just keep that in mind. So this is meiosis. This is prophase one. What we’re seeing is that the chromosomes
are coiling, and they become visible. The duplicated chromosomes join at the
centromere, that constricted area. And the nuclear membrane begins to break
down. And the centrioles move to opposite sides
of the cell. They’re analogous to what we saw during
prophase one of mitosis. The pairing of these homologous
chromosomes, is highly significant, because while they are
physically associated. They exchange genetic information in a process called recombination, or crossing
over. The pairing up of these homologous
chromosomes, is important because it facilitates the
reduction of chromosome number, by ensuring that each
daughter cell, only receives one member of each
pair. During metaphase one of meiosis, we see
that it begins when the nuclear membrane fully
disappears, the chromosomes are aligning along the equator of the cell,
and the spindle fibers are attaching to the
centromeres of the, chromosomes. Then we have anaphase one. The spindle fibers are contracting, and
they’re pulling the duplicated, pulling apart the duplicated
chromosomes at each centromere. So half of them are going to the one side,
half of them are going to the other. Identical chromosomes are migrating to
opposite sides of the cell. At the end of anaphase, we’re gonna see a complete set of chromosomes at each side
of the cell. [SOUND] Then we have the first telophase. What we’re seeing here, is that the
spindle fibers are breaking down, the endoplasmic reticulum
has formed two new nuclear membranes. Around each nuc, each compo around all
these genetic material. These chromosomes begin to uncoil, and
they become chromatin. And the cell divides, the first meiotic
division of meiosis, into two cells. So now we go after telophase one, we go
into prophase two. Once again, please recall, we have not had
an interface, so there’s been no duplication
of the genetic material. So, during the first prophase two, the
centrioles moved to opposite sides of the cell, the nuclear membranes begin
to disappear around both nuclei. The nuclear membrane fully disappears, the
chromosomes on both cells align along the equator of the cell, and the spindle fibers are attaching to the centromere of each
chromosome. The spindle fibers contract, splitting the centromeres, and separating the homologous
chromosomes. The homologous chromosomes are moving to
opposite poles of the cell. This is during anaphase two. During telophase two, a haploid set of daughter chromosomes become visible at
opposite poles. The nuclear membranes reform around all
this genetic material. The cells have undergone a second meiotic
division, so now there are four of them, and the result is four haploid daughter cells, each with one copy of each
chromosome. And the reason it’s, they’re haploid
daughter cells. Is because there is no interphase, before
the second meiotic division. [SOUND] So, this is just a graphical
illustration of some the differences between mitosis
and meiosis. During mitosis, we start out with 46
chromosomes and one cellular division, producing 46
chromosomes in each. The reason is, we have, during this
interphase here, before the cellular division, synthesis, and
duplication of the genetic material occur. Here, we start out with 46, and it
produces two cells with 46. Each of those, has an exact complement, but there’s no interphase here, and so,
when it splits into two more, it become, it only has a haploid complement of genetic
material. [SOUND] Meiosis of cells that result in
the formation of ova, or sperm, sperm, are, is known as the
gametogenesis. It’s the meiosis of cells resulting in the
formation of eggs and sperm. In males, the meiotic process is called spermatogenesis, and occurs in the male
testes. They, male germ cells are spermatogonia,
they line the tubules in the testes, and they start dividing from the
time of puberty, until death. Spermatogonia, produce daughter cells. Primary spermatocytes, spermatocy. [BLANK_AUDIO] [SOUND] Meiosis of cells that result in
the formation of ova or sperm, is known as
gametogenesis. In males, the meiotic process is called
spermatogenesis and occurs in the testes. In males, the germ cells or spermatogonia
line the tubules in the testes, and are dividing from the time they reach,
boys reach puberty, until they die. This is why men can produce offspring up
until death. The spermatogonia produce daughter cells. That are known as primary spermatocytes. These primary spermatocytes, undergo
myosis, to produce four haploid cells called
spermatids. [SOUND] Each spermatid undergoes a period of development into mature sperm. The nucleus that contains 23 chromosomes,
becomes condensed, and forms the actual head of the sperm. From the cytoplasm, the neck and the
whip-like tail develop. And the remaining cytoplasm is lost. I wanna point out that spermatogenesis is
not a very rapid process, but it actually lasts as
long as 48 days. 16 days for the first meiotic division, 16
days for the second meiotic division, an additional 16 days for conversion of
the spermatid, into mature sperm. What about in females? In females, meiosis is similar, but
slightly different. In females, the production of gametes is called oogenesis, and it occurs in the
ovaries. The germ cells, or the oagonia, divide by meiosis, and form, what are called,
primary oocytes. The cytoplasmic cleavage that occurs
during meiosis one, produces cells of unequal size, okay? So, whereas during gametogenesis in males,
it’s producing four cells of equal size. The first meiotic division, in female sex
cells, produces two cells of unequal size. The first meiotic division, yields one
large cell that contains 95% of the cytoplasm. And the second cell is called a polar
body. It’s a small cell that contains 5% or less
of the cytoplasm. In the second cellular division, there is another disproportionate distribution
of the cytoplasm. So that one large functional gamete called
the ovum, is produced. The other three cells are referred to as
polar bodies, they receive almost no cytoplasm, and are incapable of
functioning as gametes. Graphically, this is illustrated in the
following manner, we start out with a oogonia in the ovaries, it has
its first meiotic division. Produces one primary oocyte and a polar
body. This primary oocyte divides again, forms
the ovum, and another polar body. The polar body here, divides into two
polar bodies, okay? Whereas with males, you’re gonna have one
division, and you’re gonna have four spermatides,
that are produced. Now, every species is characterized by a
specific number of chromosomes. In their somatic cells. Chromosome number ranges in primates from
34 to 62. Humans are characterized by 46 chromosomes
in their somatic cells. Gorillas and chimps have as many as 48. But I wanna to make it very clear, that
chromo no, chromosome number. Is not consistent with any taxonomic
group, such as family. Closely related species may have very
different chromosome numbers, white handed gibbons are known to have 44
chromosome, crested gibbons 52. Both species belong to the same family. And sometimes you can have different species with the same number of
chromosome. Old world Patas monkeys and new world owl
monkeys both have 54. One’s found in South America, the other’s
found in Africa, and yet they have the same number
of chromosomes. The important point to take away, is that
chromosome number, does not indicate any evolutionary
relationship. Chromosomes always come in pairs. Thus human somatic cells contain 23 pairs. Gorillas have 24 pairs. One member of each pair is inherited from
your mother. The other pair, the other one from each
pair is inherited from your father. This is what we refer to as homologous
pair of chromosomes. Paternal, maternal. The members of each pair are called
homologous chromosomes. [SOUND] During the process of cell
division, chromatin. Condenses and call, coils into tightly wound discrete structures called
chromosomes. Each chromosome has a very distinctive
size, and a very distinctive shape. The shape is based in part on the position
of the centromere, that constricted area, which is that condense and constricted region of
chromosome. The size of the chromosome, is based on
the size of the DNA molecules that make up the
chromosome. During cell division, photographs of the
chromosomes can be taken, and a karyotype can be
constructed. A karyotype, shows all the chromosomes in
a single cell, and it’s often taken from a photograph, during cell
division, during metaphase. In eukaryotes, each chromosome consists of
a single DNA molecule, mixed with proteins, to form
chromatin. There are two types of proteins that are
in chromatin, called histio, histones and
non-histone pro, proteins, the histones play a major role in chromosome structure,
while the non-histone proteins are thought to be
involved in gene regulation. [SOUND] There are four main classes of
chromosomes. These are metacentric, submetric,
acrocentric, and teleocentric. The metacentric chromosomes, is where the
centromere is approximately in the center of the
chromosome. So, the arms of each chromosome are equal
in length. Subcentric, submetric chromosomes, are
ones where the centromere is located, to one side of the chromosome
centre. So arms are of slightly unequal length. In acrocentric chromosomes. The chrome centromere is near one end, and
the arms are of very unequal length. When the X and the Y connect, they usually
have acrocentric chrome centromeres at which
they connect near the tip. Teleocentric is the centromere at the very
tip of the chromosomes. And I’d like to point out we do not, it is
not believed that we have any in humans. The short arm of chromosomes is generally
referred to as the p arm because it is petite. The long arm is the Q arm for no other reason than it is the next letter in the
alphabet. So, what about chromosome structure? In eukaryotes, each chromosome consists of
a single DNA molecule. Okay. [BLANK_AUDIO]. There are two basic types of chromosomes. Autosomes, and sex chromosomes. Autosomes carry genetic information for
all physical characteristics except for anything to do with our sex
determination. Anything to do with whether we’re male or
female. The sex chromosomes in our bodies are either, X chromosomes or Y chromosome
sometimes both. Y chromosomes are primarily, primarily
function to determine whether or not we’re males. And the X chromosomes are much larger than
the Y chromosomes, and they function more like
an autosome. They are very important for determining
the female sex. In general, in humans, males have one X
and one Y. While females have, a pair of two Xs, have
a pair of Xs. Within humans, of the 23 chromosome pairs,
22 pairs are autosomes, and one pair represents a
sex chromosome. In females that’s XX and males it’s an XY. All autosomes occur in pairs. The 23rd pair is the sex chromosome. Sex chromosomes are only paired for
females who have an XX, male chromosomes, sex chromosomes are not
paired because they have 1 X and 1 Y. I wanna point out that abnormal numbers of autosomes are almost always fatal, usually
soon after conception. One exception to this happens to be Downs
Syndrome, commonly called trisomy 21, because there are three sets
of the 21st chromosome. Although abnormal numbers of sex
chromosomes, are not usually fatal, they often result sterility, and have other
reproductive consequences as well. So I want you to understand that in order for your body to function normally, it is
essential for a cell to possess both members of each
chromosome pair or a total of 46 chromosomes no more, no
less. The gene is the fundamental unit of
heredity. [BLANK_AUDIO] [SOUND] On each pair of chromosomes, there
are a number of loci, these are places or slots on each
chromosome. At each locus is a gene that affects one
or more traits. Since there are two chromosomes, there are
two genes for each trait. Similar, but slightly different forms of
the same gene, are referred to as alleles. The previous discussion has focused, on
the behavior of the chromosomes. However, the chromosome is not the gene
itself. In order to understand the gene, we need
to understand the chemical nature of the
hereditary material itself. All substances are composed of atoms. And atoms are the basic building blocks of
matter. The different types of atoms are called
elements. Of the 100 or, so elements that occur in nature, four are very important in
living organisms. Carbon C, hydrogen H, oxygen O, nitrogen
N. There are a variety of others that also
play an important role, things like calcium and phosphorus,
sulfur, chlorine, sodium, magnesium, iron, and
potassium. All of them play a very important role,
but these four we really have to have in our
lives. Atoms are generally joined together to
form molecules. The size of the molecules vary in size, depending on the number of atoms that are
involved. In molecules, atoms are generally held
together by a stable, covalent bond. A covalent bond ids basically a pair of electrons, that are shared between two
atoms. Another type of bond, between atoms is
known as a hydrogen bond. The molecules that are generally found in
living organisms are very large. This is because they contain carbon, and
carbon atoms can form long chains that sometimes consist of
thousands of atoms. And often contains, and living organisms
often have more than five or six carbon chains to carbon
atoms. Most of the molecules found in living
organisms fall into one of four categories, carbohydrates, lipids,
proteins and nucleic acids. Carbohydrates include, your sugars and
your starches, lipids include fats, oils and waxes, and new
proteins. These are some of the most important
molecules in the body. All of your, body’s biological structures
from your nerve cells to your blood cells to your bone cells, they are
made up predominately of proteins. And these are also com, comprised of long
chains of amino acids. The fourth type are nucleic acids. These are the largest molecules found in
living organisms. They are the hereditary material. Much like proteins, the nucleic acids form
large chains of unit. In the case of these nucleic acids, the
large chains are comprised of nucleotides. The nucleotide, is a complex consisting of
three lesser units. It contains a five carbon sugar, a
phosphate, and a nitrogenous base. The bases that are found in these nucleic acid, bases are of two different
varieties. The two different types of bases are
purines and pyramidines. The purines consist of connected rings of
carbon, and nitrogen atoms. The pyramidines consist of a single ring. So they’re generally much smaller. The nucleic acid based upon the pentose
sugar, ribose is called ribonucleic acid. The nucleotides that make up the RNA,
contain the following bases. It contains the purines, adenine and guanine, and the pyramidines uracil, and
cytosine. They are frequently referred to just by
their initial letters A, G, U, C. The nucleic acid that is based upon a pentose
sugar deoxyribose is called deoxyribonucleic acid, or DNA. DNA also contains adenine, guanine, and
cytosine, AGC, but instead of uracil, it contains
thymine, T. So these are the four bases contained in DNA with one difference in comparison to
RNA. [BLANK_AUDIO]. Cellular functions are directed by DNA. To understand these activities, and how
characteristics are inherited, we must first know something, about the
structure and function of DNA. It was in 1953 that Watson and Crick developed a structural and functional
model for DNA. For their work, they received the Nobel
Prize in medicine in 1962. Their work, that’s how revolutionary their
work was. Watson and Crick found that the DNA
molecule is composed of two chains of sub units called
nucleotides. These nucleotides are molecules, composed
of a base linked by a covalent bond to a sugar, which in turn, is covalently
bonded to a phosphate group. Each nucleotide contains one sugar, one phosphate, and one unit of the four bases. [SOUND] In DNA, the nucleotides are
stacked one upon each, one upon another to form a
chain. This chain is bonded along its bases to. To another complementary nucleotide chain,
and together these two chains twist to form a spiral or helical
shape. The resulting DNA molecule is double
stranded, and is described as a double helix that
resembles. A twisted ladder, following up this
analogy of a twisted ladder, you think of the sugars,
and the phosphate as the sides of the ladder, and the
nitrogenous basis, as the rungs, where you step, on
the ladder. The secret of how DNA functions lies with
the four nitrogenous bases. In the formation of the double helix, the
joining of bases is done in a very specific
manner. Chemically, only one type of base pairs
with one other type of base. Base pairs can only form, between adenine,
and thymine, and cytosine, and guanine. This base paring rule, structurally and
functionally, is the heart of the DNA molecule and is referred to as the
base pairing rule. DNA replication. Prior to the visible beginnings of
cellular division, specialized enzymes break the
bonds between the bases in the DNA leaving the two
previously joined strands of nucleotides with their
bases exposed. This separation permits the exposed bases
to attract. Unattached nucleotides that are free
floating within the cell nucleus. Since the base pairing rule states that
only one base can join to a specific other base, the attraction between bases occurs
in a very complimentary fashion. If adenine is exposed, it’s going to
attract the nucleotide carrying thymine. When this process is complete, there are two double-stranded DNA molecules that are
exactly alike. Now we have one. Now we have two DNA molecules, each
consisting of one old strand, and one new strand. Okay, this is an example, here we see the DNA molecules pairing up after it’s
split apart. And this green is going here, and this
blue is going here, and this yellow is going
here, okay? You see how nicely it works. In ad, this process that I just described
of replication is known as semiconservative replication since an old
strand is conserved in each new molecule. And another form of com, of replication,
called conservative replication. Synthesis of complementary nucleotide
chains occurs as I described. However, following synthesis, the two
newly created strands are brought together and the parental
strands also re-associate. So what happens is, the original DNA helix
is conserved. Protein synthesis, during the life of the
cell, the most important function of DNA is
protein synthesis. Proteins are complex, 3-dimensional
molecules that function through their ability to bind to other
molecules. A classic example concerns hemoglobin. The protein hemoglobin is found in your
red blood cells, and it binds to oxygen and it
allows it. To transport oxygen throughout the body by
touching through the red blood cells. Another example are enzymes. Enzymes are proteins whose function is to
initiate and enhance chemical reactions. If any of you are lac, lac, lactase
deficient. Lactase is an enzyme that breaks down
lactose. Lactose is found in sugar. I’m sorry. Is found in milk, it’s the sugar in milk. And it break lactase, the enzyme, breaks
down lactose into simple sugars your body can
digest. If you’re lactose deficient, your body
does not have the lactase enzyme to help break down
lactose. Proteins are comprised of linear chains of
amino acids. Chains of amino acids are called
polypeptides. In all, there are 20 amino acids which are
combined in different amounts, and in different
sequences to produce different proteins. Nine of the amino acids are not produced
by our bodies and need to be consumed from protein in
the foods we eat. The remaining 11, are produced from the
cells. [BLANK_AUDIO]. The most important function of DNA, is
it’s protein synthesis function. Proteins, as I’ve mentioned, are complex
3-dimensional molecules that function through their ability to bind to
other molecules. A classic example is hemoglobin. Hemoglobin in our red blood cells binds to oxygen and transports oxygen throughout
the body. Another example made that may be easier to
understand, concerns enzymes. Enzymes are specialized type of protein
whose function it is to initiate and enhance chemical
reactions. If any of you are lactase deficient, you
know that the enzyme lactase breaks down lactose, the sugar
that’s in milk, into simple sugars. If you are lactase deficient, your body
does not have this enzyme to help break down sugar which your
body can then digest. Proteins are composed of linear chains of
smaller molecules known as amino acids. Chains of amino acids are called
polypeptides. In all, there are 20 amino acids which are
combined in different amounts and different sequences, which produced different
proteins. Nine of the amino acids are not produced
by our bodies and need to be obtained from the foods
that we eat. The other 11 amino acids are directly
produced in our cells. A typical protein can be comprised of more
than 200 amino acids. This chain of amino acids is referred to
as a polypeptide. What distinguishes proteins from one
another is the number of amino acids involved and the sequence in
which they are arranged. The sequence of amino acid chains is called the primary structure of the
protein. DNA serves as a recipe for making a
protein. It is the sequence of DNA bases that
determines the order of the amino acids in a protein
molecule. In the DNA instruction, a triplet, or a
group of three bases, commonly referred to as a codon, specifies
a particular amino acid. If a triplet includes CGA, then it
specifies the amino acid alanine. If the next triplet in the chain is GTC, it refers to another amino acid
galled glutamine. There are 64 different triplets or codons,
that can be produced from this 20 amino acids. Of these, 61 of them specifically code for
specific amino acid. Three of the triplets, three of the codons
are what we call stop codons. These are UAA, UAG, and UGA. The AUG/TAC codon has two specific
functions. Encode for the amino acid methionine and
it also serves as a start codon that marks the beginning of
a polypeptide chain. 64 triplets or 64 codons may not seem like
a lot, except with the fact that we only need 20 amino
acids in our body. We only need 20 amino acids to be
specified by the genetic code, and it can specify as
many as 64. The three base code provides more than
enough possibilities to code for all of these 20 amino acids. In fact, some of these amino acids have
several different codes. For example, alanine, is specified by the sequence CGA, CGG, CGT and CGT. [BLANK_AUDIO]. CGC. [SOUND] There are 64 different triplets or codons that can be produced from these 20 amino acids. 61 actually code for amino acids. Three of them, three triplets, UAA, UAG,
and UGA all function as stop codons. The AUG and TAC codons have two functions. They code for the amino acid methionine,
and they serve as start codons that mark the beginning of
a peptide chain. 64 codons may not seem like a lot, but
only 20 amino acids are actually needed to be
specified by the genetic code. So there’s a tremendous amount of
redundancy that is produced. For example, some amino acids have several
different codes. For example, CGA, CGG, CGT, and CGC. All code for the amino acid alonine. An interesting feature of the genetic code
is that the same codons are used for the same amino acids
in all life forms. In bacteria, in plants, in animals,
including humans. We all use the same amino acids for the
same proteins. What makes oak trees different from
humans, is not differences in the DNA material, but differences in how
the DNA is arranged. The genetic code is universal. The universal nature of the genetic code
means that the genetic code was established early on in evolution and
the evolution of this planet. Now I wanna make it really clear that
protein synthesis is a lot more complicated than the
explanation I my, I just gave might imply. For one thing, protein synthesis occurs
outside the nucleus at the ribosomes in the
cytoplasm. There’s a logistic problem, because DNA
molecule is not capable of traveling outside the
nucleus. So how does it get to the ribosomes in the
cytoplasm? The first step in protein synthesis, is to
copy the DNA message in a form that can pass through the nuclear
membrane into the cytoplasm. This process is accomplished through the
formation of a molecule similar to DNA commonly called
RNA. There are three main types of RNA: ribosomal RNA, messenger RNA, and transfer
RNA. The thing you need to understand about RNA
is how it differs from DNA. First of all, RNA is single-stranded. See, there’s only one swirled strand. It contains a different type of sugar. So, instead of having dioxide ribose it
has ribose as it’s sugar. RNA contains uracil instead of thymine and
most importantly, RNA is capable of passing through the
nuclear membrane. So I said, there are three important types of RNA: ribosomal RNA, messenger RNA and
transfer RNA. Ribosomal RNA is the largest and contains, comprises approximately 80% of RNA in the
cell. During protein synthesis, it functions
during the translation stage. We have messenger RNA, which carries
genetic information from DNA in the nucleus to the ribosomes for
protein synthesis. And then we have transfer RNA. It’s the smallest of the RNA molecules,
and it carries amino acids to the ribosomes
during translation. The RNA molecule forms on the DNA template
in much the same way as new strands of DNA are assembled during
DNA replication. The double-stranded DNA molecule separates
into two single strands of DNA. The exposed bases attract free-floating
nucleotides with the help of some, an enzyme called RNA
polymerase. During protein synthesis, the nucleotides
are RNA instead of DNA. As the RNA bases arrive at the DNA
template, their nucleotides attach to one another in
a linear fashion and produce a chain of RNA molecules or
RNA nucleotides that are complementary to the DNA strand
they are reading. The RNA strand is a, that functions in
this manner, is a particular strand of RNA that is
called messenger RNA. During its assembly on the DNA molecule,
messenger RNA is transcribing the DNA code and the formation of the
messenger RNA is called transcription. Once the appropriate DNA segment has been
copied, the, messenger RNA strand, peels away from the DNA
molecule, and travels through the pores in the nuclear membrane
to the ribosome. Meanwhile, the bonds between the DNA bases
become re-established, and you have a double helix, again, in the
nucleus. But once we go back outside to the, to the
cytoplasm, the messenger RNA strand arrives at the ribosome and the ribosome
translates the code that it contains. This is called translation because the genetic code is being decoded and
implemented. As each DNA triplet specifies one amino
acid, each RNA or messenger RNA triplet also called codons, also serve
as this function. One other form of RNA, transfer RNA, is also essential to the assembly of the
protein. Once the messenger RNA arrives at the
ribosome, the messenger RNA transfers its information by
attracting transfer RNA. Transfer RNA is, of course, a
free-floating molecule. The sequence of messenger RNA containing
the sequence CGA, attracts transfer RNA molecule with a complimentary
sequence GGU. The transfer RNA sequence GGU is referred
to as the anti-codon. The result is that the amino acid prolene
specified by the RNA sequence GGU, or the DNA sequence GGT, is included in the chain of amino acids making up a particular
protein. Once the proteins are formed in the
polypap, peptide, it’s released into the
endoplasmic reticulum, where it will become chemically modified and
transported to the Golgi apparatus for excretion from the
cell. Okay, so here’s that graphical
illustration. The translation process incorporates 20
different amino acids, dictated by the three-base codons built from the
alphabet of four bases. [BLANK_AUDIO]. There are four primary characteristics of
DNA that I would like to repeat. First is the code is universal. What we see in penguins, what we see in
oak trees, what we see in cucumbers. The DNA code is universal for all living
organisms. Second, the code is in triplet. Each amino acid is specified by a sequence
of three basis. The code is commaless. There are no pauses separating the codons. There are stops and there are starts,but
there are no commas. If a single base is deleted, the entire
construction after it would be affected. Similarly if a, if a base were inserted,
it would affect the entire construction after
it as well. These are frame shift mutations, which we
will talk about when we discuss mutation. And fourthly, the code is degenerate. There are 64 possible codons to create 20
amino acids, so there’s a tremendous amount of redundancy of this
code and we need to keep that in mind. [BLANK_AUDIO]

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