Cinelecture 15 – Nucleic Acids – DNA, RNA


no DNA is a very pretty molecules, the DNA double helix but in order to understand its structure
which we’ll study in great detail later let’s just look at
a single-strand of DNA and examine how the polymer is put together so thats what we’ll do right now. and I just like to point out that I’m going to spend a little bit I’m on this because this is such an important molecule this is the hereditary material and I really would like you to
understand its structure in some detail we’ll need to know its structure when we want to understand DNA
replication and other processes later on so we’ll spend a little bit of time on this now. so what we have here is a short stretch a single stranded DNA not double-stranded DNA not double helical DNA and we can see that is made out of component nucleotides in each nucleotide in this four nucleotides stretch we’re only
showing four nucleotides stretch as an example each nucleotide has a nitrogenous base having nitrogen show in blue hydrogens are not represented here for
simplicity’s sake and then the second component is a ribos sugar there’s a ribos sugar carbon atoms are shown in gray and oxygen are in red and you can see that in oxygen lies at
the apex of the sugar and then the third component of every nucleotide is a phosphate group phosphous plus oxygens and those are the three components of every nucleotide in the a DNA polymer so the nucleotides are the monomers and DNA is the polymer so now we can consider how the linkages that connect the varies atoms of the molecule so considering the ribo sugar there’s an
oxygen at the apex of the ribo sugar there is the first carbon of the sugary ring, second third fourth and 5th carbon and now let’s indicate that there is a polarity to this strand because the linkages go from the five prime carbon to the 3 prime carbon and then on down the molecule so there is a polarity to the chain
dictated by the polarity of the sugars and the phosphates so if we look at our phosphorus atoms now you and consider what is attached to them and this molecule could go on up and down for long strecthes we’re only showing four nucleotide stretch but naturally
because we’re talking about possibly group their oxygen attached to the phosphorous atoms in the phosphate functional groups and those oxygens can have hydrogen on them that they donate to solution and that makes for an acid when hydrogen ion concentration goes up we have an acid which is donated hydrogens now
if we look at the sugar and look at the the second carbon we see that it does not have an oxygen attached to it
so it is deoxy Deoxy minus a hydroxyl group there’s no oxygen attached no OH group attached to the second carbon in RNA there is but this is DNA so we have deoxy ribo sugars and since we do have ribo sugars as a main component of every nucleotide
then we say that there’s deoxy ribo standing for the ribos and we know that the nucleic acids are so named because
they were first discovered in the nucleus even though RNA is found in the cytoplasma as well, nevertheless RNA and DNA are known as nucleic acids because DNA was discovered in the nucleus so we have deoxy ribonucleic and then the oxygens on the phosphate
remember can donate hydrogens to solution making
the phosphate groups the phosphate functional groups, acidic
because they increase the hydrogen ion concentration in an aqueous solution. Therefore we have an acid and DNA is an acid so DNA deoxyribonucleic acid now considering, showing the individual
monomers now in color we’re showing a nucleotide in blue that has A as its nitrogenous base, adenine, the yellow nucleotide has thymine as its nitrogenous base, red cytosine and green guanine as its nitrogenous base so you can see the individual monomers clearly here and if we zoom in on the nucleotide containing sizing as its nitrogenous base and number the carbons in the sugar ring 1, 2, 3, 4, 5 carbon 1 and the nitrogen of the nitrogenous base is linked to the first carbon of the sugar and that is called a glycosidic bond and its always a nitrogen of the nitrogenous base that is linked to the first carbon of the ribo-sugar and glycogen implies sugars so a glycosidic bond links the nitrogenous base to the ribosugar now if we consider
how the sugar is linked to the phosphate its linked through an oxygen to the phosphorus atom and that phosphorus atom is linked to the next oxygen that is typed, which is covalently
bound to the third carbon of the nucleotide above it and so we
have oxygen linkages are called ester
linkages and we therefore have a phosphate with two ester linkages on either side I should say a phosphorus atom with two
ester linkages on either side it and you can see now that if you remove phosphates those are the linking points for the, for the polymer. So we have a phospho- diester backbone phosphodiester linkage there’s the phosphate and there’s one ester linkage and there’s the other one so we have a phosphodiester linkage that connects the backbone, connects the
monomers in the backbone so that we can then consider the molecule as a sugar phosphate backbone here shown
in purple sugars and phosphates, sugar and phosphates and that’s backbone and then the nitrogenous bases hang off the sugars and in double helic DNA they are available for hydrogen bonding to bases from the other strand of DNA in a double helix note that the nitrogenous bases are not
connected to each other except covalently speaking they are not
connected to each other except through the phosphate, the sugar phosphate backbone. alright then having done it now makes it
easy to understand the schematic representations that you often see so here is a single-stranded a short stretch a single stranded DNA
shown schematically with the number carbons of the sugar rings the nitrogenous bases heres the glycosidic bond and the phosphates connecting the sugars through a diester linkage the oxygen aren’t shown here notice that we are deoxy on the
two front carbon no oxygen there and these are the nitrogenous bases this uracil is only found in RNA not DNA, DNA has 4 nirtogenous bases to purines at adenine and guanine these are double ringed aromatic rings, aromatic ring means that there are carbong rings with mostly with double
bonds and these electrons and these double bonds can resonate around the ring they are called aromatics so we have to purines with the double aromatic rings and two pyrimidines cytosine and thymine smaller single aromatic rings those are in DNA G, T, C or A is possible at any given position along a long stretch of DNA we’ll talk about the genetic code later
so we know that DNA encodes amino acids
it encodes proteins and DNA exists in a double helix and there are base pairing rules so we have one strand of DNA here, another strand here the nitrogenous bases are kind of like in the center of the molecule along the long axis and they are arranged, the two strands of DNA that wrapped around each other arranged such that hydrogen bonds form between adenine and thymine and between guanine and cytosine. There’s adenine and thymine but only between those base pairs adenine can’t pair with guanine or cytosine cytosine can’t pair with adenine or thymine T’s are always paired with A’s and G’s are always paired with C’s here’s an A-T base pair there so we’ll talk more about that a little later in the course. RNA is similar to DNA but it contains ribose instead of deoxyribose so there is a hydroxy group on the two prime carbon the second carbon of the ribose sugar and RNA contains uracil instead of thymine and RNA is usually a single strand not double-stranded. Although some double stranded RNA do exist and RNA is the carrier of information from DNA to
specify the manufacture of a particular sequence amino acids in
proteins again a process we’ll study later so here is DNA and RNA compared double helical DNA, thymine whereas RNA has uracil instead of thymine single-stranded RNA, double-stranded
DNA hydrogen-bonded base pairs in the
interior and of course DNA, two prime carbon is dioxy whereas, the two prime
carbon in RNA has a hydroxy group on it not shown here. there are other
nucleotides than the ones found in the DNA for example ATP shown here adenine, adenosine rather which means there is an adenine hydrogenous base and the scene implies that there’s sugar
attached to that nitrogenous base so here we have adenosine adenine plus the sugar, ribosugar note that it is not dioxy and 3 phosphates so this stands for ATP stands for adenine here tri I’m sorry adenosine rather, adenosine thats these moieties together these two trophosphate, 3 phosphates, adenosine triphosphate and this is the primary energy currency
of the cell because these breakage off of phosphates here releases energy that can be used and can be harnessed by enzymes to catalysed biochemical reactions ATP is the energy currency of the cell and they’re also NAD and FAD that are electron carriers and we will see
that the passage of electrons in redox reactions, know we talked about reduction oxidation so NAD can be reduced to it can accept basically an electron on a hydrogen and it can become NADH and become reduced NAD by accepting an electron and FAD
can accept a H and become reduced FAD and then that electron can passed on to other molecules and in so doing that we can release
energy that can be harnessed for various purposes as we will see so that something to keep
in mind will see that again now we’re ready to tackle proteins and
so we’ll go on to the next part for that

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