Transcription and Gene Expression

In order to properly function and survive,
living things must express their genes to make protein. Some genes need to constantly
express their proteins, but other proteins only need to be made some of the time. Prokaryotes
and eukaryotes have different ways of regulating their gene expression. Prokaryotes rely on
variations in their environment to regulate their gene expression. For example, if lactose
is present in the environment, it will trigger the expression of lactase so the bacteria
can break down the lactose. If lactose is absent, the gene is not expressed. Eukaryotes
also regulate genes in response to their environment. Each cell of a eukaryote only expresses a
small number of genes in the nucleus that pertain to its job. Transcription factors
are proteins that regulate gene expression by binding to specific sequences that are
associated with a transcription initiation site. Transcription factors can be activators
or repressors. Along the DNA strand there are regions of DNA that are called enhancers
and regions called are silencers. Enhancers are regions of DNA that increase the rate
of transcription when an activator binds to them. Silencers are regions of DNA that decrease
the rate of transcription when a repressor binds to them. The environment of the cell
and even of the whole organism can effect the gene expression. The Siamese Cat is a
perfect example. Siamese cats have a mutated allele that codes for the enzyme tyrosinase,
the first step in the production of fur pigment, which is affected by temperature. When the
temperature drops, gene expression increases and the pigment in the fur increases. When
the temperature increases, gene expression is reduced and the pigment in the hairs decrease.
Nucleosomes also play a role in gene expression. When the tails of the histones are acetylated,
the nucleosomes become less tightly packed. In this loosened form, transcription can occur,
which is the first part of gene expression. When the tails of the histones are deacetylated,
the nucleosomes pack in tighter, stopping gene expression. Additionally, methylated
cytosine will tighten the nucleosomes while unmethylated cytosine will loosen it. So,
active (open) chromatin and unmethylated cytosine along with acetylated histones will activate
a gene and turn it “on.” Silent (condensed) chromatin and methylated cytosine along with
deacetylated histones inactivate a gene and turn it “off.”
Genes are the sequences of DNA which transcribe into RNA, typically becoming protein during
translation. Genes have three main parts, the promoter, which is the transcription initiation
site, the coding sequence which is transcribed, and the terminator is the site where transcription
ends. DNA is double stranded, but only one strand is transcribed for protein synthesis.
The coding strand is called the antisense strand. The non-coding strand is the sense
strand. Transcription begins at the promoter on the antisense strand. The RNA forms in
the 5 prime to prime direction, reading the antisense strand in the 3 prime to 5 prime
direction. RNA polymerase binds to the promoter, unzips DNA by breaking the hydrogen bonds
and covalently joins the RNA nucleotides together. Base pairing between DNA and RNA is still
between C and G, and A and T, but on the mRNA strand, uracil replaces thymine. When messenger
RNA is formed in prokaryotes, it is immediately ready for transcription. But in eukaryotes,
the mRNA is referred to as pre-mRNA and needs to be modified to become mature mRNA. Sections
of mRNA will be removed and then the mRNA will be spliced back together to make mature
mRNA. The sections that are removed are called intervening sequences or introns, and the
sections that are kept are called exons. After the introns are removed and the exons are
spliced, the mRNA is mature and ready for translation into protein. Eukaryotes can selectively
remove exons of a gene to form different proteins from the same gene. This is called alternative
splicing. This greatly increases the variety of proteins that can be produced. Strangely,
only 1% of DNA actually codes for protein. The rest of the DNA is called non-coding DNA
and used to even be called “junk DNA” in the past. But just because the DNA doesn’t
code for protein, doesn’t mean it has no purpose. Here are 5 different uses for non-coding
DNA, with the acrostic, STING to help you remember them. STING stands for satellite
DNA, telomeres, introns, noncoding RNA genes and gene regulatory sequences. Satellite DNA
includes tandem repeats which are the portions used for DNA profiling. Telomeres are repetitive
DNA at the ends of chromosomes that protect the chromosomes from deterioration during
replication. Introns are non-coding sequences within genes. Noncoding RNA genes can form
structural RNA like the tRNA molecules, and gene regulatory sequences include regions
of the DNA that are used in the process of transcription such as promoters, enhancers
and silencers. Thanks for watching this episode of Teacher’s Pet, don’t forget to like
and subscribe and follow me on twitter @SciencePet

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