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