UNIT 3: Molecular Genetics
Chapter 7: Genes and Proteins Synthesis
pg. 310 -
7.2: Transcription: DNA-Directed RNA Synthesis
pg. 319 - 324
Initiation
Promoter – is a nucleotide sequence that lies just before a gene and allows
for the binding of RNA polymerase.
TATA box – is a region of the promoter that enables the binding of RNA
polymerase.
In prokaryotes and eukaryotes RNA polymerase will bind to the DNA
molecule at a promoter region just upstream from the actual gene. In
eukaryotes the promoter region contains a TATA box (high concentration of
A and T) that the RNA polymerase attaches, where prokaryotes contain a
TATAAT sequence.
Since adenine and thymine are complementary and are bonded together by
two hydrogen bonds, it is easier to break these bonds as opposed to breaking
the three hydrogen bonds between cytosine and guanine.
RNA polymerase used less energy when unwinding and unzipping the DNA
molecule. At this point the portion of the DNA molecule containing the gene
to be transcribed into RNA is exposed and is called the transcriptional unit.
Elongation
Coding strand – is the DNA strand that is not being copied but contains the
same sequence as the new RNA molecule.
When the DNA molecule is unzipped the process of producing a pre-mRNA
begins without the need of a primer. The pre-mRNA molecule is produced in
5′ to 3′ direction, reading the template DNA strand from a 3′ to 5′ direction.
The template strand is read, the opposite DNA strand is known as the coding
strand, it will the same as the pre-mRNA sequence, except the coding strand
has thymine while the pre-mRNA has uracil.
As the DNA continuous to unzip, RNA nucleotides molecules are being
attached one by one.
If the cell requires a particular protein, they usually need to produce
thousands or even millions of copies. Instead of producing the protein one
event at a time, a second RNA polymerase may follow the first, if there is
room at the promoter region, and produce another pre-mRNA. This can
occur many times, producing many pre-mRNAs.
Termination
Termination sequence - is a sequence of bases at the end of a gene that
signals the RNA polymerase to stop transcribing.
Once the RNA polymerase reaches the end of the gene, RNA polymerase
recognizes a termination sequence, and transcription stops. In eukaryotes
have a termination sequence identified by a string adenine bases. The new
formed pre-mRNA now disassociates from the DNA template strand.
Figure 1: Transcribing a gene into precursor mRNA in eukaryotes. There are three stages: initiation,
elongation, and termination. RNA polymerase moves along the gene, separating the two DNA strands to
allow RNA synthesis in the 5′ → 3′ direction. The 3′ → 5′ DNA strand is used as a template.
Post-transcriptional Modifications
Poly (A) tail – is a chain of adenine nucleotides that are added to the 3′ end
of the pre-mRNA molecule to protect it from enzymes in the cytosol.
5′ cap – is a sequence o seven Gs that is added to the start of a pre-mRNA
molecule; ribosomes recognize this site and use it as the site of initial
attachment.
Exon – is a sequence of DNA or RNA that codes fort part of a gene.
Intron – is a non-coding sequence of DNA or RNA.
Spliceosome – is an enzyme-protein complex that removes introns from
mRNA.
Small ribonucleoprotein (snRNP) – is a protein that binds to introns and
signals them for removal.
Alternative splicing – is a process that produces different mRNAs from premRNA (exons and introns), allowing more than one possible polypeptide to
be made from a single gene.
The newly transcribed RNA molecule (pre-mRNA) or primary transcript
must be modified prior to leaving the nucleus and entering the cytosol’s
harsh environment, where enzymes can derogate the mRNA.
One of the modifications is the addition of a poly (A) tail. A chain of 50 to
250 adenine nucleotides are attached to the 3′ end of the pre-mRNA by an
enzyme called poly-A polymerase. The poly (a) tail protects the end of the
mRNA molecule as it enters the cytosol.
A second modification is the addition of the 5′ cap. The 5′ cap consists of
seven guanine nucleotides being attached the 5′ portion of the pre-mRNA.
The guanines act as the initial attachment point to the ribosome and gives
direction to reading the mRNA.
The last modification is the removal of non-coding regions from the premRNA, and the reattachment of coded regions, in order, to created mRNA.
The non-coding regions are interspersed between coding regions along the
pre-mRNA; non-coding regions are called introns and the coding regions are
called exons.
Eukaryotes contain introns in their genetic codes, but prokaryotes do not
contain introns, therefore this post-transcriptional modification does not
occur.
Spliceosomes are enzymes responsible for splicing, removing the introns
and combined the exons together.
Alternative splicing can occur where the exons are resembled in a different
order, creating a number and variety of proteins from one gene. From the
one pre-mRNA molecule, a family of proteins can be produced, which have
different functions. This explains where there are approximately 20 000
genes and over 100 000 proteins in humans.
Figure 2: The relationship between a eukaryote protein-coding gene, the pre-mRNA transcribed from it,
and the mRNA processes from the pre-mRNA.
Transcription in Eukaryotes versus Prokaryotes
Eukaryote transcription is similar to prokaryotes but not identical. One
difference is in the molecule of RNA polymerase. Prokaryotes have one type
that transcribes everything, but in eukaryotes there are three types, where
RNA polymerase II transcribes genes that produce proteins, Type I and III
transcribes non-protein coding genes (tRNA and rRNA).
Table 1: Compare Eukaryote to Prokaryote, pg. 323
Variable
Prokaryotes
Eukaryotes
Location
- throughout the cell
- in the nucleus
Enzymes
- one type of RNA polymerase
- 3 types of RNA polymerase
Elongation
- 15 to 20 nucleotides per second
- fast
- 5 to 8 nucleotides per second
- slow
Promoters
- less complex
- more complex, found upstream
Termination - proteins cleave mRNA
- polyuracil site, protein bind here
Introns/
Exons
- no introns
- introns and exons
Product
- mRNA ready to translate
- pre-mRNA needs to be modified