Hey it's professor Dave, let's talk about DNA transcription and translation.
Now that we understand the structure of DNA
it's time to understand exactly how this molecule codes for a particular organism.
How is it that a single cell containing a specific set of genetic material will
result in the development of a fish or a cat or a human? To understand this
phenomenon we have to learn about transcription and translation. This is
the collective process by which the genetic code is read by enzymes in order
to produce all of the proteins in an organism. A chromosome is a very long
molecule consisting of many millions of base pairs. Most of these bases don't do
too much, but certain portions of the chromosome are special. They are called
genes. These are the parts that code for different things. In a human a gene will be
on average around 10 to 50 thousand base pairs long, though the longest is
two-and-a-half million base pairs, and when a gene is expressed a specific
protein is produced.
So how does this work? The first step is called transcription. This is the process
by which enzymes use one of the strands of DNA within a gene as a template to
produce a messenger RNA, or mRNA. To do this, RNA polymerase, with the help of
proteins called transcription factors, binds to a specific sequence within the
gene, which is called the promoter, and pries the two strands apart. One of the
strands will serve as the template strand, or antisense strand, meaning it
will be used to generate the mRNA, and the other is the nontemplate strand or
the sense strand. RNA polymerase doesn't need a primer, it simply initiates mRNA
synthesis at the start codon, and then moves downstream along the gene in a process
called elongation, synthesizing the mRNA as it goes,
reading the antisense strand from 3' to 5' and generating the mRNA
from the 5' end, attaching RNA nucleotides to the 3' end as it goes.
This is very similar to the way DNA polymerase synthesizes DNA as it moves
along the template strand, the main difference here is that RNA is being
synthesized, which as we recall will be ribose rather than deoxyribose, and it
will have uracil instead of thymine. Unlike replication, RNA polymerase zips
DNA back up as it goes
keeping only 10 to 20 bases exposed at a time. Once RNA polymerase reaches the end
of the gene, termination occurs, the enzyme detaches from the gene and the
DNA is returned to its original state. But we have produced an mRNA. This
carries with it the information encoded in the gene, and after a few quick
modifications during RNA processing it will leave the nucleus, where all the
genetic material or chromatin is, and move into the cytoplasm, where it will
find a ribosome. This is where translation occurs. During translation
the mRNA acts as a code for a specific protein. This happens because each set of
three bases on the mRNA, which we call codons, will code for a specific
anticodon, which will be carried by a specific transfer RNA, or tRNA, and each
different tRNA is covalently linked to a particular amino acid. The arrangement of
the nucleotides into these codons is called the reading frame. Since there are
four bases and each codon has three letters, 4^3 gives us 64 different
possible codons, which is more than enough to code for all the amino acids
we need. Here is a table of all the mRNA codons and the amino acids they code for.
Notice that there is some redundancy, with multiple codons resulting in the
same amino acid, but there is no ambiguity. Each codon corresponds to a
particular amino acid. Notice also that some of these codons are special. AUG is
the start codon, which initiates translation by coding for methionine,
and these three are stop codons. These are the ones that terminate translation.
Translation will occur inside a ribosome.
The small ribosomal subunit binds to an mRNA and an initiator tRNA, which
adheres to the start codon. Then the large ribosomal subunit joins to
complete the translation initiation complex. Then, the tRNA that corresponds
to the next codon after the start codon will enter the ribosome. This will carry
with it an amino acid, which becomes covalently bound to the methionine
from the initiator tRNA. The first tRNA detaches and leaves the ribosome, which
has shifted over, making room for the next tRNA. The new amino acid links to
the first two, and this process continues all the way down the mRNA. As tRNAs enter
and exit the ribosome in a sequence that is dictated by the codons on the mRNA, a
polypeptide chain will grow. This continues until a stop codon is reached,
at which point the completed polypeptide will swim away, most likely entering one
of the cell organelles for folding and further modification. So in this two-step
process, DNA is transcribed into an mRNA, and then this mRNA is translated into a
protein, all simply by obeying the base pairing that occurs in nucleic acids, and
since every gene codes for a specific protein, and proteins make up most of
what you are, from your muscle tissue and organ tissue, to all of your receptors
and enzymes, this is how DNA carries the code for a living organism.
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