Transcription and Translation: From DNA to Protein

Transcription and Translation: From DNA to Protein


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. Thanks for watching, guys. Subscribe to my channel for more tutorials, and as always, feel free to email me: