Genes and Inheritance [10]: Transcription (A Level Biology)


Uploaded by freeeschool on 02.09.2012

Transcript:
A Level Biology: Genes and Inheritance 10 - Transcription
Hi! Welcome to my 10th video on the series about Genes. Today, we are going to be looking
at Protein synthesis, specifically Transcription.
The two major processes involved in Protein synthesis are Protein Transcription and Translation.
What we are looking mainly today is Transcription. In transcription, the relevant gene in the
DNA and the nucleus is copied into a molecule of RNA called mRNA or messenger RNA. The DNA
is always found in the nucleus and RNA is a single strand of genetic material, whereas
DNA is double-stranded. mRNA is able to move from the nucleus into the cytoplasm of the
cell. The reason this is useful is because the thing that actually builds the protein
is found in the cytoplasm.
This represents some DNA found in the nucleus of a cell. As you can see, it is made into
three parts: This is the phosphate group, this is your ribose sugar and this in the
middle here, and this is one of your nucleotide bases. This shows a double-stranded section
of DNA.
Within transcription, what happens first is the two strands of DNA actually unzip and
they begin to move apart. Now, they’re held together although they were held together
by hydrogen bonds and so those hydrogen bonds are broken, which means they’re free to
move apart. These two polynucleotide strands separate. Nucleotide being in the nucleotide
bases and poly meaning many because there are lots of them. There are four. One of the
strands, that’s referred to as sense strand and that’s usually the one on the right
in diagrams.
The other strand is actually called the antisense strand, but it is from the sense strand and
this one here that we are going to take a template and that mRNA is going to come in
and bind and so it can then take a template and then move into the cytoplasm of the cell.
What then happens is free nucleotide bases come in and bind. With mRNA, single-stranded
but it has a slightly different make-up to DNA. Instead of having the base Thymine, it
has base Uricil, so it is represent by U.
If the first base comes in and binds, second, third and fourth. You’ll see that the U
is basically a replacement for the T. In fact, the U binds with the A but still cystine still
binds with the guanine, guanine still binds with the cystine and adenine still with thymine.
Now, what then happens is the phosphate, the circular pieces here and the ribose sugars
form bonds. Then this whole structure here is what’s known as mRNA. That’s now free
to move from the nucleus out into the cytoplasm. What then occurs is that the messenger RNA
is then free to detach from the sense RNA and it can go and leave the nucleus and then
go into the cytoplasm as it is happening there. What then can occur is the DNA can then bind
back together. This process is repeated hundreds of thousands of times.
We can think of the DNA being the like blueprints. The blueprints, you normally keep in one location
where everybody knows they can go and get them. The DNA acts as a blueprint for everything
and it stays within the nucleus. It is the mRNA which is able to take a template and
then move into the cytoplasm for the next phase of protein synthesis which is translation.
In summary, in transcription, the first thing that happens is that the double-stranded DNA
uncoils and unzips so you are left with two strands. You then have free nucleotide bases
are able to bind with the sense strand of the DNA, which is usually the right hand one.
Those free nucleotide bases then join up in themselves and then they form this chemical
called mRNA which is known as messenger RNA. That’s then free to move from the nucleus
to move into the cytoplasm where it can build the protein. Before it does that though, it
has got to remove the sections within the mRNA that don’t code for proteins and those
sections are called introns and so what’s left behind is some messenger RNA that only
codes for proteins. Those sections that actually code as called exons.
[end of audio – 04:41] A Level Biology: Genes and Inheritance 10
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