OCR A2 Biology: DNA, Coding for Proteins, Protein Synthesis

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Key Definitions for this topic:

Gene: Length of DNA that codes for one (or more) Polypeptides

Polypeptide: a polymer consisting of a chain of amino acids residues joined by peptide bonds.

Genome: The entire DNA sequence of an organism. E.g. the human genome consists of about 3 billion nucleotide base pairs.

Protein: A Large polypeptide- usually 100 or more amino acids. Some proteins consist of one polypeptide chain and some consist of more than one or more polypeptide chain.

Transcription: the creationof a single-stranded mRNA copy of the DNA coding strand

Translation: the assembly of polypeptides(proteins) at ribosomes



Genes are lengths of DNA which code for one(or more) polypeptide chains. Specifically it is the sequence of nucleotide bases that determine the polypeptide chain. It is known as a unit of heredity. Most genes are found on the linear chromosomes within the nucleus, however a few can be found in mitochondria.Genes take up specific areas or a locus on the chromosome.

Genes code for polypeptides like collagen, keration, haemoglobin, immunoglobins, antigens, channel proteins, electron carriers, enzymes and many others.

The genetic code is the sequence of nucelotide bases on a gene. The code has a number of specific characteristics:

1) Triplet code: A sequence of 3 bases codes for one amino acid. As there are four bases arranged in threes the different type of triplet seqeunces avaliable is 64. Why? 4 to the power of 3 = 64. As there are only 20 naturally occuring amino acids 64 choices is more than enough.

2) Degenerate Code: All amino acids except methionine have more than one code.

3) Stop codes. Some codes indicate the end of a polypeptide chain

4) Widespread but not universal. E.g. base sequence TCT codes for the amino acid serine in any organism, whereas in mammalian mitochondria there are two codes for methionine. But the fact that the same sequences code for the same amino acids is very useful in genetic engineering.

Genes are found in linear chromosomes inside the nucleus, so why does protein synthesis take place out side the nucleus on the ribosomes? DNA is too large to leave the nucleus through the nuclear pore so a copy must be made that can leave. This copy is called mRNA or Messenger RNA. Another reason to produce a copy is that DNA is valuable similar to books in a refernece library. In the cell, or library, there is only one copy of that particular gene( or book). The DNA cannot be damaged as it the only copy of the gene (book) the cell (library) possess. SO in short.... to prevent damage occuring to the genes a copy is made.

Stages of Protein Synthesis:

 1) Transciptipn



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