F215 Module 1 Cellular Control DNA - Apoptosis

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  • Created by: Lexi
  • Created on: 01-02-13 10:34

State that genes code for polypeptides, including enzymes

A gene is a length of DNA that codes for one or more polypeptides. Examples of such polypeptides are haemoglobin and enzymes

Explain the meaning of the term genetic code

The sequence of nucleotide bases on a gene provide a code with instructions for the construction of a protein. The genetic code has 4 main characteristics:

  • It is a triplet code - a sequence of 3 nucleotide bases codes for an amino acid. There are 4 bases that can be arranged in these groups of 3, meaning that there are 64 possible combinations
  • It is a degenerate code - all amino acids except for methionine have more than one code
  • Some codes don't correspond to an amino acid but indicate 'stop' i.e. the end of the polypeptide chain
  • 'It is widespread but not universal' - Some base sequences code for the same amino acid in any organism, for example, TCT = serine. However there are some variations

Describe, with the aid of diagrams, the way in which a nucleotide sequence codes for the amino acid sequence in a polypeptide

Through TRANSCRIPTION, the 1st stage of protein synthesis

1. The length of DNA that makes up the gene dips into the nucleolus, allowing it to unwind and unzip. Hydrogen bonds between complementary bases break

2. Activated RNA nucleotides bind to their exposed complementary bases with hydrogen bonds. This is catalysed by RNA polymerase

3. x2 extra phosphoryl groups are released in order to release energy for bonding adjacent nucleotides

4. The mRNA produced is complementary to the nucleotide base sequence on the template strand of the DNA and is therefore a copy of the base sequence on the coding strand of the length of DNA  

5. The mRNA is released from the DNA and passes out of the nucleus, through a pore in the nuclear envelope, to a ribosome

Describe, with the aid of diagrams, how the sequence of nucleotides within a gene is used to construct a polypeptide, including the roles of messenger RNA, trnasfer RNA and ribosomes

TRNASCRIPTION is the 1st stage of protein synthesis

TRANSLATION is the 2nd stage, occurring at ribosomes, which are in the rough endoplasmic reticulum or free in the cytoplasm

Ribosomes are assembled in the nucleolus of eukaryote cells, from rRNA and protein; each is made up of 2x subunits and there is a groove into which the length of mRNA, with the code for the sequence of amino acids, can fit.The sequence of amino acids is critical because:

  • It forms the primary structure of the protein
  • Primary structure determines tertiary structure
  • Tertiary structure is what allows a protein to function
  • If the tertiary structure is altered, the protein can no longer function so effectively, if at all e.g. changed active site shape

tRNA is made in the nucleus and passes into the cytoplam; these are lengths of RNA that fold into hairpin shapes and have 3x exposed basses at one end where a particular amino acid can bind. At the other end…


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