5.1.1 Cellular Control
(a) Genes code for polypeptides (including enzymes)
(b) Genetic Code - is a sequence of nucleotide bases on a length of DNA that provides instructions for the construction of a polypeptide or protein.
(c) and (d) The way a nucleotide sequence codes for the amino acid sequence in a polypeptide:
1. A gene to be transcribed unwinds and unzips. To do this the length of DNA that makes up the gene dips into the nucleolus. Hydrogen bonds between complementary bases break.
2. Activated RNA nucleotides bind (with hydrogen bonds) to their exposed complementary bases. U binds with A, G with C and A with T on the template strand. This is catalysed by the enzyme RNA POLYMERASE.
3. The two extra phosphoryl groups (phosphates) are released. This releases 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 on 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.
1. A molecule of mRNA binds to the ribosome. Two codons (6 bases) are attached to the small subunit of the ribosome and exposed to the large subunit. Using ATP energy and an enzyme, a tRNA with the anti-codon forms hydrogen bonds with this codon.
2. A 2nd tRNA, bearing a different amino acid, binds to the second exposed codon with its complementary anticodon.
3. A peptide bond forms between the two adjacent amino acids. An enzyme, present in the small ribosome subunit, catalyses the reaction.
4. The ribosome now moves along the mRNA, reading the next codon. As the 3rd tRNA brings another amino acid, the first tRNA leaves and is able to collect and bring another of its amino acids.
5. The polypeptide chain grows until a stop codon is reached.
(e) Mutations cause changes to the sequence of nucleotides in DNA molecules.
(f) The effects of Mutations can be beneficial,neutral or harmful:
Beneficial; e.g. When humans migrated North in the Environment of Evolutionary Adaptation (EEA), there was less sun so less Vitamin D was available, humans with a paler skin therefore would be able to synthesise more Vitamin D, meaning less chance of developing rickets or a smaller pelvis (problematic during childbirth). Whereas those with darker pigmentation wouldn't be able to synthesise enough Vitamin D.
Neutral; If the mutated gene alters its base sequence, it may become an alternate version of the same gene. It may produce no change if:
-The mutation is a non-coding region of the DNA
-It is silent mutation (although the base triplet has changed, it still codes for the same amino acid, so the protein is unchanged)
-If the mutation does cause a change in protein structure and therefore has a different characteristic, but gives no particular advantage or disadvantage to the organism e.g…