Control, Genome and Environment
Cellular Control & Variation
(a) state that genes code for polypeptides, including enzymes; (b) explain the meaning of the term genetic code;
The sequence of the bases on a gene is a code with instructions for the construction of proteins. It has a number of characteristics:
It is a triplet code- three bases code of an amino acid
It is a degenerate code- All amino acids bar one have more than one code Some codes don’t code for amino acids but are ‘stop’ codons- they indicate the end of the polypeptide chain
It is widespread but not universal- Codons generally always code for the same amino acid in every organism, but this is not always the case.
(c) describe, with the aid of diagrams, the way in which a nucleotide sequence codes for the amino acid sequence in a polypeptide;
1. Free RNA nucleotides are activated, two extra phosphoryl groups are added to make ATP, GTP, CTP and UTP.
2. The gene to be transcribed unwinds and unzips. To do this the length of DNA that makes up the gene dips into the nucleolus & the hydrogen bonds between the nucleotide bases break. 3. Activated RNA nucleotides binds, using Hydrogen Bonds, with their complementary exposed bases on the template strand. This is catalysed by RNA polymerase 4. The two extra phosphoryl are released, releasing energy for bonding two adjacent nucleotides The mRNA produced is complementary to the nucleotide base sequence on the template strand of DNA and therefore is a copy of the base sequence on the coding strand 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
(d) 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, transfer RNA and ribosomes; 1. A molecule of mRNA binds to a ribosome. Two codons are attached to the small subunit of the ribosome and exposed to the large subunit. The first exposed mRNA codon is always AUG. Using ATP energy and an enzyme, a tRNA molecule with the amino acid methionine and the anticodon UAC forms hydrogen bonds with this codon
2. A seconds tRNA molecule, bearing a different amino acid, binds to the second exposed condon with its complementary anticodon
3. A peptide bonds forms between the two adjacent amino acids. This is catalysed by an enzyme in the small ribosomal sub unit
4. The ribosome now moves along the mRNA reading the next codon. A third tRNA brings another amino acid and a peptide bonds forms between it and the dipeptide. 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, for which there are no corresponding tRNAs and the polypeptide chain is complete
(e) state that mutations cause changes to the sequence of nucleotides in DNA molecules; (f) explain how mutations can have beneficial, neutral or harmful effects on the way a protein functions; Beneficial
The mutation changes the sequence of amino acids and therefore the phenotype, but this gives the organism an advantageous characteristic
E.g. Paler skin in more temperate climates absorbs more vitamin D Neutral
It is a mutation in a non-coding region of the DNA
It is a silent mutation- although the base triplet has changed, it still codes for the same amino acid and so the protein is unchanged.
The mutation changes the sequence of amino acids and therefore the phenotype, and the resulting characteristic is harmful
E.g. paler skin in a hotter climate burns more easily
(g) state that cyclic AMP activates proteins by altering their three-dimensional structure;
Control, Genome and Environment
(h) explain genetic control of protein production in a prokaryote using the lac operon; E. coli grown in a culture medium with no lactose can be placed in a growth medium with lactose. At first they cannot metabolise the...
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