Chapter 1 - Cellular Control
a.State that genes code for polypeptides including enzymes.
Gene is a length of DNA. A sequence of nucleotide bases that code for one or more polypeptides. There are about 25,000 genes in a human genome.
Each gene occupies a specific place/locus on the chromosome.
Polypeptides that are coded for by genes:
Structural proteins-collagen and keratin.
Cell surface receptors.
Actin and myosin in the muscle cells.
Tubulin proteins in the cytoskeleton.
b. Explain the meaning of the genetic code.
Genetic code consists of two words...
Triplet code is three nucleotide bases code for one amino acid.
Degenerate code is All amino acids by methionine have more than one code.
Some codes indicate “stop”/the end of the polypeptide chain.
Widespread but not universal-in some organisms some codes code for different things but not all.
c. Describe the way in which a nucleotide sequence codes for the amino acid sequence in a polypeptide.
1. Hydrogen bonds between the complementary bases on the DNA break.
2. The gene that is being transcribed unwinds and unzips.
3. The activated RNA nucleotides bind with hydrogen bonds complementary with the exposed bases on the template strand of DNA. This is catalysed by RNA Polymerase.
4. Two extra phospholipid groups are released to release energy for bonding adjacent nucleotides.
5. mRNA is produced that is complementary to the template strand of DNA.
6. mRNA is released from the DNA by passing through the nuclear pore to a ribosome.
d. Describe how the sequence of nucleotides within a gene is used to construct a polypeptide, including the roles of messenger RNA, transfer RNA and ribosomes in Transcription and Translation.
- A molecule of mRNA binds to a ribosome. Two codons (six bases) attach to the small subunit and are exposed to the large subunit. Using energy and an enzyme, tRNA and the correct anticodon bases form hydrogen bonds with the codon.
- A second tRNA, with complementary anticodons, bind with the second exposed codon.
- A peptide bond forms between the two amino acids which is catalysed by an enzyme present in the small ribosomal subunit.
- The ribosome moves along the mRNA to read the next codon. A third tRNA brings another amino acid. A peptide bond forms between it and the other amino acids. The first tRNA leaves so that it can bring another amino acid.
- The polypeptide chain grows until a stop codon is reached. This is because there are no corresponding tRNA’s.
e. State that mutations cause some changes to the sequence of nucleotides in DNA molecules.
- Mutation is a random change to the genetic materials.
- Point mutation is where one base pair replaces another. Also known as substitutions.
- Insertion/deletion mutations is where one or more nucleotide pairs are inserted or deleted from a length of DNA. Also known as Frameshift mutations.
- Different types of substitutions
Nonsense mutation leads to the termination of a polypeptide.