3.1 genes

Question 1

what is a gene?

Answer 1

• particular section of dna that when transcribed and translated, forms a specific polypeptide
• heritable factor that consists of a length of dna and influences a specific characteristic
• section of dna that codes for formation of polypeptide
• controls a specific characteristic of life when expressed
• a unit of inheritance (information) and a physical segment of dna occupying a particular location on a particular chromosome (locus; plural: loci)

Question 2

what are alleles?

Answer 2

• alleles: specific forms of a gene occupying the same gene locus / position as other alleles of that gene
• genes code for a particular trait, but can come in various alternate forms that specifically code for a specific trait
• e.g. gene for hair colour; allele for black hair colour and allele for brown hair colour
• differ from one another by a few bases / small differences in base sequence
• alleles encode for a v similar function which is caused by type of protein produced; thus bases within gene cannot be too different

Question 3

how are new alleles formed?

Answer 3

• by mutation
• mutation: permanent change in base sequence of dna
• when gene is mutated, protein product after transcription and translation may have different amino acid at location
• may alter the shape of the protein –> diff in shape of protein may affect function (esp in enzymes)

Question 4

what is an example of allele mutation?

Answer 4

ptc

• few point mutations occurred and transferred to offspring
• renders a person unable to taste ptc (phenylthiocarbamide)

other mutations can alter rate / type of reaction catalysed

Question 5

what are point mutations?

Answer 5

• aka base substitution mutations [insertion of incorrect nucleotide]
• only affects 1 nucleotide
• 1 nucleotide replaced with another nucleotide due to mutation taking place
• reading frame of gene not altered

types of mutation:

• missense mutation: change in base sequence caused 1 different amino acid to be produced
• nonsense mutation: change in base sequence caused stop codon to be produced; polypeptide produced is shortened

Question 6

what is sickle cell anemia caused by?

Answer 6

• result of SINGLE base substitution mutation; change to base sequence of mrna transcribed from it and change to sequence of polypeptide in hemoglobin
• inherited blood disorder
• co-dominant alleles disease (allele for sickled-cell and normal are co-dominant / both expressed)
• results from single amino acid substitution in one of the genes that make haemoglobin
• haemoglobin consists of heme (made of carbon), iron, 2 ⍺ chains and 2 β chains
• both ⍺ and β subunit are secondary structure (nothing to do w sequence but has to do w hydrogen bonding)
• gene defect is a mutation of a single nucleotide of β-globin gene, results in glutamic acid being substituted by valine at position 6
• the 9 β-chains are affected by the sickle-cell mutation
• to form a normal β-chain, the particular triplet base pairing in the dna the C–T–C on the coding strand of dna being transcribed to mrna triplet G–A–G, which gives glutamic acid
• in sickle-cell mutation, adenine replaces thymine in coding strand, becomes C–A–C on dna which is transcribed to mrna triplet G–U–G, which gives valine
• slight change in primary structure of protein can result in overall shape and structure of protein and can affect function of protein
• homozygous individuals: all haemoglobin abnormal
• symptoms: physical weakness, pain, organ damage, paralysis, carry less oxygen
• heterozygous individuals: healthy but suffer from some symptoms
• red blood cells of affected can appear to be sickle-shaped

Question 7

what is genome?

Answer 7

• whole of genetic information of organism
• genome: all dna present in individual & summation of all genes present in individual
• genes define species of individual and includes smaller and more subtle differences between individuals based on expressed traits