3.4.3 Genetic diversity can arise as a result of mutation or during meiosis Flashcards
What is a gene mutation?
- A random change in the DNA base/nucleotides sequence.
- results in the formation of a new allele
Where can mutations occur?
- somatic (body cells)
- germ line (gametes). Germ line mutations cause discontinuous variation
What is the difference between gene (point) mutations and chromosome mutations?
Gene mutations change one or more nucleotide bases whereas chromosome mutations result in changes in whole chromosomes
What are the 3 types of gene mutation?
- addition mutations (bases are added to the sequence)
- deletion mutations (bases are removed from the sequence)
- substitution mutations (bases are swapped in the sequence)
What is a non-sense mutation
- non-sense mutation: causes a stop condon to be coded for. Leads to truncated protein that can’t function properly.
Explain a mis-sense mutation
- mis-sense mutation: causes different amino acid coded for
- so changes the sequence of amino acids (primary structure)
- which changes types of bonds formed
- which changes way polypeptide folds (tertiary structure)
- so changes function of protein
Explain a silent mutation
- has no effect because the same amino acid is still coded for
- because genetic code is degenerate
What is a frameshift mutation?
- caused by addition and deletion mutations
- reading frame is changed, changing the entire amino acid sequence coded for after the mutation
What are the causes of mutations?
- spontaneously/randomly during DNA replication
- mutenic agents (‘mutangens’) can increase mutation rate
- including: high energy radiation and chemicals that alter DNA structure or interfere with transcription
What are the advantages of mutations?
Can create genetic diversity which is required for natural selection and speciation
What are the disadvantages of mutations?
- can create less advantageous alleles
- can cause cancer by disrupting cell division
What 2 types of genes control cell division?
- proto-oncogenes stimulate cell division
- tumour suppressor genes slow/inhibit cell division
What happens if proto-oncogenes mutate?
- mutate into oncogenes
- results in receptors on cell surface membrane being permanently activated so cell division is permanently switched on
OR - the oncogene itself codes for growth factors
- these are then produced in excess, continually stimulating cell division
What happens if a tumour suppressor gene mutates?
- it becomes inactivated
- can’t inhibit cell division any longer
- so the cell divides constantly
Meiosis overview
- produces daughter cells that are genetically different to each other
- involves two nuclear divisions
- creates 4 haploid daughter cells (gametes) from a single diploid parent cell
- can fuse with another haploid gamete during random fertilisation
What are haploid and diploid cells
Haploid - only one set of chromosomes. n
Diploid - two copies of each chromosomes (homologous). 2n
How is genetic variation introduced?
- independent segregation of homologous chromosomes
- crossing over between homologous chromosomes
- random fertilisation
Describe independent segregation
- homologous pairs of chromosomes line up opposite each other at the equator of the cell
- Random which side of the equator the maternal and paternal chromosomes lie
- pairs separated so one of each homologous pair ends up in daughter cell
How do you calculate the number of possible chromosomes combinations in the daughter cells/gametes? A zygote?
- 2^n
- n = number of homologous pairs
- 2^23 in humans
- when you take into account random fertilisation… (zygote)
- (2^n)^2
- in humans (2^23)^2
- doesn’t take into account crossing over
Describe crossing over
- homologous pairs of chromosomes associate
- parts of chromatid become twisted around each other, chiasma forms
- sections/lengths of chromatids with different alleles exchanged
- new combination of alleles on the chromatids
What mechanism is responsible for chromosomes mutations?
- non disjunction
- due to individual homologous chromosomes not separating during meiosis
What are the two outcomes of chromosome mutations?
- changes in number if individual chromosomes - organisms have an extra copy of an individual chromosome
- polyploidy: changes in whole sets of chromosomes- when organisms have 3 or more sets of chromosomes
When do gene mutations occur? When do chromosomes mutations occur?
- gene - during DNA replication
- chromosome - during meiosis
Describe chromosome mutations
- mechanism: non-disjunction
- due to individual homologous chromosomes not separating during meiosis
- organisms have extra copy of an INDIVIDUAL chromosome (downs syndrome)
- OR polyploidy- changes in whole sets of chromosomes
- organisms have 3 or more sets of chromosomes
Why does a mutation in a gamete affect all body cells?
- mutation in gamete that formed zygote
- all cells derived from zygote by mitosis
- mitosis produces genetically identical cells
Why might a gene mutation not have an effect?
- genetic code is degenerate
- new amino acid is coded for but this does not affect tertiary structure
- new allele is recessive so has no effect on phenotype
