Genetic diversity can arise as a result of mutation or during meiosis Flashcards
Define gene mutations
- Change in the base sequence of DNA of a chromosome
What are the 3 types of mutations?
- Point mutation
- Frame shift mutation
- Chromosome mutation
Describe point mutations
- Single base substitution - changes the codon so that codon codes for a different amino acid
Describe frame shift mutations
- base pair addition or deletion - changes the triplet of bases downstream from the mutation resulting in a frameshift so primary structure of
polypeptide may be completely different
Describe chromosome mutation
- change in chromosome structure and number
What does base deletion cause?
- frame shift
Which mutation is sickle cell anaemia caused by?
- base substitution in the DNA, resulting in a change of amino acid
What is phenylketonuria-PKU?
- disease where the person lacks the enzyme phenylalanine hydroxylase
What does lack of enzyme phenylalanine hydroxylase cause?
- build up of phenylalanine
What are the symptoms of phenylketonuria-PKU?
- Paleness, as no melanin is produced
- Phenylalanine accumulates in blood and tissue fluid – can cause brain damage in children
What is a chromosome mutation that causes individuals to have an abnormal number of chromosomes called?
- aneuploidy
What do aneuploid cells occur as a result of?
- chromosome breakage or nondisjunction errors that happen during meiosis.
What is nondisjunction?
- failure of homologous chromosomes to separate properly during cell division
What does nondisjunction cause?
- individuals with either extra or missing chromosomes
What can sex chromosome abnormalities that result from nondisjunction lead to?
- conditions such as Klinefelter and Turner syndromes
Describe Klinefelter syndrome
- males have one or more extra X sex chromosomes
Describe turner syndrome
- In Turner syndrome, females have only one X sex chromosome
What is an example of a condition that occurs due to nondisjunction in autosomal (non sex) cells?
- Down syndrome
Describe Down syndrome
- have an extra chromosome on autosomal chromosome 21
- translocation of part chromosome 21
What are mutagenic agents?
- can increase the rate of mutation
What is a diploid?
- containing two complete sets of chromosomes, one from each parent.
What is a haploid?
- Containing one complete set of chromosomes
What does meiosis produce?
- daughter cells that are genetically different from each other
Describe the process of meiosis
- Prophase 1 - Homologous chromosomes
pair up, Chromosomes are visible and made of two sister chromatids joined by centromere, Crossing over of, homologous chromosomes
occur - Metaphase 1- Homologous chromosomes
line up on the equator of the cell - Anaphase 1- Homologous chromosomes part
company and are pulled to opposite poles of the cell
Describe the differences between meiosis and mitosis
- meiosis produces four genetically different haploid cells from a diploid cell whereas mitosis produces two genetically identical diploid cells from diploid cell
- meiosis undergoes two nuclear divisions whereas mitosis undergoes one division
- meiosis undergoes crossing over in prophase 1 results in further genetic variation in daughter cell whereas mitosis does not undergo crossing over
What does independent segregation of homologous chromosomes result in?
- genetically different daughter cells
What does crossing over between homologous chromosomes result in?
- further genetic variation among daughter cells
What is meiosis?
- process of nuclear division in which the
chromosome number is halved and variation is introduced into the 4 haploid nuclei produced (gametes)
Why is meiosis important?
- ensures that, when the gametes fuse at fertilisation, the normal diploid number of chromosomes is maintained
What is the formula used to calculate the possible number of different combinations of chromosomes following meiosis?
- 2^n
What does n represent in 2^n?
- haploid number
What is the formula used to calculate the possible number of different combinations of chromosomes following random fertilisation?
- (2^n)^2
Which two mechanisms in meiosis introduce variation?
- Independent segregation of homologous chromosomes
- Crossing over between homologous chromosomes
When does independent segregation and crossing over occur?
- in meiosis 1
How to identify meiosis in a life cycle?
- find point where diploid (2n) becomes haploid cell (n)
What is aneuploidy?
- changes in number of individual chromosomes
Which 2 forms can chromosome non disjunction occur in?
- Polyploidy
- Aneuploidy
What is polyploidy?
- changes in whole sets of chromosomes following random
What does polyploidy look like?
- three or more sets of chromosomes rather than usual two
Describe how polyploidy occurs in meiosis 1
- 3 homologous chromosomes
- each homologous pair is doubled due to DNA replication in interphase
- non disjunction in meiosis 1 all chromosomes fail to separate equally
- normal division in meiosis 2 chromatids separate equally
- 2n gamete (diploid), 2n gamete (diploid) and no chromosomes in gamete
- diploid gamete fuses with haploid gamete to form a triploid
Describe how polyploidy occurs in meiosis 2
- 3 homologous chromosomes rather than
- each homologous pair is doubled due to DNA replication in interphase
- normal division in meiosis 1 chromosome separate equally
- normal meiosis 2 in one side and non disjunction in meiosis 2 all chromatids fail to separate equally
- n gamete (haploid), n gamete (haploid), no chromosomes and 2n gamete (diploid)
Describe how aneuploidy occurs in meiosis 1
- 3 homologous chromosomes
- each homologous pair is doubled due to DNA replication in interphase
- non disjunction in meiosis 1 one chromosome does not separate equally
- normal division in meiosis 2 chromatids separate equally
- n+1 haploid gamete with one additional chromosome, n+1 haploid gamete with one additional chromosome, n-1 haploid gamete missing one chromosome, n-1 haploid gamete missing one chromosome
- n+1 gamete (haploid) fuse with n gamete (haploid) and produce 2n+1 zygote (diploid)
- if n-1 gamete (haploid) fuse with n gamete (haploid) produces 2n-1 zygote (diploid)
Describe how aneuploidy occurs in meiosis 2
- 3 homologous chromosomes
- each homologous pair is doubled due to DNA replication in interphase
- normal division in meiosis 1
- non disjunction in meiosis 2 one chromatid fails to separate equally
- n-1 haploid gamete missing one chromosome, n+1 haploid gamete with one additional chromosome, n haploid gamete and n haploid gamete
