4.3 Genetic diversity can arise as a result of mutation or during meiosis Flashcards
Does meiosis create haploid or diploid cells?
haploid
How many divisions occur in meiosis?
2
In humans, how many chromosomes do haploid cells have?
23
What type of cells does meiosis form?
gametes
When does the first division in meiosis occur?
after interphase (after the chromosomes have duplicated in DNA replication)
Summarise what happens to chromosomes in meiosis I:
homologous chromosomes are separated from each other
Summarise what happens to chromosomes in meiosis II:
the sister chromatids from each chromosome are separated
What happens to the chromosomes in prophase I?
they condense, and become visible
What happens to the nuclear envelope in prophase I?
breaks down
What happens to the spindle fibres in prophase I?
they form from the centrioles
What process occurs in prophase I?
crossing over
What happens to the chromosomes in metaphase I?
each chromosome lines up to their homologous partner at the equator
What happens to the spindle fibres in metaphase I?
they attach to the chromosomes, by attaching to the centromeres
What process happens during metaphase I?
independent assortment
What happens to the chromosomes during anaphase I?
each chromosome from the homologous pair is pulled apart to opposite ends of the cell
Why is anaphase I random?
one out of the pair of homologous chromosome will end up on each side
What happens during telophase I?
nuclear envelope reforms around each set of chromosomes and the chromosomes decondense
What happens during cytokinesis?
the cytoplasm divides in two
Do plant cells undergo cytokinesis in meiosis I?
NO
Does interphase occur between meiosis I and meiosis II?
NO
What happens to the chromosomes during prophase II?
The DNA condenses and becomes visible
What happens to the nuclear envelope during prophase II?
breaks down
What happens to the spindle fibre during prophase II?
new spindle fibres form
Are the chromatids on each chromosome identical in prophase II? Why/Why not?
NO - due to crossing over in meiosis I
What happens to the chromosomes during metaphase II?
they line up at the centre and are randomly assorted to each pole
What happens to the spindle fibres during metaphase II?
they attach to the centromere of each chromosome
What happens to the spindle fibres during anaphase II?
they contract
What happens to the chromatids during anaphase II?
they are pulled to opposite poles (due to the spindle fibres contracting)
What happens during telophase II?
the nuclear envelope reforms; the chromosomes de-condense
What happens during cytokinesis?
the 2 cells split in two -> making 4 cells
Do independent assortment + crossing over happen in meiosis I or meiosis II?
meiosis I
What are homologous chromosomes?
chromosomes with the exact set of genes on them, but different alleles
What is independent segregation?
The random division of homologous chromosomes into separate daughter cells during meiosis I
How does independent segregation increase genetic variation?
there is a large number of possible combinations of chromosomes in the daughter cell produced
2^n (n = number of homologous pairs)
What is crossing over?
the exchange of homologous portions of non-sister chromatids
How does crossing over increase genetic variation?
new combinations of alleles are formed
Does random fertilisation increase genetic variation?
yes - there are 2^n possible chromosome combinations + it is random which egg + sperm will fuse in fertilisation.
Define the term mutagenic agent.
A factor that increases the rate of mutations
Apart from mutation, explain one other way genetic variation within a species is increased.
- random fertilisation
- produces new allele combinations
Explain how the chromosome number is halved during meiosis.
- homologous chromosomes
- one of each pair goes to each daughter cell
Describe the process of crossing over and explain how it increases genetic diversity.
- Homologous pairs of chromosomes form a bivalent
- Chiasmata form
- lengths of non-sister chromatids / alleles are exchanged
- producing new combinations of alleles
Name two ways in which meiosis produces genetic variation.
- Independent segregation of homologous chromosomes
- crossing over/formation of chiasmata
Define ‘gene mutation’ and explain how a gene mutation can have no effect on an individual.
- Change in the base sequence of DNA
- results in the formation of new allele
- has no effect because: genetic code is degenerate (so amino acid sequence may not change)
Define ‘gene mutation’ and explain how a gene mutation can have a positive effect on an individual.
- Change in the base sequence of DNA
- results in the formation of new allele
- effect = results in change in polypeptide that positively changes the properties of the protein
What is the formula for calculating combinations of gametes?
2n (where n is the number of different homologous pairs of chromosomes)
What is the formula for calculating combination of offspring?
(2n)2 (where n is the number of different homologous pairs of chromosomes)
What is a mutation?
a change in the base sequence of DNA
What does a chromosome mutation lead to?
a change in the number of chromosomes present in the zygote after fertilisation has occurred
What is non-disjunction?
the bivalent that in anaphase 1 does not separate leading to a gamete with one more than the haploid number (so wrong number of chromosomes in diploid cell after fertilisation)
What does a gene mutation lead to?
This is a change in the DNA base sequence forming a new version of a gene or allele and thus a non-functional protein
What is a neutral mutation?
a gene mutation that has no effect on survival or reproduction
When does gene mutation occur?
during DNA replication
What are point mutations?
gene mutations that involve changes in one nucleotide
What are the 3 types of point mutation?
- Base deletion - causes from shift to the left
- Base substitution - no frame shift
- Addition - frame shift to the right
How does gene mutation lead to a non-functional mutation?
- DNA triplets are altered
- Different mRNA codon
- Different amino acid sequence
- Non functional protein
What are 3 other types of gene mutation?
- Duplication - many bases added so frame shift to the right
- Inversion - order of bases in a codon reverses
- Translocation - sequence of bases leaves a chromosome and joins another
What are mutagenic agents?
Environmental factors that increase the rate of mutation e.g. tobacco smoke, UV rays
(i) During which part of the cell cycle are gene mutations most likely to occur? and (ii) Suggest an explanation for your answer
________
Interphase/S/synthesis phase
DNA/gene replication/synthesis occurs.
(b) Describe how the change in chromosome number in Patau syndrome was produced. (more chromosomes)
b) 1. In meiosis;
2. Homologous chromosomes/sister chromatids do not separate
non disjunction
(c) Explain why all the cells of the body will have this mutation.
c) 1. Mutation/extra chromosome in gamete/egg/sperm;
2. All cells derived from a single cell/zygote by mitosis.
Define the term exon.
Base/nucleotide/triplet sequence coding for polypeptide.
(a) Explain how a single base substitution causes a change in the structure of this polypeptide.
Do not include details of transcription and translation in your answer.
a) 1. Change in sequence of amino acids/primary structure;
2. Change in hydrogen/ionic/disulfide bonds;
3. Alters tertiary/3D structure.
(b) The faulty allele that causes Ellis-van Creveld syndrome is the result of a mutation of a gene called EVC. This mutation leads to the production of a protein that has one amino acid missing.
(i) Suggest how a mutation can lead to the production of a protein that has one amino acid missing.
i) Loss of 3 bases/triplet
(ii) Suggest how the production of a protein with one amino acid missing may lead to a genetic disorder such as Ellis-van Creveld syndrome.
- Change in tertiary structure/active site;**
- Faulty/non-functional protein/enzyme.
