OCR F215 Meiosis and Variation

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F215 Module 1: Cellular Control and Variation
5.1.2 Meiosis and Variation
(a) describe, with aid of diagrams and photographs, the behaviour of the nuclear envelope, cell membrane
and centrioles (name of the main stages are expected but not the subdivisions of prophase)
Meiosis ­ is a reduction division. The resulting daughter cells have half the original number of chromosomes.
They are haploid and can be used for sexual reproduction.
To maintain the original chromosome number, the chromosome number in gametes needs to be halved. Thus
when two haploid gametes join at fertilisation, the resulting zygote is diploid.
Meiosis I Meiosis II
Prophase Chromosomes condense and undergo Chromosomes condense and undergo
supercoiling ­ they shorten and thicken . supercoiling ­ they shorten and thicken .
Homologous chromosomes pair up Nucleolus disappears and the nuclear
(synapsis ) to form a bivalent . envelope disintegrates again.
Nonsister chromatids attach at chiasmata The spindle reforms at right angles to the
­ they may swap sections of chromatids previous spindle axis.
called crossing over .
Nucleolus disappears and the nuclear
envelope disintegrates .
A spindle forms made of protein
microtubules .
Metaphas Chromosomes arrange themselves Chromosomes arrange themselves
e randomly (independent assortment of randomly (independent assortment of
chromosomes ) on the equator of the chromatids ) on the equator of the spindle
spindle and attach to the spindle fibres. and attach to the spindle fibres .
Anaphase The homologous chromosomes in each Centromeres divide .
bivalent are pulled by the spindle fibres to Chromatids are pulled by the spindle fibres
opposite poles . to opposite poles .
Telophase In animal cells , new nuclear envelopes Nuclear envelopes reform around the
form and the cell divides by cytokinesis . haploid daughter nuclei.
There is a brief interphase and the In animals , the two cells now divide to give
chromosomes uncoil . four haploid cells.
In plant cells , the cell goes straight from In plants, a tetrad of four haploid cells is
anaphase I into meiosis II ­ no telophase I . formed.
What are homologous chromosomes?:
Chromosomes are the same size.
Chromosomes have the same position of centromere .
Chromosomes carry the same genes at the same loci,
but different alleles ­ one maternal and one paternal.

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F215 Module 1: Cellular Control and Variation
(b) explain the terms:
Allele A version of a gene there may be a difference in the base sequence that is
expressed as a slightly different polypeptide.
Locus The position of a gene on a chromosome. Alleles of a gene are found at the same
locus on each chromosome in a pair.
Phenotype The characteristics expressed in an organism ­ determined by the genotype and the
environment in which it has developed.…read more

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F215 Module 1: Cellular Control and Variation
Random mutation DNA mutation may occur during interphase when DNA replicates.
1. Insertion/deletion mutations ­one of more nucleotide pairs are inserted or
deleted from a length of DNA, causing a frameshift ­ the amino acid sequence
is altered after the insertion/deletion point.
2. Point mutations/Substitution ­ one base pair replaces another.
Nonsense ­ introduces a premature stop codon, stopping translation early,
giving a truncated polypeptide.…read more

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F215 Module 1: Cellular Control and Variation
W
Heterozygous individuals with genotype CR C have red and white hairs ­ roan coat.
(f) describe the interactions between loci (epistasis)
(g) predict phenotypic ratios in problems involving epistasis
Epistasis ­ the interaction of different gene loci so that one gene locus masks or supresses the expression of
another gene locus, reducing phenotypic variation. The genes may work against each other resulting in masking,
or may work together in a complementary fashion.…read more

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F215 Module 1: Cellular Control and Variation
The gene loci must complement each other ­ if one gene codes for an intermediate colourless
pigment and the second locus codes for an enzyme that converts the intermediate compound
to the final purple pigment.…read more

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F215 Module 1: Cellular Control and Variation
Genetic variation is caused by spontaneous mutation, due to sexual reproduction.
The selection pressure causes some individuals to have a selective advantage over the others.
They are more likely to survive and pass on their allele to their offspring, causing an increase in allele
frequency.
(m) use the HardyWeinberg principle to calculate allele frequencies in populations
Population ­ the number of individuals of the same species that can reproduce to produce fertile offspring.…read more

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F215 Module 1: Cellular Control and Variation
A large population of organisms may be split into subgroups by various isolating mechanisms. This leaves two
subpopulations, isolated from each other. In each case different alleles will be eliminated or increased within
subpopulation. Eventually the subpopulations will not be able to interbreed and will be different species.…read more

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F215 Module 1: Cellular Control and Variation
Natural Selection Artificial Selection
Natural selection is a mechanism for evolution. In artificial selection:
Genetic variation is caused by spontaneous Humans select the organisms with useful
mutation, due to sexual reproduction. characteristics.
The selection pressure causes some individuals to Humans allow those with these useful
have a selective advantage over the others. characteristics to breed and prevent the ones
They are more likely to survive and pass on their without the characteristics from breeding.…read more

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F215 Module 1: Cellular Control and Variation
Each year, in the UK, the Camden and Chorleywood Food Research Association (C&CFRA), formerly The Flour
Milling and Baking Research Association, surveys the wheat varieties grown in the UK and classifies them
according to their suitability for making bread or biscuits or for use in animal feed.…read more

Comments

Mona

Thank you! This is really helpful 

Mona

Thank you! This is really helpful 

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