OCR A2 UNIT F215 MEIOSIS AND VARIATION

OCR A2 UNIT F215 MEIOSIS AND VARIATION

Specification:

a) Describe, with the aid of diagrams and photographs, the behaviour of chromosomes during meiosis, and the associated behaviour of the nuclear envelope, cell surface membrane and centrioles. (names of the main stages are expected, but not the subdivisions of prophase)

b) Explain the terms allele, locus, phenotype, genotype, dominant, codominant and recessive

c) Explain the terms linkage and crossing-over

d) Explain how meiosis and fertilisation can lead to variation through the independent assortment of alleles

IMPORTANCE OF MEIOSIS:

· Nuclear division that occurs in the sex organs to produce gametes

· Process involves a reduction in the chromosome number of a diploid cell

· The daughter cells (gametes) contain the haploid number of chromosomes compared with the parent cell, with the diploid number

· This halving of the chromosome number is important so that the diploid number can be restored during fertilisation of gametes

· Each parent cell divides to form 4 gametes

· Meiosis is a source of genetic variation in the gamete cells

SOURCES OF GENETIC VARIATION IN MEIOSIS:

· Chiasmata and crossing over in prophase I

· Independent assortment of homologous chromosomes in metaphase I

Important Definitions

Haploid (n) refers to cells or organisms with only one set of chromosomes per cell

Diploid (2n) refers to cells or organisms with two sets of chromosomes per cell

Homologous Chromosomes

· Diploid (2n) cells of an organism have the full complement of chromosomes

· In diploid cells, the chromosomes are in pairs called homologous pairs

· Each species has a specific diploid number. Human cells have a diploid number of 23 pairs

Features of Homologous Chromosomes

· They are usually the same size and shape

· Their centromeres are in the same position

· They have the same genes w/ same gene loci (position along the chromosome)

· The homologous chromosomes of each pair are not genetically identical since the alleles of the genes could be different

· In the first cell of the organism produced by sexual reproduction, one chromosome of each homologous pair was derived from the mother and the other from the father

The left hand diagram below is from a photograph of all the chromosomes from a human male diploid cell

The right hand diagram shows the same chromosomes arranged into homologous pairs (note the X and Y sex chromosomes No 23)

Identifying Homologous Chromosomes

(1) State the diploid number of this cell...6..........................................

(2) How many pairs of homologous chromosomes are in this cell? 6...

(3) Using 3 different coloured pencils, indicate the different homologous pairs

Gene

A gene is a length of DNA that codes for the synthesis of a polypeptide. A gene determines a specific feature of an organism

Gene Locus

The position of the gene on a chromosome is called the gene locus

Alleles

Alleles are alternative forms of a gene. Each gene locus can only contain one allele

Crossing-over

The process by which a chromatid breaks during prophase I of meiosis I and rejoins to a non-sister chromatid of its homologous chromosome, so that the non-sister chromatids exchange alleles

LIFE CYCLE OF A HUMAN TO REPRESENT HAPLOID AND DIPLOID STAGES

State the number of chromosomes and whether haploid or diploid, in the following cells;

Human sperm cell there is 46 chromosomes and it is diploid

Human ovum (egg cell)……………………………………………………..

Human zygote………………………………………………………………..

Human embryo cell………………………………………………………….

Human testis/ovary tissue cell……………………………………………..

CELL CYCLE OF CELLS IN REPRODUCTIVE ORGANS THAT UNDERGO MEIOSIS

A cell in the ovary/ testis/anther/ovule that undergoes meiosis will go through a cell cycle including interphase as follows

· Interphase (G1, S and G2)

· Meiosis I

· Cytokinesis

· Meiosis II

· Cytokinesis

Meiosis I and meiosis II represent a double division of the nucleus to produce 4 haploid cells from 1 diploid parent cell

The stages of meiosis are as follows:

· Prophase I

· Metaphase I

· Anaphase I

· Telophase I

· Prophase II

· Metaphase II

· Anaphase II

· Telophase II

Details of Events in Each Stage

Interphase

· Chromosomes not visible because they are decondensed and not supercoiled

· Cell is preparing for meiosis and cytokinesis

· DNA replication occurs in S phase

· Organelles are synthesised, increased protein synthesis and ATP synthesis occur in G1 and G2 phases

Prophase I

Early Prophase I

· Chromosomes take up stains and can be seen with a light microscope

· They become more visible as they condense by supercoiling, becoming shorter and thicker

Early Prophase I

· As the chromosomes thicken, each chromosome is seen as a pair of sister chromatids, joined by a centromere

Late prophase I

· Homologous chromosomes pair up, by synapsis, forming bivalents

· Each member of the homologous pair has the same genes at the same loci

· Each homologous pair consists of one maternal and one paternal chromosome

· The non-sister chromatids may wrap around each other and attach at points called chiasmata.

· The non-sister chromatids may exchange sections of chromatids with each other – a process called crossing over. This is one source of genetic variation

· The centrioles (present in animal cells only) migrate to the poles of the cell and the spindle fibres form from the centrioles

· The nuclear envelope and nucleolus break down

Metaphase I

Metaphase I

· The bivalents line up at the equator (middle) of the cell, with centromeres attached to the spindle fibres

· The bivalents are arranged randomly. Each member chromosome of an homologous pair face opposite poles

· Independent assortment of homologous chromosomes is also a source of genetic variation

Anaphase I

Anaphase I

· Separation of homologous chromosomes, to opposite poles of the cell

· Chromosomes separate along the spindle fibres that shorten to pull homologous chromosomes apart

· The centromeres do not divide

· Chromatids, modified by crossing over, remain modified

· Independent separation of homologous chromosomes is a source of genetic variation

Telophase I

Telophase I

· Chromosomes arrive at the poles of the cell (one of each homologous pair at each pole)

· The chromosomes may decondense and become less visible

· Spindle fibres disintegrate

· In animal cells, nuclear envelopes and nucleoli reform at each end of the cell

Cytokinesis I

· In animal cells, division of the cytoplasm produces 2 haploid cells, each containing one of each of the homologous pairs of chromosomes

· Cytokinesis in animal cells involves the pinching in of the plasma membrane at the equator forming a cleavage furrow. The initial formation of this furrow can be seen in the cell diagram under telophase I above

Note that in most plants cells, the cell goes from Anaphase I to Prophase II directly

Meiosis II

In Meiosis II, the equator of the cells and the poles are at right angles to their positions during Meiosis I

Prophase II

Prophase II

· Chromosomes condense and become visible

· Each chromosome is seen as a pair of chromatids joined by a centromere

· If they reformed in telophase I, the nuclear envelopes and nucleoli disintegrate

· The centrioles migrate to opposite poles (animal cells only)

· Spindle fibres form, at right angles to the previous fibres in meiosis I

Metaphase II

Metaphase II

· The chromosomes line up along the equator with centromeres attached to the spindle fibres

· The chromatids of each chromosome may not be genetically identical, because of crossing over, and are arranged independently at the equator

· Independent assortment of chromatids at the equator is a source of genetic variation

Anaphase II

Anaphase II

· Centromeres divide

· The chromatids separate, pulled apart by shortening of the spindle fibres

· The chromatids are pulled to opposite poles

· The independent separation of ‘sister’ chromatids is a source of genetic variation

Telophase II

· The chromatids (now chromosomes) have reached the poles

· Spindle fibres disintegrate

· The chromosomes decondense becoming less visible

· The nuclear envelope and nucleolus reform

Cytokinesis II

· Division of the cytoplasm occurs in both cells produced from cytokinesis I, producing 4 haploid gametes, sometimes referred to as a tetrad

· Each cell has 1 of each homologous chromosome

· It is highly likely that all 4 cells will be genetically different

How Meiosis leads to Genetic Variation

1. Crossing over during prophase I – exchange of alleles between non-sister chromatids

2. Independent assortment of maternal and paternal homologous chromosomes in metaphase I

3. Independent assortment of sister chromatids during metaphase II

4. Random chromosome mutation (e.g. non-disjunction)

How Fertilisation leads to Genetic Variation

1. Random mating

2. Random fertilisation of male and female gametes

Crossing Over

· During prophase I, homologous chromosomes pair up forming bivalents

· Non-sister chromatids wrap around each other very tightly, attaching at chiasmata. Label the chiasmata in the diagram below, indicating which chromatids are involved in the formation of each

· The chromatids may break at the position of the chiasmata. If this happens, the broken ends may rejoin to the non-sister chromatid in the same bivalent. This leads to crossing over – the exchange of alleles between non-sister chromatids

· Crossing over results in new combinations of alleles in the chromatids that will become chromosomes in the haploid gametes

· During metaphase I, the chiasmata remain intact and hold the maternal and paternal homologous chromosomes together on the spindle

· During anaphase I, the chiasmata break and one chromosome (made up of 2 chromatids) of each homologous pair is pulled to opposite poles

· On average 2-3 cross-over events occur on each homologous pair

Independent Assortment of Homologous Chromosomes during Metaphase I and Independent Separation of Homologous Chromosomes during Anaphase I

· Maternal and paternal homologous chromosomes are randomly distributed at the spindle equator during metaphase I

· This random distribution leads to independent separation of the maternal and paternal homologous chromosomes during anaphase I

· Each daughter nucleus contains a different mixture of maternal and paternal homologous chromosomes

The cell below contains three homologous pairs of chromosomes at metaphase I

There are four possible outcomes when two daughter cells are produced during meiosis I

Complete the cells below by drawing in the possible combinations of maternal and paternal chromosomes in the two daughter nuclei after anaphase I and telophase I, using red and blue pencils

Independent Assortment of Chromatids at Metaphase II and Independent Separation of Chromatids during Anaphase II

· Because of crossing over in prophase I, ‘sister’ chromatids are no longer genetically identical

· The distribution of the chromatids along the equator is random during metaphase II and this will determine the independent separation of ‘genetically non-identical sister chromatids’ during anaphase II

The diagram below shows one daughter cell containing three chromosomes after meiosis I. Crossing over has occurred in all three chromosomes

Two haploid gametes are produced from each haploid daughter cell produced in Meiosis I

There are four different ways in which these 3 chromosomes could be distributed at the equator in metaphase II and therefore, four different independent ways in which the chromatids could be separated in anaphase II

Draw these possible combinations in the cells below, using red, blue and green pencils

1.

2.

3.

4.

Random Mating and Fertilisation

· Any adult female of a species can mate with any adult male of the same species. This is random mating.

· In sexual reproduction, the nuclei of two haploid gametes must fuse to restore the diploid number of the zygote.

· This fusion of gametes (fertilisation) is completely random and adds to genetic variation within a population.

Random Chromosome Mutation

· Chromosome mutations may occur during meiosis such as non-disjunction

· If the mutation occurs in the production of gametes and a mutated gamete is involved in fertilisation, the- mutation will be present in every body cell of the offspring

Comparison of mitosis and meiosis

FEATURES

MITOSIS

MEIOSIS

Involves DNA replication in interphase

X

Involves organelle replication in interphase

X

Involve spindle formation

X

X

Only one division of the nucleus

X

Two divisions of the nucleus

X

Important during growth of an organism

X

Produces clones of cells

X

Important in asexual reproduction

X

Introduces genetic variation

X

Occurs in the sex organs

X

X

Homologous chromosomes pair up in prophase I

X

X

Daughter nuclei have the same number of chromosomes as the parent nucleus

X

Crossing over may occur in prophase I

X

Chiasmata are never formed

X

Four daughter cells are produced

X

Answers to questions on pages 13 and 14

Four possible combinations after random separation of three pairs of homologous chromosomes during meiosis I

Four possible combinations after random separation of ‘sister’ chromatids during meiosis II

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