meiosis 3.3. today’s class focus on gamete production (meiosis) and how cells go from being...

MEIOSIS

3.3

Today’s Class• Focus on gamete production (meiosis) and how cells go from being diploid to

haploid

• Curriculum– 3.3 U1 One diploid nucleus divides by meiosis to produce 4 haploid nuclei– 3.3 U2 The halving of the chromosome number allows a sexual life cycle with fusion of

gametes– 3.3 U3 DNA is replicated before meiosis so that all chromosomes consists of two sister

chromatids– 3.3 U4 The early stages of meiosis involve pairing of homologous chromosomes and

crossing over followed by condensation– 3.3 U5 Orientation of pairs of homologous chromosomes prior to separation is random– 3.3 U6 Separation of pairs of homologous chromosomes in the first division of meiosis– 3.3 U7 Crossing over and random orientation promotes genetic variation– 3.3 U8 Fusion of gametes from different parents promotes genetic variation

10.1 Meiosis HL 3

10.1 Meiosis HL 4

10.1 Meiosis HL 5

10.1 Meiosis HL 6

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An homologous pair of chromosomes…

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An homologous pair of chromosomes……replicates during S-phase interphase…

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…giving two pairs of sister chromatids, each joined at the centromere.

centromere

sister chromatids

An homologous pair of chromosomes……replicates during S-phase interphase…

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The homologous pair associates during prophase I, through synapsis…

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The homologous pair associates during prophase I, through synapsis…

…making a bivalent.

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Crossing-over might take place between non-sister chromatids in prophase I…

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Crossing-over might take place between non-sister chromatids in prophase I…

…leading to recombination of alleles.

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In anaphase I, the homologous pair is separated but the sister chromatids remain attached.

This is the reduction division.

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Check your language. This image shows…

A. Four separate chromosomes.

B. A bivalent.

C. One pair of sister chromatids.

D. Non-disjunction.

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A. Four separate chromosomes.

B. A bivalent.

C. One pair of sister chromatids.

D. Non-disjunction.

Check your language. This image shows…

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A. Two separate chromosomes.

B. A bivalent.

C. One pair of sister chromatids.

D. Crossing-over.

Check your language. This image shows…

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A. Two separate chromosomes.

B. A bivalent.

C. One pair of sister chromatids.

D. Crossing-over.

Check your language. This image shows…

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A. Two separate chromosomes.

B. A bivalent.

C. One pair of sister chromatids.

D. Homologous chromosomes.

Check your language. This image shows…

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A. Two separate chromosomes.

B. A bivalent.

C. One pair of sister chromatids.

D. Homologous chromosomes.

Check your language. This image shows…

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A. 8 separate chromosomes.

B. Two bivalents.

C. Two pairs of sister chromatids.

D. Two homologous chromosomes.

Check your language. This image shows…

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A. 8 separate chromosomes.

B. Two bivalents.

C. Two pairs of sister chromatids.

D. Two homologous chromosomes.

Check your language. This image shows…

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InterphaseIn the S-phase of the interphase before meiosis begins, DNA replication takes place.

Chromosomes are replicated and these copies are attached to each other at the centromere.

The attached chromosome and its copy are known as sister chromatids.

Following S-phase, further growth and preparation take place for meiosis.

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Prophase IThe homologous chromosomes associate with each other, to form bivalents.

The pairs of sister chromatids are joined by the centromere. Non-sister chromatids are next to each other but not joined.

This bivalent is composed of:- One pair of homologous chromosomes- Which have replicated to form two pairs of

sister chromatids.

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Prophase I

Crossing-over between non-sister chromatids can take place. This results in recombination of alleles and is a source of genetic variation in gametes.

The homologous chromosomes associate with each other, to form bivalents.

The pairs of sister chromatids are joined by the centromere. Non-sister chromatids are next to each other but not joined.

The homologous pair is separated in anaphase I. The joined sister chromatids are separated in anaphase II.

This bivalent is composed of:- One pair of homologous chromosomes- Which have replicated to form two pairs of

sister chromatids.

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Metaphase IThe bivalents line up at the equator.

Random orientation occurs and is a significant source of genetic variation.

There are 2n possible orientations in metaphase I and II. That is 223 in humans – or 8,388,068 different combinations in gametes!

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Anaphase ISpindle fibres contract.

Homologous pairs are separated and pulled to opposing poles.

This is the reduction division.

Non-disjunction here will affect the chromosome number of all four gametes.

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Telophase INew nuclei form and the cytoplasm begins to divide by cytokinesis.

The nuclei are no longer diploid.

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Interphase IIThere is no Synthesis phase in Interphase II.

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Prophase IIThe nuclei break down.

No crossing-over occurs.

Chromosomes condense & becomevisible.

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Metaphase IIPairs of sister chromatids align at the equator. Spindle fibres form and attach at the centromeres.

Random orientation again contributes to variation in the gametes, though not to such an extent as in metaphase I.

This is because there is only a difference between chromatids where crossing-over has taken place.

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Metaphase I vs II: Genetic Variation

Lots of variation in gametes produced• Random orientation of homologous

pairs, which may have a great diversity in alleles present

• Therefore many possible combinations of alleles could be pulled to each pole

Some variation in gametes produced• Random orientation of sister chromatids• Variation only in regions where crossing

over has taken place in prophase I (recombination of alleles)

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Anaphase IISpindle fibres contract and the centromeres are broken.

The pairs of sister chromatids are pulled to opposing poles.

Non-disjunction here will lead to two gametes containing the wrong chromosome number.

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Telophase IINew haploid nuclei are formed.

Cytokinesis begins, splitting the cells.

The end result of meiosis is four haploid gamete cells.

Fertilisation of these haploid gametes will produce a diploid zygote.

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Which phase of meiosis is shown? Why?

A. Interphase

B. Prophase I

C. Metaphase I

D. Metaphase II

Reason:

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Which phase of meiosis is shown? Why?

A. Interphase

B. Prophase I

C. Metaphase I

D. Metaphase II

Reason: • Homologous pairs are aligned (at equator), so must

be metaphase. • Crossing-over has already taken place, so must be

after prophase I. • Homologous pairs have not yet separated, so must

be still in meiosis I (metaphase I).

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Which phase of meiosis is shown? Why?

A. Interphase

B. Prophase I

C. Metaphase I

D. Metaphase II

Reason:

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Which phase of meiosis is shown? Why?

A. Interphase

B. Prophase I

C. Metaphase I

D. Metaphase II

Reason: • Homologous pairs have associated. • Crossing-over has taken place. • Homologous pairs have not aligned at the equator.

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Which phase of meiosis is shown? Why?

A. Interphase

B. Prophase I

C. Metaphase I

D. Metaphase II

Reason:

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Which phase of meiosis is shown? Why?

A. Interphase

B. Prophase I

C. Metaphase I

D. Metaphase II

Reason: • Homologous pairs have not yet associated. • Replication has taken place. • Crossing-over has not yet taken place.

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Outline the differences between the behaviour of chromosomes in Mitosis and Meiosis

5 marks

Mitosis Meiosis

One division Two divisions

Diploid cells produced Haploid gametes produced

No crossing-over in prophase Crossing-over in prophase I

Homologous pairs do not associate and line up at the equator in metaphase

Homologous pairs associate as bivalents and lined up at the equator in metaphase I

Sister chromatids separate in anaphase Homologous pairs separate in anaphase ISister chromatids separate in anaphase II

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Genetic VariationCrossing-over in prophase ILeads to recombination of alleles on the chromosomes.

Random orientation in metaphase IHuge number of maternal/paternal chromosome combinations possible in the final gametes. There are over 8million possible orientation in humans (223 orientations)

Random orientation in metaphase IIFurther genetic variation arises where there are genetic differences between sister chromatids as a result of crossing-over in prophase I.

is almost infinite as a result of meiosis.

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Genetic VariationCrossing-over in prophase ILeads to recombination of alleles on the chromosomes.

Random orientation in metaphase IHuge number of maternal/paternal chromosome combinations possible in the final gametes. There are over 8million possible orientation in humans (223 orientations)

Random orientation in metaphase IIFurther genetic variation arises where there are genetic differences between sister chromatids as a result of crossing-over in prophase I.

is almost infinite as a result of meiosis.

Even more variation!Random fertilisation during sexual reproduction ensures even greater variation within the population.(FUSION OF GAMETES)

http://learn.genetics.utah.edu/content/disorders/whataregd/down/

http://learn.genetics.utah.edu/content/disorders/whataregd/down/

• Gametes contain two copies or no copies of a particular chromosome.

• Offspring have an extra or missing chromosome.

Figure 15.12a, b

Meiosis I

Nondisjunction

Meiosis II

Nondisjunction

Gametes

n + 1n + 1 n 1 n – 1 n + 1 n –1 n nNumber of chromosomes

Nondisjunction of homologouschromosomes in meiosis I

Nondisjunction of sisterchromatids in meiosis II

(a) (b)

Non Disjunction

• Down’s Syndrome – Trisomy 21– The person has 3 (instead of 2)

21st chromosomes

• Age of parents vs. Down Syndrome

• Do the DBQ on pg. 167 – 168: “Parental age and non-disjunction”

• Karyotype: a property of a cell – the number and type of chromosomes present in the nucleus.

• Karyogram:picture of chromosomes arranged in pairs,according to their size and structure(banding patterns).

Chromosomal abnormalities

• Trisomy 18, Trisomy 13

• Turner’s Syndrome – females with only one X

• Klinefelter’s Syndrome – males with XXY

• Karyotyping is used for pre-natal (before birth) diagnosis of chromosome abnormalities.

• Where do we get the cells for doing a karyotype?

1) amniocentesis• Extract amniotic fluid,• Inside are some of the

baby’s cells

• Risks:– Miscarriage 1 in 200 to

1 in 400– Accuracy: 99.4%

2) chorionic villus sampling

• Tissue sample from the placenta’s projections into the uterus wall

• Risks?– Slightly higher chance of

miscarriage than amniocentesis because it is done earlier in pregnancy.

– Accuracy: 98%