3.3 Meiosis Alleles segregate during meiosis allowing new combinations to be formed by the fusion of gametes.

3.3 Meiosis

Alleles segregate during meiosis allowing new combinations to be formed by the fusion of gametes

One diploid nucleus divides by meiosis to produce four haploid nuclei.

Q: List a similarity and two differences between mitosis and meiosis.

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

5 marks

Mitosis Meiosis

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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

No chiasmata formation Chiasmata form

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




The halving of the chromosome number allows a sexual life cycle with fusion of gametes.

Q: Why is halving the chromosome number necessary in sexual reproduction?

Fusion of gametes from different parents promotes genetic variation.

Q: How does sexual reproduction increase diversity?

DNA is replicated before meiosis so that all chromosomes consist of two sister chromatids.

Q: What is the difference between chromosomes and chromatids?


An homologous pair of chromosomes…





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





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

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

centromere

sister chromatids




The early stages of meiosis involve pairing of homologous chromosomes and crossing over

followed by condensation.Q: During which stage does crossing-over happen? Draw it using two different colored pencils/pens.

Orientation of pairs of homologous chromosomes prior to separation is random.


The homologous pair associates during prophase I, through synapsis…





The homologous pair associates during prophase I, through synapsis…

…making a bivalent.





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





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

…leading to recombination of alleles.





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

This is the reduction division.





Check your language. This image shows…

A. Four separate chromosomes.

B. A bivalent.

C. One pair of sister chromatids.

D. Non-disjunction.






A. Four separate chromosomes.

B. A bivalent.


D. Non-disjunction.






A. Two separate chromosomes.

B. A bivalent.


D. Crossing-over.







B. A bivalent.


D. Crossing-over.







B. A bivalent.


D. Homologous chromosomes.







B. A bivalent.


D. Homologous chromosomes.






A. 8 separate chromosomes.

B. Two bivalents.

C. Two pairs of sister chromatids.

D. Two homologous chromosomes.






A. 8 separate chromosomes.

B. Two bivalents.

C. Two pairs of sister chromatids.

D. Two homologous chromosomes.





Meiosis Is a reduction division from diploid somatic cells (2n) to produce haploid gametes (n).

The reduction is in the chromosome number in each nucleus.





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.





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.





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.





Crossing-Over Increases genetic variation through recombination of linked alleles.

SynapsisHomologous chromosomes associate

Chiasma FormationNeighbouring non-sister chromatids

are cut at the same point. A Holliday junction forms as the DNA

of the cut sections attach to the open end of the opposite non-sister

chromatid.

RecombinationAs a result, alleles are

swapped between non-sister chromatids.





Prophase I

Crossing-over is more likely to occur between genes which are further apart. In this example, there will be more recombination between D and E than between C and D.

During prophase, the nuclear membrane also breaks down and the centrioles migrate to the poles of the cell.

The homologous chromosomes associate with each other.

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





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!





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.





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

The nuclei are no longer diploid.

They each contain one pair of sister chromatids for each of the species’ chromosomes.

If crossing-over and recombination has occurred then the sister chromatids will not be exact copies.





InterphaseThere is no Synthesis phase in Interphase II.





Prophase IIThe nuclei break down.

No crossing-over occurs.





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.





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)





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.





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.





Which phase of meiosis is shown? Why?

A. Interphase

B. Prophase I

C. Metaphase I

D. Metaphase II

Reason:






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).






A. Interphase

B. Prophase I

C. Metaphase I

D. Metaphase II

Reason:






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.






A. Interphase

B. Prophase I

C. Metaphase I

D. Metaphase II

Reason:






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.





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.





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.




3 applications!

• Application: Non-disjunction can cause Down syndrome and other chromosome abnormalities.

• Application: Studies showing age of parents influences chances of non-disjunction. An understanding of karyotypes has allowed diagnoses to be made for the purposes of genetic counselling. This raises ethical issues over selective abortion of female fetuses in some countries.

• Application: Description of methods used to obtain cells for karyotype analysis e.g. chorionic villus sampling and amniocentesis and the associated risks.

http://learn.genetics.utah.edu/units/disorders/karyotype/downsyndrome.cfm

http://www.youtube.com/watch?v=Jt3cX2vuMyM

http://www.medindia.net/animation/amniocentesis.asp

http://www.youtube.com/watch?v=BOttNyd9qgI

http://www.youtube.com/watch?v=0XUZsvTkEnw

Skill: Drawing diagrams to show the stages of meiosis resulting in the formation of four haploid cells.

3.3 Meiosis Alleles segregate during meiosis allowing new combinations to be formed by the fusion of gametes.

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