http://sciencevideos.wordpress.com 10.1 meiosis hl1

http://sciencevideos.wordpress.com 10.1 Meiosis HL 1

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http://www.lewport.wnyric.org/jwanamaker/animations/meiosis.html

http://www.biostudio.com/d_%20Meiosis.htm


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.





An homologous pair of chromosomes…





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 homologous pair associates during prophase I, through synapsis…

…making a bivalent.





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

…leading to recombination of alleles.





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.





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.





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

Crossing over leads to more variation in gametes.

This is the standard notation for writing genotypes of alleles on linked genes. More of this later when we study 10.2 Dihybrid crosses and gene linkage.

http://sciencevideos.wordpress.com/ahl/10-genetics-ahl/dihybrid-crosses-and-gene-linkage-ahl/







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!





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

This is the reduction division.





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.




http://www.biostudio.com/d_%20Meiotic%20Nondisjunction%20Meiosis%20I.htm

http://www.biostudio.com/d_%20Meiotic%20Nondisjunction%20Meiosis%20II.htm

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


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.





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

5 marks

Mitosis Meiosis





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





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.





Mendel’s Law of Independent Assortment“The presence of an allele of one of the genes in a gamete has no influence over which allele of another gene is present.”

A and B are different genes on different chromosomes.

A is dominant over a. B is dominant over b.

This only holds true for unlinked genes (genes on different chromosomes).





Random Orientation vs Independent Assortment“The presence of an allele of one of the genes in a gamete has no influence over which allele of another gene is present.”

Random Orientation refers to the behaviour of homologous pairs of chromosomes (metaphase I) or pairs of sister chromatids (metaphase II) in meiosis.

Independent assortment refers to the behaviour of alleles of unlinked genes as a result of gamete production (meiosis).

Due to random orientation of the chromosomes in metaphase I, the alleles of these unlinked genes have become independently assorted into the gametes.

Animation from Sumanas:http://www.sumanasinc.com/webcontent/animations/content/independentassortment.html




http://www.sumanasinc.com/webcontent/animations/content/independentassortment.html



Mendel and Meiosis “The presence of an allele of one of the genes in a gamete has no influence over which allele of another gene is present.”

Animation from Sumanas:http://www.sumanasinc.com/webcontent/animations/content/independentassortment.html

Mendel deduced that characteristics were determined by the interaction between pairs of alleles long before the details of meiosis were known.

Where Mendel states that pairs of alleles of a gene separate independently during gamete production, we can now attribute this to random orientation of chromosomes during metaphase I.

Mendel made this deduction when working with pea plants. He investigated two separate traits (colour and shape) and performed many test crosses, recording the ratios of phenotypes produced in subsequent generations.

It was rather fortunate that these two traits happened to be on separate chromosomes (unlinked genes)! Remember back then he did not know about the contents of the nucleus. Chromosomes and DNA were yet to be discovered.

We will use his work as an example of dihybrid crosses in the next section.







Chromosome Collection






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.






A. 8 separate chromosomes.

B. Two bivalents.

C. Two pairs of sister chromatids.

D. Two homologous chromosomes.





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.




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