Meiosis HL1

Meiosis HL7 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.

Meiosis HL8 Interphase In 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.

Meiosis HL9 An homologous pair of chromosomes…

Meiosis HL10 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

Meiosis HL11 The homologous pair associates during prophase I, through synapsis… …making a bivalent.

Meiosis HL12 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.

Meiosis HL13 Crossing-over might take place between non-sister chromatids in prophase I… …leading to recombination of alleles.

Meiosis HL14 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.

Meiosis HL15 Crossing-Over Increases genetic variation through recombination of linked alleles. Synapsis Homologous chromosomes associate Chiasma Formation Neighbouring 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. Recombination As a result, alleles are swapped between non- sister chromatids.

Meiosis HL16 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.10.2 Dihybrid crosses and gene linkage

Meiosis HL17 Metaphase I The bivalents line up at the equator. Random orientation occurs and is a significant source of genetic variation. There are 2 n possible orientations in metaphase I and II. That is 2 23 in humans – or 8,388,068 different combinations in gametes!

Meiosis HL18 In anaphase I, the homologous pair is separated but the sister chromatids remain attached. This is the reduction division.

Meiosis HL19 Anaphase I Spindle 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.

Meiosis HL25 Telophase I New 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.

Meiosis HL26 Interphase There is no Synthesis phase in Interphase II.

Meiosis HL27 Prophase II The nuclei break down. No crossing-over occurs.

Meiosis HL28 Metaphase II Pairs 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.

Meiosis HL29 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)

Meiosis HL30 Anaphase II Spindle 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.

Meiosis HL31 Telophase II New 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.

Meiosis HL32 Outline the differences between the behaviour of chromosomes in Mitosis and Meiosis 5 marks

Meiosis HL33 Outline the differences between the behaviour of chromosomes in Mitosis and Meiosis 5 marks

Meiosis HL34 Genetic Variation Crossing-over in prophase I Leads to recombination of alleles on the chromosomes. Random orientation in metaphase I Huge number of maternal/paternal chromosome combinations possible in the final gametes. There are over 8million possible orientation in humans (2 23 orientations) Random orientation in metaphase II Further 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.

Meiosis HL35 Genetic Variation Crossing-over in prophase I Leads to recombination of alleles on the chromosomes. Random orientation in metaphase I Huge number of maternal/paternal chromosome combinations possible in the final gametes. There are over 8million possible orientation in humans (2 23 orientations) Random orientation in metaphase II Further 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.

Meiosis HL36 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).

Meiosis HL37 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:

Meiosis HL38 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: 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.

Meiosis HL39 Chromosome Collection

Meiosis HL40 Check your language. This image shows… A.Four separate chromosomes. B.A bivalent. C.One pair of sister chromatids. D.Non-disjunction.

Meiosis HL41 Check your language. This image shows… A.Four separate chromosomes. B.A bivalent. C.One pair of sister chromatids. D.Non-disjunction.

Meiosis HL42 Check your language. This image shows… A.Two separate chromosomes. B.A bivalent. C.One pair of sister chromatids. D.Crossing-over.

Meiosis HL43 Check your language. This image shows… A.Two separate chromosomes. B.A bivalent. C.One pair of sister chromatids. D.Crossing-over.

Meiosis HL44 Check your language. This image shows… A.Two separate chromosomes. B.A bivalent. C.One pair of sister chromatids. D.Homologous chromosomes.

Meiosis HL45 Check your language. This image shows… A.Two separate chromosomes. B.A bivalent. C.One pair of sister chromatids. D.Homologous chromosomes.

Meiosis HL46 Check your language. This image shows… A.8 separate chromosomes. B.Two bivalents. C.Two pairs of sister chromatids. D.Two homologous chromosomes.

Meiosis HL47 Check your language. This image shows… A.8 separate chromosomes. B.Two bivalents. C.Two pairs of sister chromatids. D.Two homologous chromosomes.

Meiosis HL48 Which phase of meiosis is shown? Why? A.Interphase B.Prophase I C.Metaphase I D.Metaphase II Reason:

Meiosis HL49 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).

Meiosis HL50 Which phase of meiosis is shown? Why? A.Interphase B.Prophase I C.Metaphase I D.Metaphase II Reason:

Meiosis HL51 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.

Meiosis HL52 Which phase of meiosis is shown? Why? A.Interphase B.Prophase I C.Metaphase I D.Metaphase II Reason:

Meiosis HL53 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.

For more IB Biology resources: This is a Creative Commons presentation 10.1 Meiosis HL 54