Topics 3.2 and 3.3 Chromosomes and Meiosis (Core)

Eukaryote vs. Prokaryote DNA Key Differences: How many strands of DNA in prokaryotes? Eukaryotes? Organizational differences? What is the stimulus for these differences?

Different chromosomes within an organism contain different amounts of genes as well as genes for different traits Chromosomes in eukaryotes are linear molecules that are associated with histone proteins Notice the difference between these two human chromosomes.

Can you describe the difference between a karyotype and a karyogram? The karyotype of this individual is 46, XY

Every somatic cell in your body is a diploid cell. Every gamete in your body is a haploid cell Shown above are karyograms for a diploid and a haploid cell. What is the difference?

Two types of chromosomes in a cell: Autosomes (Outlined in Blue) Sex Chromosomes (Outlined in Red) How would this karyogram be different if the organism were different? Say drosophila melanogaster? Autosomes – determine characteristics that are common to both sexes Sex Chromosomes – are the chromosomes that determine characteristics for the development of sexual characteristics (in addition to some other general genes)

3.3 - Meiosis The miracle of variation within a species

Check out this animation from Johnkyrk.com, and see if you can identify what is happening in each of the steps of Meiosis

11 A homologous pair of chromosomes…

12 A homologous pair of chromosomes… …replicates during S-phase of interphase… DNA is replicated before meiosis so that all chromosomes consist of two sister chromatids.

13 A homologous pair of chromosomes… …replicates during S-phase of interphase… …giving two pairs of sister chromatids, each joined at the centromere. centromere sister chromatids

14 The homologous pair associates during prophase I, through synapsis…

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

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

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

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

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

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.

21 Prophase I 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. This bivalent is composed of: -One pair of homologous chromosomes -Which have replicated to form two pairs of sister chromatids.

22 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.

23 Crossing-Over Increases genetic variation through recombination of linked alleles. Homologous chromosomes associate Chiasmata Formation Neighboring non-sister chromatids are cut at the same point. 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.

24 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

25 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.

26 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!

27 Anaphase I Spindle fibers 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 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.

32 Interphase There is no Synthesis phase in Interphase II.

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

34 Metaphase II Pairs of sister chromatids align at the equator. Spindle fibers 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.

35 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)

36 Anaphase II Spindle fibers 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.

37 Telophase II New haploid nuclei are formed. Cytokinesis begins, splitting the cells. The end result of meiosis is four haploid gamete cells. Fertilization of these haploid gametes will produce a diploid zygote.

49 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.

50 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 fertilization during sexual reproduction ensures even greater variation within the population.

51 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).

52 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 behavior of homologous pairs of chromosomes (metaphase I) or pairs of sister chromatids (metaphase II) in meiosis. Independent assortment refers to the behavior 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:

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.