Topics 3.2 and 3.3 Chromosomes and Meiosis (Core)

Prokaryotic DNA is comprised of one circular chromosome. Prokaryotes and Eukaryotes have differences in their DNA. Prokaryotic DNA is comprised of one circular chromosome. Eukaryotic DNA is composed of linear DNA molecules, associated with histones. These are called chromosomes Plasmids are also present in prokaryotic cells, which are strands of naked DNA scientists can use in genetic transfer.

Chromosome 1 – 3958 genes Chromosome 11 – 2364 genes Different chromosomes within an organism contain different amounts of genes as well as genes for different traits Chromosome 1 – 3958 genes Chromosome 11 – 2364 genes Notice the difference between these two human chromosomes.

Ultimately, the number of chromosomes in the nuclei of the cells of an organism is what determines its characteristics. Determine the number of chromosomes of the following species: Species Common Name Number of Chromosomes Homo sapiens Humans Pan troglodytes Chimpanzee Canis familiaris Dog Oryza sativa Asian Rice Parascaris equorum Equine Roundworm

Ultimately, the number of chromosomes in the nuclei of the cells of an organism is what determines its characteristics. Determine the number of chromosomes of the following species: Species Common Name Number of Chromosomes Homo sapiens Humans 46 Pan troglodytes Chimpanzee 48 Canis familiaris Dog 78 Oryza sativa Asian Rice 24 Parascaris equorum Equine Roundworm 2 Scientists are able to see the number of chromosomes present through a procedure known as Karyogramming, which generates an image like this:

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

Every somatic cell in your body is a diploid cell. Ploidy is the term we use to refer to the number of copies of a genome that are present in the nucleus of a cell. 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 in humans. What is the difference?

Two types of chromosomes in a cell: How would this karyogram be different if the organism were different? Say drosophila melanogaster? Autosomes (Outlined in Blue) Sex Chromosomes (Outlined in Red) 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)

The miracle of variation within a species 3.3 - Meiosis The miracle of variation within a species

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

A homologous pair of chromosomes…

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.

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

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.

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.

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.

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

Neighboring non-sister chromatids are cut at the same point. 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.

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.

Prophase I 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 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.

Metaphase I The 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 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.

Interphase There is no Synthesis phase in Interphase II.

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

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.

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 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.

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.

Inquire and Share Activity Your Task – To research one of the following methods of prenatal testing, and design a whiteboard to display important information about the procedure and the bioethics surrounding it. Prenatal Testing Methods: Nuchal Translucency Scan Amniocentesis Chorionic Villus Sampling Maternal Blood Sampling Maternal Serum Alpha-Fetoprotein Whiteboard Components: We will share and talk about specific important parts of each procedure. Needed for IB Procedure Name Who gets it? Availability Cost vs Effectiveness Bioethics - Pros Bioethics - Cons

Genetic Variation is almost infinite as a result of meiosis. 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 (223 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.

Genetic Variation is almost infinite as a result of meiosis. 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 (223 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. Even more variation! Random fertilization 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 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: 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.” 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 (color 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. Animation from Sumanas: http://www.sumanasinc.com/webcontent/animations/content/independentassortment.html