11-4 Meiosis. Chromosome Number

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1 11-4 Meiosis Chromosome Number Sexual reproduction shuffles and recombines genes from two parents. During gametogenesis, genes are segregated and assorted (shuffled) into gemetes, and at fertilization, into offspring. Different species have a different number of chromosomes. Other apes have 48! There is NO relationship between the number of chromosomes and the complexity of the organism. Some plants have hundreds! Drosophila (fruit flies) have 8 chromosomes, 4 from each parent. Homologous chromosomes are chromosome pairs (one chromosome from each parent) that are the same in length, genes, and centromere location, however they may have different alleles. A diploid (2N) cell or nucleus contains two complete sets of chromosomes, one from each parent. N = 1 chromosome set In animals, like Drosophila, all somatic cells (body cells) are diploid. The diploid number for Drosophila is 8. A haploid (N) cell or nucleus contains one complete set of chromosomes. In sexually reproducing organisms, only gametes are haploid, because when gametes fuse at fertilization they must produce a diploid zygote. Gametogenesis is by MEIOSIS!

2 A karyotype is the ordered arrangement of chromosomes from one cell, in homologous pairs, according to size and banding pattern. This is a human karyotype. The diploid number for this species is 46 (23 pairs). Haploid is 23 (no pairs). 1 of 23 Homologous Pairs Amneocentesis is the proceedure used to collect the cell from which this karyotype was made. It is the extraction of amniotic fluid that surrounds the developing fetus. All are Autosomes, except the sex chromosomes. These are all the Mitotic Duplicated Chromosomes from the cell below. A mitotic cell obtained by amneocentesis was broken open to spill the mitotic chromosomes. The photo below was taken and each chromosome was cut out and arranged into this karyotype. Each mitotic duplicated chromosome is composed of two sister chromatids (DNA molecules) attached at the centromere. For each homologous pair, one chromosome came from the mother and one from the father ME! Mitotic Cell Sex Chromosomes 23 It s a girl! Mitotic Duplicated Chromosomes

3 Meiosis I ( NOT Mitosis-like) Interphase I Prophase I Metaphase I Anaphase I Telophase I and Cytokinesis Meiosis is nuclear/cell division that reduces one diploid, somatic cell to 4 haploid gametes by separating the homologous pairs. There are 2 rounds of nuclear/cell division involved in meiosis. The first is shown here Meiosis I. Duplicated chromatin condenses into duplicated chromosomes in a diploid somatic cell. The chromosomes form homologous pairs and line up along the metaphase plate. Homologous chromosomes separate into two haploid daughter cells.

4 Interphase I is the same as interphase preceeding mitosis (G1, S, G2). In Prophase I, duplicated chromatin condenses into duplicated chromosomes as the nuclear envelope breaks down and the centrosomes move toward oposite poles forming the spindle. Homologous chromosomes pair up (synapsis) forming tetrads (4 chromatids) that exchange genes (crossing-over). Somatic Parent Cell Interphase I Prophase I Spindle Microtubules Centrosomes with Centrioles Nucleus with Duplicated Chromatin Homologous Pairs of Duplicated Chromosomes in Tetrads, Crossing Over

5 Homologous Pair Tetrad Crossing Over Nonsister Chromatids Chiasma Crossing over (recombination) occurs during Prophase I. This does not happen in mitosis! When homologous chromosomes pair up (synapsis) forming tetrads, non-sister chromatids will cross-over forming chiasmata and exchange homologous segments (recombine). The letters on the chromatids represent chromatid segments, each containing many genes! DNA is being recombined (recombinant DNA). Crossover and recombination is the RULE not the exception! This explains why most genes appear to assort independently, even if they are on the same chromosome. The ultimate result of crossing over at the organism level is VARIATION in gametes! At the population level, VARIATION between individuals! VARIATION is the raw material of natural selection and evolution! Recombinant DNA

6 During Metaphase I, homologous chromosome pairs in tetrads line up together, one of each pair on opposite sides of the metaphase plate. Homologous Pairs of Duplicated Chromosomes in Tetrads Metaphase I Crossovers! The chromosomes line up RANDOMLY. This results in additional gene shuffling and VARIATION! During Anaphase I, homologous DUPLICATED chromosomes separate. Anaphase I The result of this separation reduces the number of chromosomes in half, so that the resulting daughter cells will be haploid. Duplicated Chromosomes (2 Sister Chromatids)

7 Telophase I and cytokinesis I occur the same as in mitosis, except that the resulting daughter cells are HAPLOID, GENETICALLY DIFFERENT and still contain duplicated chromosomes. Telophase I & Cytokinesis Duplicated Chromosomes (2 Sister Chromatids) Note: Nuclear envelopes do not reform in all species. On to Meiosis II

8 From Meiosis I Meiosis II (Mitosis- ) LIKE Telophase I and Cytokinesis I Prophase II Metaphase II Anaphase II Telophase II and Cytokinesis II There is no interphase II that precedes meiosis II. That means no DNA synthesis, because the chromosomes are already duplicated! But the centrosomes/centrioles must replicate! Both HAPLOID cells from Meiosis I enter Meiosis II. During Prophase II the nuclear envelopes break down and the centrosomes begin to form the spindle. During Metaphase II, the duplicated chromosomes line up along the metaphase plate. During Anaphase II, the chromatids separate, becoming unduplicated chromosomes. Telopohase II and Cytokinesis result in 4 haploid cells, each GENETICALLY UNIQUE to each other, the parent cell, and all gametes on Earth!!!