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Meiosis Dr. Jessica Pamment Overview Overview
• Role of meiosis in sexual reproduction • Comparison of meiosis and mitosis • What happens when meiosis goes wrong • Genetic variation produced in sexual life cycles contributes to evolution Sexual Reproduction Sexual Reproduction
• Parents pass on genes to their offspring as a result of fertilization of their gametes phenotype • The inherited genes result in the observed • Genes are passed on in gametes Chromosomes in Human Cells Chromosomes in Human Cells
• Somatic cells have 46 chromosomes, 23 pairs • Each chromosome pair is a made up of 2 homologous chromosomes • Set of chromosomes is made up of a pair of sex chromosomes and 22 pairs of autosomes • Gametes have 23 chromosomes Somatic cells vs. Gametes Somatic cells vs. Gametes
• Somatic cells are diploid cells that undergo mitosis. They include every cell in your body except body’s reproductive cells (gametes) • Gametes are haploid cells that are derived from diploid cells through the process of meiosis Karyotyping Karyotyping APPLICATION TECHNIQUE
5 µm Pair of homologous replicated chromosomes Centromere Sister chromatids TECHNIQUE
5 µm Pair of homologous replicated chromosomes Centromere Sister chromatids One duplicated chromosome The X and the Y Terminology Terminology
• Number of chromosomes in a single set n • Cell with two chromosome sets is a diploid • Diploid number for humans is 2n= 46 • Gametes contain a single set, n • Gametes are haploid cells • Each sexually reproducing species has a characteristic diploid and haploid number Describing Chromosomes Describing Chromosomes
Key 2n = 6 Maternal set of chromosomes (n = 3) Paternal set of chromosomes (n = 3) Two sister chromatids
Centromere Two nonsister chromatids in a homologous pair Pair of homologous chromosomes The Human Life Cycle Haploid gametes (n = 23) Egg cell n n Sperm cell MEIOSIS FERTILIZATION Multicellular diploid adults (2n = 46) MITOSIS and development 2n Diploid zygote (2n = 46) Key Haploid (n) Diploid (2n) Sex Outcomes in Human Reproduction Sex Outcomes in Human Reproduction Sex Outcomes in Human Reproduction Sex Outcomes in Human Reproduction Meiosis Meiosis
• A form of cell division • Preceded by replication of chromosomes • Consists of two consecutive cell divisions, meiosis I and II • Results in four daughter cells, each with one set of chromosomes Meiosis Meiosis Interphase Homologous pair of chromosomes in diploid parent cell Chromosomes replicate Homologous pair of replicated chromosomes Diploid cell with replicated chromosomes Meiosis Meiosis Interphase Homologous pair of chromosomes in diploid parent cell Chromosomes replicate Diploid cell with replicated chromosomes Meiosis I 1 Homologous chromosomes separate Haploid cells with replicated chromosomes Meiosis Meiosis Interphase
Homologous pair of chromosomes Chromosomes replicate Homologous pair of replicated chromosomes Diploid cell with replicated chromosomes Meiosis I 1 Homologous chromosomes separate Meiosis II
2 Sister chromatids separate Haploid cells with unreplicated chromosomes Crossing over and Genetic Diversity Mechanisms of Genetic Variation in Mechanisms of Genetic Variation in Sexually Reproducing Organisms 1. Crossing Over 1. Independent Assortment 1. Random Fertilization Independent Assortment and Genetic Diversity The Meiotic Division of an Animal Cell The Meiotic Division of an Animal Cell
Meiosis I: separates homologs Meiosis II: separates chromatids Prophase I Metaphase I Anaphase I Telophase I and Cytokinesis Prophase II Metaphase II Anaphase II Telophase II and Cytokinesis Summary of Meiosis Summary of Meiosis
• Chromosome sets reduced from diploid to haploid • The two stages of cell divisions produces four haploid daughter cells • During meiosis I homologs are separated • During meiosis II sister chromatids are separated • Genetic variation introduced by crossing over MITOSIS Parent cell Stage 1 Chromosome replication 2n = 6 MEIOSIS Chiasma MEIOSIS I Stage 1 Homologous chromosome pair Chromosome replication Stage 2 Stage 2 Stage 3/4 Daughter cells of meiosis I 2n Daughter cells of mitosis 2n Stage 3/4 Haploid n=3 MEIOSIS II n n n n Daughter cells of meiosis II Significance of Meiosis Significance of Meiosis
• Meiosis introduces genetic variation into a population and thus contributes to evolution • Meiosis results in reshuffling of alleles • Alleles are different versions of a gene When Meiosis goes Wrong When Meiosis goes Wrong
• Meiotic spindle distributes chromosomes to daughter cells • Members of a chromosome pair can fail to separate • Results in gametes with abnormal numbers of chromosomes Fertilization after failure of chromosomes to separate in mother mother Maternal age and Down Syndrome Maternal age and Down Syndrome Down Syndrome Down Syndrome
• A trisomy: extra chromosome 21 • Affects 1 out of every 700 children • Characteristic facial features; heart defects; short stature Trisomy 21 and Down Syndrome Trisomy Down Syndrome Down Syndrome Triploidy Triploidy Abnormal number of sex Abnormal number of sex chromosomes
• Has less of an effect than abnormal number of autosomes • Y chromosome very small • Cells normally operate with only one functioning X chromosome Turner Syndrome (X0) Summary Summary
• Asexual reproduction produces identical daughter cells by mitosis • Sexual reproduction combines sets of genes from different parents forming genetically diverse organisms • Meiosis reduces the number of chromosome sets from diploid to haploid: it is the basis of sexual reproduction Summary
• Meiosis consists of two cell divisions which results in four haploid daughter cells • Genetic variation is introduced during meiosis due to crossing over, independent assortment of chromosomes, and random fertilization F H Biology and Society
• As women age, the cells in their ovaries that ultimately ;produce eggs also age. The older a woman is when she becomes pregnant the older that egg cell is and the more likely it is that chromosomal damage has occurred. If this is the case and the embryo has a chromosomal abnormality, the mother’s system may spontaneously abort. Biology and Society contd…..
• However, the aborting mechanism also becomes defective with age. This is one reason why older women are more likely to have children born with birth defects than younger women. At menopause women stops releasing eggs. reproductive technology to have their own biological child? • What do you think of older women using Issues to consider….
• How risky is pregnancy at an older age? • If the body has naturally stopped releasing eggs, should this be overridden by technology? • How likely is it that the child will have a birth defect? • How expensive is the technology? Video of meiosis Video of meiosis
ReplicationandDistributionofDNA duringMeiosis6524853 • With are the two sources of genetic diversity in meiosis?
A. binary fission and regeneration B. crossing over and independent assortment C. vegetative reproduction and regeneration D. Oogenesis and crossing over • • Meiosis and sexual reproduction have fostered diversity in the natural world because they: A. are so accurate in copying genetic information B. are nearly certain to produce differences among offspring C. Are such ancient processes D. Produce genetic replicas, one generation after another E. Are used by such a wide range of organisms. • Asexual reproduction is…… and always A.Found in only a few types of results in…….
organisms….offspring that are different from parents B.Found in all types of organisms…offspring that are sometimes genetically identical to parents C.Reproduction that results from a fusion of two reproductive cells… D.Found more commonly in simple organisms than in complex ones…offspring that are genetically identical to parents ...
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