Biology Chapter 6 Dr. Altstiel Meiosis and Mendel Biology Chapter 6 Dr. Altstiel

Meiosis Objectives Summarize the events that occur during meiosis. Relate crossing-over, independent assortment, and random fertilization to genetic variation. Compare spermatogenesis and oogenesis.

Number and structure of Chromosomes affect development Humans have 2 copies of 23 different chromosomes (46 total) Homologous chromosomes are similar in size, shape and genes Each one in a set comes from mom; other from dad

Chromosome Number Karyotype – a set of chromosomes. Humans have 23 pairs of chromosomes = 46. Diploid – when cells in the body contain 23 pairs of chromosomes. Practically all cells in the body are diploid. Diploid Number = 2n Humans, 2n = 46 Haploid – cells that contain only one copy of each chromosome. Haploid Number = n Humans, n = 23 Zygote – fusion of two haploid gametes by a process called fertilization – a diploid zygote is a fertilized egg.

Diploid cells (somatic body cells) contains two sets of chromosomes Haploid cells (sex cells) contains one set of chromosomes Two haploid sex cells come together during fertilization and make a diploid zygote

Sex Chromosome Determines whether you are male or female. Sex chromosomes designated as either X or Y. Female = XX Can only produce eggs that contain an X chromosome. Male = XY Can produce sperm with either an X or Y chromosome. Therefore, male’s sperm is sex determiner. Autosomes – chromosomes not directly involved in determining the sex of an individual (22 pairs).

Formation of Haploid Cells Meiosis – a form of cell division that halves the number of chromosomes when forming specialized reproductive cells, such as gametes and spores. Involves two divisions of the nucleus – meiosis I and meiosis II.

Meiosis Meiosis is a two-stage form of nuclear division in which the chromosome number is halved. Meiosis I – reduces the chromosome number by half. Meiosis II – separates identical copies of chromosomes.

Steps of Meiosis Prophase I – chromosomes condense, and nuclear envelope breaks down. Crossing-over may occur. Crossing-over – when a portion of a chromatid on one homologous chromosome are broken and exchanged with the corresponding chromatid portions of the other homologous chromosome. Metaphase I – Pairs of homologous chromosomes moved by the spindle to the equator of the cell. Anaphase I – Homologous chromosomes separate. Telophase I – Individual chromosomes gather at each of the poles. Cytokinesis occurs resulting in two cells.

The result of meiosis is 4 haploid cells. Steps of Meiosis Cont. Prophase II – a new spindle forms around the chromosomes. Metaphase II – Chromosomes line up along the equator. Anaphase II – the centromeres divide, and chromatids move to opposite poles of the cell. Telophase II – a nuclear envelope forms around each set of chromosomes. Spindle breaks down, and cytokinesis occurs. The result of meiosis is 4 haploid cells.

Meiosis I Prophase I DNA replication precedes the start of meiosis I. Homologous chromosomes pair and form synapses, a step unique to meiosis. The paired chromosomes are called bivalents A bivalent has two chromosomes and four chromatids, with one chromosome coming from each parent.

Meiosis I Continued Metaphase I Bivalents align at the metaphase plate. The orientation is random, with either parental homologue on a side. This means that there is a 50-50 chance for the daughter cells to get either the mother's or father's homologue for each chromosome.

Meiosis I Continued Anaphase I Chromosomes, each with two chromatids, move to separate poles. Each of the daughter cells is now haploid (23 chromosomes), but each chromosome has two chromatids.

Meiosis I Continued Telophase I Nuclear envelopes may reform, or the cell may quickly start meiosis 2.

Meiosis I Continued Cytokinesis Analogous to mitosis where two complete daughter cells form.

Meiosis II

Meiosis II Continued

Meiosis II Continued

Meiosis II Continued

Meiosis II Continued

Phases of Meiosis

Directed Reading: Meiosis Class Work Directed Reading: Meiosis

Meiosis and Genetic Variation Three Mechanisms that make key contributions to genetic variation: Independent assortment Crossing-over Random fertilization

Independent Assortment Each human gamete receives one chromosome from each of the 23 pairs of homologous chromosomes. Which of the two chromosomes passed to the offspring is a matter of chance. Independent Assortment – the random distribution of homologous chromosomes during meiosis Each of the 23 pairs of chromosomes segregate independently. Possibility of 223 different gene combinations. (8,388,608 possible combinations)

Crossing-Over DNA exchange that occurs during crossing-over adds more recombination to independent assortment of chromosomes. Number of possible genetic combinations that can occur among gametes… virtually unlimited.

Crossing-Over DNA is exchanged between homologous chromosomes, resulting in genetic recombination.

Random Fertilization The zygote that forms a new individual is created by the random joining of two gametes (each produced independently). Number of possible genetic outcomes is (223)2 = 7.04 X 1013 .

Meiosis and Gamete Formation Gametogenesis – meiosis is the primary event in the formation of gametes. Spermatogenesis – process by which sperm is produced in male animals. Occurs in the testes. After completion of meiosis, four haploid cells are produced. Sperm – four cells change in form and develop a tail. Oogenesis – process by which gametes are produced in female animals. Occurs in the ovaries. Cytokinesis following meiosis I – cytoplasm divides unevenly resulting in a large cell (will become the ovum) and three polar bodies.

Homework Correct Directed Reading for Section 1

Sexual and Asexual Reproduction Asexual Reproduction – a single parent passes copies of all of its genes to each of its offspring – no fusion of haploid cells such as gametes. Clone – an organism genetically identical to its parents. Sexual Reproduction – two parents each form reproductive cells that have ½ the number of chromosomes. Offspring have traits of both parents.

Alternation of Generations Life-cycle that regularly alternates between a haploid and a diploid phase. Plants, algae, some protists. Sporophyte - diploid phase in the life-cycle of plants – the name of the spores that are produced. Spore – a haploid reproductive cell produced by meiosis – give rise to multicellular individual called a gametophyte w/o joining with another cell. Gametophyte – haploid phase – produces gametes by mitosis. Gametes fuse and give rise to the diploid phase.