Meiosis and Sexual Life Cycles

Sexual Reproduction Reproduction = process by which a new generation of cells or multicellular individuals is produced. Sexual reproduction requires meiosis, formation of haploid gametes (eggs and sperm), and fertilization (union of a haploid egg and a haploid sperm to form a diploid zygote).

The Role of Meiosis in Sexual Life Cycles Ploidy refers to the number of chromosome sets in a cell. –Diploid (2n) - condition in which cells contain two sets of chromosomes (one set from each parent) –Haploid (n) - cells contain one set of chromosomes. In animals, these cells are called gametes (sperm and egg cells), in plants – pollen and egg. Meiosis is the process by which the chromosome number is halved during gamete formation. Thus, meiosis reduces chromosome number from diploid to haploid. The human life cycle -- follows the same basic pattern found in all sexually-reproducing organisms; meiosis and fertilization result in alternation between haploid and diploid condition.

Review of Terminology: Autosome -- any chromosome except sex chromosomes. Homologue -- a pair of chromosomes that have the same size, centromere position, and banding pattern –With one exception (sex chromosomes in a male), homologues carry the same genetic loci. –However, the sequence of the two genes may differ at a particular locus on the two chromosomes. Different forms of a gene for the same trait are called alleles. Sex Chromosomes -- dissimilar chromosomes that determine an individual’s sex. –Human females have a pair of X chromosomes. –Human males have one X and one Y chromosome. –Thus humans have 22 pairs of autosomes and one pair of sex chromosomes.

paternal homologue maternal homologue one homologous pair of chromosomes Activity Biozone:Eukaryote chromosome structure

X Y 46 chromosomes from a human male. Each chromosome is in the duplicated state. They are arranged as pairs of homologous chromosomes, with chromosome 1 the largest and chromosome 22 the smallest. Note the small size of the Y chromosome as compared to the X chromosome.

Skin Color Height Homologous chromosomes mean that diploid cells have two copies of a gene for each trait, with one copy coming from mom and one copy coming from dad! However, the sequences of the two genes do not have to be identical. Alternative forms of genes for the same trait at the same location on a chromosome are called alleles.

Meiosis occurs as two consecutive cell divisions: Meiosis I (splitting pairs of homologous chromosomes) and Meiosis II (splitting sister chromatids apart). 2n n

Meiosis: Two Divisions Meiosis I Meiosis II

Key Differences Between Meiosis and Mitosis Mitosis is characterized by just one division: diploid state is retained and 2 daughter cells are genetically identical to parental cell. Meiosis is two successive nuclear divisions: Meiosis I is a reduction division (one diploid cell divides to form two haploid cells). Meiosis II is just like mitosis, except that the cells are haploid at the beginning of Meiosis II. Overall, meiosis produces four haploid daughter cells (which will become gametes), which are all genetically different from one another. In males 4 sperm/pollen are produced. In females one egg survives. Nb Haploid = 1 allele set, Diploid = paired allele set

Meiosis I cuts chromosome number in half: cells change from diploid to haploid state. Note that each chromosome is still duplicated, though. plasma membrane newly forming microtubules in the cytoplasm spindle equator (midway between the two poles) one pair of homologous chromosomes PROPHASE IMETAPHASE IANAPHASE ITELOPHASE I MEIOSIS I

Meiosis II is similar to mitosis. Duplicated chromosomes line up randomly at metaphase, and sister chromatids split apart to form two separate chromosomes. PROPHASE IIMETAPHASE IIANAPHASE IITELOPHASE II MEIOSIS II

See the following Web sites for a description and animation of meiosis and a meiosis problem set! The stages of meiotic cell division. Interphase I –Chromosomes replicate as for mitosis. –Each duplicated chromosome consists of two identical sister chromatids attached at their centromeres. –Centrosomes duplicate.

Prophase I Chromosomes condense. Homologous chromosomes come together as pairs (called synapsis). Crossing-over occurs between nonsister chromatids of homologous chromosomes (see later). Centrosomes migrate to opposite poles of cell and spindle microtubules begin to form. Nuclear envelope and nucleoli disperse. Chromosomes begin to move to center of the cell. Prophase I occupies >90% of the time required for meiosis.

MEIOSIS - PROPHASE I

Pairs of homologous chromosomes are aligned in center of cell. Each pair is aligned so that centromeres of homologues point toward opposite poles. Each homologue is attached to kinetochore microtubles emerging from the pole it faces. MEIOSIS - METAPHASE I

MEIOSIS - ANAPHASE I & TELOPHASE I

Anaphase I Homologous chromosomes separate and the chromosomes are moved toward opposite poles by the spindle apparatus. Telophase I and Cytokinesis I The spindle apparatus continues to separate homologous chromosomes until the chromosomes reach the opposite poles. Cytokinesis occurs simultaneously with telophase I, forming two haploid daughter cells (but with each chromosome still in its duplicated state).

Meiosis II-- This second meiotic division separates sister chromatids of each chromosome. Prophase II Spindle apparatus forms and chromosomes move toward the center of the cell. Metaphase II Chromosomes align, randomly and singly, at the center of the cell.

MEIOSIS - PROPHASE II & METAPHASE II

Anaphase II Centromeres of sister chromatids separate and move towards opposite poles. Telophase II and Cytokinesis Nuclei form at opposite poles of the cell. Cytokinesis occurs producing four haploid daughter cells. MEIOSIS - ANAPHASE II & TELOPHASE II

Fig , p n n germ cell each chromosome duplicating during interphase MEIOSIS I separation of homologues MEIOSIS II separation of sister chromatids diploid number restored at fertilization zygote gametes

Variation and genetic differences make us unique. How is genetic diversity accomplished?

Evolutionary adaptation depends upon a population’s genetic variation. This is the basis for Darwin’s theory of evolution by natural selection, which states that individuals with certain traits may be better suited to survive and reproduce than individuals with different traits. There are two sources of genetic variation: 1. Mutations: random changes or relatively rare mistakes made during DNA replication in a gene. 2. Sexual reproduction (meiosis and fertilization)

1. Crossing-Over during Prophase I of Meiosis.

Multiple features of meiosis and fertilization result in genetic diversity within a species. 1. Crossing over during prophase I of meiosis. In each homologous pair (a complex of four chromatids), nonsister chromatids are linked by X-shaped chiasmata, sites where homologous strand exchange or crossing- over occurs. The exchange of genetic material between homologues occurs during prophase of meiosis I. This produces chromosomes that contain genes from both homologues (e.g., a mixture of genes from both parents). In humans, there is an average of two or three crossovers per chromosome pair during meiosis.

The effect of crossing-over on generating genetic diversity in gametes.

2. Independent Assortment

The random distribution of maternal and paternal homologues to the gametes. Since each homologous pair lines up randomly during Metaphase I and assorts independently from all the others, the process produces 2 n possible combinations, where n is the haploid number. In humans, there are 2 23 or ~8 million possibilities of chromosome assortments in any gamete. Genetic variation results from the reshuffling of chromosomes, because each homologue will carry different genetic information (alleles) at many of their corresponding loci.

3. Random fusion of gametes during fertilization. In humans, when an individual ovum (representing one of eight million combinations) is fertilized by a sperm (representing one of eight million combinations), the resulting zygote can have one of 64 trillion possible diploid combinations. This is not even considering variations from crossing over!

Fig , p n n germ cell each chromosome duplicating during interphase MEIOSIS I separation of homologues MEIOSIS II separation of sister chromatids diploid number restored at fertilization zygote gametes