Meiosis Summary Objectives: 1)Explain what haploid, diploid, gamete and crossing over means 2)Explain the phases of meiosis 3)Explain how meiosis insures variability in offspring.

Gamete – a sex cells that have a haploid number of chromosomes Haploid – the number of chromosomes in a haploid cell (gamete) Haploid Cell – cells containing a half set of chromosomes or only one member of each of the 23 sets or pairs of chromosomes; this means that there is a chromosome from only one parent for each of the 23 pairs Diploid – cells containing twice the number of chromosomes in a gamete; in the human diploid cell this number is 46; the symbol is 2n Diploid Cell – cells containing a full set of chromosomes; 23 sets or 23x2=46 chromosomes; one member of each of the 23 sets comes from each parent

ASEXUAL REPRODUCTION Asexual reproduction is based on mitotic division of the nucleus. It may be a unicellular organism reproducing itself, or: Cells of multicellular organisms that break off to form a new individual The offspring are clones—genetically identical to the parent.

Sexual Reproduction Sexual reproduction: the offspring are not identical to the parents. Sexual reproduction results in shuffling of genetic information in a population. No two individuals have exactly the same genes. There is random selection of half a parents chromosomes that go into the haploid gamete; two such gametes fuse—offspring are not exactly like parents.

Purpose: i.To produce gametes that have half the number of chromosomes (23) as the parent cell. ii. When a sperm cell (n) unites/fertilizes an egg (n) cell it forms a new cell called a zygote (2n). iii.To ensure genetic diversity through a process of crossing-over or exchange of genetic information between non-sister chromatids

Some more terms for you to know Somatic cells—body cells not specialized for reproduction. Each somatic cell contains homologous pairs of chromosomes with corresponding genes. Each parent contributes one homolog.

Meiosis produces… In females haploid cells called eggs or ova In males haploid cells called sperm

Meiosis involves 2 stages of nuclear division, Meiosis I and Meiosis II, occurring successively The first stage (Meiosis I) is significantly different from Mitosis The second stage (Meiosis II) is virtually identical to Mitosis Only Meiosis I starts with interphase (chromosomes duplicate 46 to 92)

Your Turn You will attempt to model the phases of meiosis using modeling clay. In groups of two read through you instructions carefully. Following each step carefully. When you are done, you should be able to explain the phases of meiosis.

Quiz 1.How does the process of “crossing over” in prophase I, ensure that the resulting cells are genetically different from the parent cell? 2.What is a tetrad? 3.How is the product of meiosis different from mitosis? 4.Are the products of meiosis diploid or haploid? 5.What is gametogenesis?

Prophase I Chromosomes condense Chromosomes come together (synapsis) in homologous pairs to form tetrads Crossing over occurs

Metaphase I Chromosomes (homologous pairs) line up at equatorial plate

Anaphase I Homologous pairs separate Move to opposite ends of cell

Telophase I Nucleus completes division Nuclear membrane reforms Cleavage furrow Chromosomes in each nuclei are NOT identical

Prophase II New spindle fibers form Chromosomes condense

Metaphase II Chromatids line up at the equatorial plate

Anaphase II Chromatids separate Move to opposite poles

Telophase II Cytoplasm separates

Product Result is 4 haploid cells –half the number of chromosomes as parent cell

Meiosis Summary

Prophase I

Metaphase I

Anaphase I

Telophase I

Prophase II

Metaphase II

Anaphase II

Telophase II

Your turn 1)With a partner, discuss what two stages are responsible for creating genetic variability 2)From what you have learned today, what role does meiosis play in transmitting genes/characteristics from parents to child? 3)How can you tell (genetically) if a person is a male or female? One step further… Compare mitosis and meiosis describing at least 3 differences.

MitosisMeiosis Purpose To ensure growth & to repair cells of the organism To ensure the procreation of the species Type of Cell produced Somatic (body) cells Gametes (sex) cells -Female: egg/ovum -Male: sperm Number of Nuclear Divisions 1 nuclear division 4 phases 2 nuclear divisions Meiosis I – 4 phases Meiosis II – 4 phases Number of Cells Produced 2 new identical daughter cells (identical to each other & parent) 4 new non-identical daughter cells Number of chromosomes in each new cell 46 chromosomes23 chromosomes Name given to this type of cell Diploid cell -complete set of chromosomes Haploid cell -a half set of chromosomes Symbol given to this type of cell 2nn