Ch 26 – Reproduction, Part II Learning Objectives continued Today’s topic: Meiosis

Meiosis (the development of egg or sperm cells)

Review Questions From A&P I 1. What is the biological term for body cell division? 2. Cells are generally found to be in Interphase (NOTE: Interphase marks the beginning of the cell cycle/ growth and division). What are the other 4 phases of mitosis? * You may want to spend some time on the side reviewing mitosis if you have forgotten it! This will be beneficial in remembering terms such as DNA, chromosomes, spindle fibers, centromeres, etc.

Predict How many chromosomes do human body cells, like skin, have? How many chromosomes do human sex cells (gametes) have? How many chromosomes do other living things have?

Class Discussion Why do you think human sex cells only have 23 chromosomes instead of 46? egg sperm NOTE: Each cell is mathematically expressed as 23n. The letters “n” represent a known variable. 23n

Human Life Cycle Two sets of chromosomes (one from each parent) is necessary to create life. Normal chromosome no. in body cells = 2n (___) Also called diploid. Gametes = n (___ chromosomes) or haploid

Quick Check What is the haploid (n) chromosome number in human gametes? What is the diploid (2n) chromosome number in human skin cells?

New Vocabulary Practice Instructions: Before we launch into a discussion of meiosis, it will be to your advantage to familiarize yourself with ‘special’/ new vocabulary. Estimated Time: minutes 1.Diploid 2.Haploid 3.Homologous chromosomes 4.Paternal 5.Maternal 6.Meiosis 7.Tetrads 8.Synapsis 9.Crossovers 10. Sister chromatids a.Referring to the mother’s side of the gene pool b.Chromosomes that are structurally similar c.Human body cell chromosome number of 46 (2n) d.How sex cells form and develop e.Little groups of 4 chromatids f.Pairing of homologous chromosomes during the first meiotic division g.Referring to the father’s side of the gene pool h.Human sex cell chromosome number of 23 (n) i.Identical copies of chromosomes j.Allows for the exchange of genetic material; contributes to diversity of an individual

Meiosis Meiosis = two nuclear divisions, meiosis I and meiosis II. Results in _____ daughter cells (with ____ the number of chromosomes as body cells) Chromosomes replicate prior to meiosis I

Tetrads line up at the spindle _______ during metaphase I In anaphase I, homologous chromosomes still composed of joined sister chromatids are distributed to ___________ of cell. At the end of meiosis I each daughter cell has:Two copies of either a maternal or paternal chromosome. A 2n amount of DNA and haploid number of chromosomes In telophase I: The nuclear membranes re-form around the chromosomal masses. The spindle breaks down. The chromatin reappears, forming two ___________. In meiosis I, homologous pairs of chromosomes undergo synapsis and form tetrads with their homologous partners. Crossover, the exchange of genetic material among tetrads, occurs during synapsis.

Mirrors mitosis except that chromosomes are not replicated before it begins Meiosis accomplishes two tasks: –It _________ the chromosome number by half (2n to n) –It introduces genetic variability

Quick Check/ Review 1.Gametes contain 23 chromosomes and are known as ___ or _______? (Answer choices: n, 2n, haploid, or diploid) 2. Why must gametes have only 23 chromosomes? 3. How does mitosis differ from meiosis? Instructions: Outline an answer in your notes. Be prepared to share your answers with the class.

Oogenesis Production of ______ sex cells by meiosis In the fetal period, oogonia (2n ovarian stem cells) multiply by mitosis and store nutrients Primary oocytes begin meiosis but stall in prophase I

Oogenesis continued At puberty, one activated primary oocyte produces two _______ cells –The first polar body –The secondary oocyte The secondary _______ arrests in metaphase II and is ovulated If penetrated by sperm the second oocyte completes meiosis II, yielding: –One large ovum (the functional gamete) –A tiny second polar body

Oogenesis

Spermatogenesis The sequence of events that produces _______ in the seminiferous tubules of the testes Each cell starts with two sets of chromosomes (one maternal, one paternal) and is said to be _______ (2n chromosomal number) Humans have 23 pairs of homologous chromosomes Gametes only have 23 chromosomes and are said to be _______ (n chromosomal number) Gamete formation is by meiosis, in which the number of chromosomes is halved (from 2n to n)

Spermatogenesis

Court’s In Session: Case of the Dividing Cell See class handouts Approx Time: 30 minutes (Case Day 1) 35 minutes (Case Day 2)