Meiosis Chapter 6

MITOSIS vs MEIOSIS Page 161 in the textbook

Introduction to Meiosis Chromosomes and meiosis

Body Cells and Germ Cells We have two types of cells: 1. Somatic Cells (body cells) 2. Germ Cells that develop into gametes or sex cells Ova or eggs in females and spermatozoa (sperm) in males. DNA from gametes is passed on to offspring. Each of our somatic cells (body cells) has 46 chromosomes (23 pairs). All somatic cells are genetically identical to each other. Cells within the organism differ from one another because of which genes are expressed NOT because the genes are different.

Homologous Chromosomes A koala has 16 chromosomes Glove analogy: Suppose you had 8 pairs of gloves for a total of 16 gloves that could be divided into 2 sets; 8 right and 8 left. Similarly the koala has 8 pairs of chromosomes for a total of 16 that can be divided into 8 sets. 8 from mom and 8 from dad. Just as you use both gloves when it’s cold out, your cells use both sets of chromosomes to function properly.

Homologous Pair Together, each pair of chromosomes (gloves) is referred to as a homologous pair. Homologous means “having the same structure”. Homologous chromosomes are 2 chromosomes: 1 from mom and 1 from dad. They have the same length and general appearance.

Homologous Chromosomes They carry genetic information about the same traits, arranged in the same order. The two copies may differ! EX: If you have a gene that influences blood cholesterol levels on chromosome #8 you will have one copy from mom and one copy from dad. It is possible that one copy is associated with higher levels and one copy is associated with lower levels.

Karyotype (a display of chromosomes in a cell) For convenience scientist have assigned a number to each pair of homologous chromosomes. Ordered from largest to smallest.

Karyotype Activity https://bio.rutgers.edu/~gb101/lab10_meiosis/meiosis_web/karyotype1/karyo_frame1.html Activity shows how geneticists use karyotypes to diagnosis chromosomal abnormality.

Autosomes Chromosomes 1-22 make up your autosomes. Chromosomes that contain genes for characteristics not directly related to the sex of an organism.

The 23rd pair: Sex Chromosomes Most sexually reproducing organisms have sex chromosomes They directly control the development of sexual characteristics Humans have 2 very different sex chromosomes: X and Y How sex is determined varies by species In all mammals (including humans) the sex is determined by the XY system.

Sex Chromosomes XX is female / XY is male Although the X and Y pair with each other they are not homologous (only small homologous portions). The X is larger and contains numerous genes, many unrelated to sexual dev. The Y contains the genes that directly control the development of the testes and other male traits. Y is the smallest with fewest genes.

Asexual Reproduction Single parent Offspring are clones

Sexual Reproduction 2 parents Offspring have combo of parent traits

Stages of Sexual Reproduction Meiosis: making gametes Fertilization: combining gametes

Benefits of Sexual Reproduction Pass on genes Variation Evolutionary success

Sexual Reproduction Involves the fusion (fertilization) of 2 gametes, resulting in off spring that are a mixture of both parents When fertilization occurs the nuclei of the ovum (egg) and sperm fuse to form 1 nucleus.

The new nucleus (zygote): Must have the correct number of chromosomes for a healthy new organism to develop. THEREFORE both the ovum (egg) and sperm need only half the usual number of chromosomes (one chromosome from each homologous pair).

Diploid Cells Somatic cells (body cells) are diploid 2 copies of each chromosome (one from mom and one from dad) Homologous chromosomes 2n (humans = 46)

Haploid Cells Gametes are haploid 1 set of chromosomes Combo of mother and father 1n (humans = 23)

Question #1 If a diploid cell had 36 chromosomes, how many chromosomes would be in the gametes (haploid cells)? 18

Question #2 In a haploid cell with 18 chromosomes how many different chromosomes are there? 18

So, how do we get those haploid cells? MEIOSIS

Meiosis Process that produces gametes Cells divide twice (2n  1n) Random mix of mom and dad DNA to each gamete Produces 4 haploid cells

Primordial Germ Cell Cell set aside early in embryonic development that is a precursor to germ cells that give rise to gametes (ova and sperm). These cells are waiting in INTERPHASE for a signal to start meiosis. (They go through interphase before they enter meiosis.) So, the DNA has already been duplicated prior to MEIOSIS.

Reminder: Homologous chromosomes and sister chromatids Homologous chromosomes are divided during meiosis I and the sister chromatids are not divided until meiosis II.

Germ Cell Interphase Prior to meiosis, all chromosomes are duplicated in a process similar to chromosome duplication in the S phase of the cell cycle prior to mitosis.

MEIOSIS I DNA has already been copied in interphase

Prophase I Nuclear membrane breaks down Centrioles begin to move to opposite poles of cell Spindle fibers begin to assemble Homologous chromosomes pair up Crossing over occurs (random pieces of DNA exchanged between homologous chromosomes)

Kahn Academy: Meiosis www.kahnacademy.org Search MEIOSOS. Start at 10:00 for review of crossing over AND an explanation of how the chromosomes from the mom and the dad line up randomly along the equator of the cell during metaphase I creating even more genetic variation.

Metaphase I Homologous chromosome pairs randomly line up along the middle of the cell by spindle fibers

Metaphase I (continued) In humans, 23 homoologous pairs of chromosomes line up randomly along each side of the cell equator. This mixes up the chromosomal combinations even more, contibuting to genetic diversity. Human meiosis may result in 2 to the 23rd power or 8,388,608 possible combinations of chromosomes!

Anaphase I Homologous pairs separate and move to opposite poles of cell Sister chromatids remain together with the centromere intact

Telophase I / Cytokinesis Nuclear membrane re-forms Spindle fibers disassemble 2 haploid daughter cells formed End result: 2 cells, each having a unique combination of 23 duplicated chromosomes coming from both parents.

Meiosis II Divides sister chromatids Results in undoubled chromosomes DNA is NOT copied again between meiosis I and meiosis II.

Prophase II Nuclear membrane breaks down Centrioles move to opposite poles of the cell Spindle fibers assemble

Metaphase II Chromosomes line up in middle of cell

Anaphase II Centromeres break down Sister chromatids move to ends of cell

Telophase II/ Cytokinesis Nuclear membranes reform Spindle fibers gone 4 haploid daughter cells formed All different with a combination of chromosomes from mom and dad

Meiosis II

Amoeba Sisters ‪Meiosis: The Great Divide https://www.youtube.com/watch?v=toWK0fIyFlY BTW: there is an excellent diagram of meiosis in your textbook on page 164!

Quizlet LEARN section for review. Number 1-15: https://quizlet.com/1233278/learn TEST: https://quizlet.com/1233278/test “