Meiosis Notes

1st…. A Review of Chromosomes http://learn.genetics.utah.edu/content/begin/scale/

Chromosome Numbers In a human, a body cell contains 46 chromosomes.

Chromosomes come in homologous pairs, thus genes come in pairs. Homologous pairs – matching genes – one from female parent and one from male parent When homologous chromosomes are paired together, this is referred to as a tetrad. Example: Humans have 46 chromosomes or 23 pairs. One set from dad 23 in sperm One set from mom 23 in egg Chromosome Numbers

Chromosome Numbers Cells with both sets of homologous chromosomes are called diploid. These cells are represented by 2N. In humans, the diploid number is 46, so 2N=46. In sexually reproducing organisms, the gametes (sex cells – sperm and egg) contain one set of chromosomes. They are called haploid, and are represented by the symbol N. In humans, the haploid is 23, so N=23.

Match the Information! 23 CHROMOSOMES GAMETES N SEX CELLS 2N HAPLOID DIPLOID N SEX CELLS 2N SOMATIC CELLS BODY CELLS YOU NEED TO GET OUT OF PPT SLIDE SHOW TO MOVE THE WORDS. OR YOU CAN TRANSFER IT INTO SMARTBOARD. 46 CHROMOSOMES SEXUAL REPRODUCTION ASEXUAL REPRODUCTION

MEIOSIS

PHASES OF MEIOSIS Meiosis is a process of reduction division where the number of chromosomes per cell is cut in half through the separation of homologous chromosomes in a diploid cell to produce 4 haploid cells. Meiosis is divided into two process Meiosis I Meiosis II

PHASES OF MEIOSIS I During interphase, DNA and organelles are copied. PRE-MEIOSIS - INTERPHASE During interphase, DNA and organelles are copied. 46 chromosomes  92 chromosomes (diploid) Don’t forget….centrioles are also copied!

PHASES OF MEIOSIS I PROPHASE I Spindle fibers appear; nuclear membrane disappears. Homologous chromosomes are paired, forming a tetrad Each tetrad contains 4 sister chromatids Crossing over occurs. Portions of chromatids are exchanged, contributing to genetic variation in offspring

CROSSING OVER As homologous chromosomes pair up and form tetrads in Meiosis I, they actually exchange portions of their chromatids. Crossing over is the reason for genetic diversity! It produces new combinations of genes.

PHASES OF MEIOSIS I METAPHASE I Tetrads/homologous chromosomes line up in the middle of the cell Spindle fibers attach

PHASES OF MEIOSIS I ANAPHASE I Spindle fibers pull the homologous chromosomes toward opposite ends of the cell

PHASES OF MEIOSIS I Cytokinesis occurs; two new nuclear envelopes form TELOPHASE I Cytokinesis occurs; two new nuclear envelopes form End result is two daughter cells, each with 46 chromosomes

PHASES OF MEIOSIS II PROPHASE II Spindle fibers appear; nuclear envelope disappears. Question of the day: why is there not another interphase?

PHASES OF MEIOSIS II Chromosomes line up in the middle of the cell METAPHASE II Chromosomes line up in the middle of the cell

PHASES OF MEIOSIS II ANAPHASE II Sister chromatids separate and move toward opposite ends of the cell

PHASES OF MEIOSIS II TELOPHASE II Cytokinesis occurs; four new nuclear envelopes form End result is four daughter cells, each with 23 haploid chromosomes

PHASES OF MEIOSIS II - CYTOKINESIS MALES FEMALES In males, cytokinesis makes four equal cells that develop heads and tails – sperm – called spermatogenesis FEMALES In females, cytokinesis is unequal; one cell gets most of the cytoplasm to become the mature egg, while the other three usually die and are known as polar bodies. Called oogenesis. Egg and Sperm are called gametes.

http:www.cellsalive.com/meiosis.htm

Chromosome Separation Problems Failure of the homologous chromosomes to separate normally during meiosis -- nondisjunction 1. missing a chromosome – monosomy (45 chromosomes) 2. having an extra chromosome – trisomy (47 chromosomes) 3. most embryos fail to survive, but some do

(47 chromosomes) – Down Syndrome Short, round face, upper eyelids cover the inner part of the eye, mental retardation – trisomy 21 -- three #21 chromosomes (47 chromosomes) – Down Syndrome

Male with longer-than-average limbs, sterile – XXY Male with longer-than-average limbs, sterile – XXY Klinefelter’s Syndrome

Female with short stature, webbed neck, sterile – XO Turner’s Syndrome

deletion / translocation / duplication / inversion Parts of chromosomes break off and reattach in different ways, occurs in meiosis: deletion / translocation / duplication / inversion Deletion Duplication Inversion Translocation

Detecting chromosome mutations: 1. Picture of individual’s chromosomes – karyotype 2. Amniotic fluid surrounding an embryo is removed for analysis (done 3½ to 4 months of pregnancy) – amniocentesis 3. Analysis of chorionic villi which grows between mother’s uterus and placenta (done 2 months of pregnancy – chorionic villi sampling (CVS)

COMPARISONS! MEIOSIS MITOSIS Occurs in sex cells ONLY During first meiotic division, crossing over occurs During second meiotic division, chromosome duplication does not occur Produces four cells, each with half a set of complete chromosomes Cells are haploid (N = 23) Cells are GENETICALLY DIFFERENT from each other Occurs in ALL body cells Chromosomes are duplicated Chromosomes are pulled apart by spindle fibers An identical, complete set of chromosomes moves towards each side of cell Cytokinesis End product are two IDENTICAL cells Daughter cells are diploid 2N = 46