Ms. Henriksen’s Biology Meiosis Ms. Henriksen’s Biology

Why do cells divide instead of just growing bigger? 2 Reasons: The larger a cell becomes, the more DNA it needs. If a cell is very big, it is hard to move enough nutrients and wastes across the membrane to meet its needs. (not enough cell membrane surface area for diffusion)

Homologous chromosomes Definition: each chromosome has a matching partner from the other parent. One from your mother, one from your father. Both have the same genes Example: Shoes Homologous chromosomes are like your shoes. They are very similar but not identical. This means you have two copies of all of your genes! This is called diploid! Homologous chromosomes dad dad When doubled it looks like mom mom

Haploid and Diploid Diploid Haploid Di = 2 dad dad Diploid Di = 2 Two sets of each chromosome or gene One from mom, one from dad Normal cells are diploid (2N) Haploid Only one copy of each gene Seen in sex cells (eggs and sperm) as a result of meiosis Key to keeping chromosome numbers the same in all offspring - not doubling mom mom Haploid/diploid

1 A cell that has 46 chromosomes is known to be diploid. How many chromosomes would a haploid cell from the same organism have? 92 46 23 16 3

Do you remember Mitosis Prophase The Stages of Mitosis Mitosis asexual reproduction 2 identical sister cells Diploid cells (two sets of each gene) I PMAT C Metaphase Interphase Anaphase Telophase cytokenisis

Meiosis sexual reproduction happens only in reproductive organs 4 haploid cells Important to maintaining chromosome numbers in offspring What would happen if diploid cells fused in reproduction? 46 chromosomes + 46 chromosomes = 92 92 + 92 = 184 . . . . And so on Gives variation to new offspring because genes are separated randomly and then parental offspring is mixed to form new fertilized cell Meiosis what it is

Meiosis is like Mitosis in two rounds All chromosomes are doubled in interphase Parent cell – chromosome pair (diploid) Chromosomes copied, crossing over occurs 1st division - pairs split Meiosis I Meiosis order Meiosis II 2nd division – produces 4 gamete cells with ½ the original no. of chromosomes (haploid)

Meiosis stages Similar to mitosis, but chromosomes line up with homologous pairs first in Meiosis I Interphase I Meiosis I Prophase I Metaphase I Anaphase I Telophase I Meiosis II Prophase II Metaphase II Anaphase II Telophase II Meiosis Meiosis order Mitosis

Facts about Meiosis Meiosis happens in reproductive organs Ovaries, testis, pistle, stamen Produces eggs and sperm Chromosomes can cross over to give genetic variation. Your genes are not identical to your parents Important for maintaining chromosomes numbers If not for haploid cells we would add DNA with each generation Errors in meiosis can lead to problems in offspring Down’s syndrome is caused by an extra copy of a chromosome Crossing-over occurs when tetrads exchange portions of their chromatids. Meiosis where

Down’s syndrome Can you see the extra 21st chromosome? Is this person male or female? Karyograph picture

2 Unlike mitosis, meiosis in male mammals results in the formation of One haploid cell Three diploid cells Four diploid cells Four haploid cells 4

3 Genes for the same trait (the copies from each parent) are separated from each other during the process of Mitosis Meiosis I Meiosis II Interphase 2

Compare but Don’t Confuse Mitosis Results Two identical cells Diploid (2N) cells Growth and replacement of cells Asexual reproduction Meiosis Results Four unique cells Haploid (N) cells Produces gametes (sex cells) for reproduction of offspring Sexual reproduction Keeps chromosome numbers the same in offspring. Allows genetic variation (different traits) in offspring (rather than clones!)

4 Which of the following is not part of meiosis Produces diploid cells Anaphase I Sexual reproduction Maintain chromosome # in offspring 1

5 Definition: Division of the cytoplasm during cell division Mitosis Meiosis Cytokenisis Telophase 3

6 Definition: threadlike structure within the nucleus that contains the organisms genes. Spindle Chromosome Nuclear envelop Homologous 2

7 Meiosis is the process by which gametes are produced. In which of the following human organs does meiosis occur? Skin Pancreas Liver Testis 4

8 The reduction of the chromosome number during meiosis is most important for: A keeping the amount of DNA in the cell at a minimum level preventing the nucleus from becoming larger with each cell division maintaining the chromosome number during sexual reproduction allowing the growth of the cell without increasing the DNA content 3