MEIOSIS Unit 4 – Growth & Reproduction
Dec 21, 2015
MEIOSIS
Unit 4 – Growth & Reproduction
Terms to Know – (add to vocab page)
Homologous Chromosome –corresponding (matching) chromosomes from a female and male parent.
Diploid Cell (2n) – a cell that contains both sets of homologous chromosomes
Haploid Cell (n) – a cell that contains one set of homologous chromosomes
Gene – segment of DNA that codes for a trait
Replicated Chromosome – a chromosome that has doubled it’s DNA
Non-Replicated Chromosome – a chromosome that has split and contains only one copy of DNA.
What is Meiosis and do we need it? A form of cell division happening in
sexually reproducing organisms consisting of two consecutive cell divisions (meiosis I and meiosis II).
Produces 4 sex cells Men 4 sperm cells (in the testes) Women 1 egg and 3 polar bodies (in
the ovaries)
Meiosis Overview
Meiosis I: One diploid cell (46 replicated chromosomes/23
pairs) becomes two haplod cells (23 replicated chromosomes.) (start with 92 chromatids)
Meiosis II: Two haploid cells (23 replicated chromosomes)
becomes 4 haploid cells (23 non-replicated chromosomes.)
Why do we need Meiosis?
Distributes maternal and paternal chromosomes evenly and randomly (genetic variation).
Meiosis is necessary to half the number of chromosomes going into the sex cells.
Why half the chromosomes in gametes? At fertilization the male and female sex
cells will provide ½ of the chromosomes each – so the offspring has genes from both parents
Why do we need Meiosis? continued.
23
2346
Female vs. Male
Oogenesis
egg production
Spermatogenesis
Sperm Production
What does Meiosis accomplish?
1. Meiosis takes a cell with two copies of every chromosome (diploid) and makes cells with a single copy of every chromosome (haploid).
2. Meiosis scrambles the specific forms of each gene that each sex cell (egg or sperm) receives through crossing over and independent assortment.
Steps in Meiosis
PMAT2 similar cells23 chromosomes
Separates homologous pairs
Haploid
PMAT4 similar cells23 chromosomesSeparates replicated chromosomes
Haploid
Meiosis I – starts diploid Meiosis II – starts haploid
Same steps as Mitosis, divides twice to produce 4 daughter cells instead of 2.Daughter cells are not identical but similar
Interphase/Prophase I
Interphase Cells duplicate their
DNA Prophase I
sister chromatids pair up forming a homologous chromosome
Crossing Over When two
chromosomes exchange portions of their chromatids
Occurs during Prophase I
Metaphase I
Metaphase I Spindle fibers
attach to the centromeres on each chromosome
Homologous chromosomes line up together.
Anaphase I
Anaphase I• Spindle fibers
pull the homologous chromosomes toward opposite ends of the cell.
Telophase I/Cytokinesis
Telophase I/Cytokinesis
Meiosis I results in 2 haploid (n) daughter cells.
Each with half the number of chromosomes as the original cell (23)
Still replicated chromosomes.
Prophase II
Prophase II 2 haploid (n)
daughter cells. Each with half the number of chromosomes as the original cell.
Interphase does NOT happen again.
Same steps as normal prophase.
Metaphase II
Metaphase II The
chromosomes line up in a similar way to the metaphase stage of mitosis. Occurs in BOTH cells.
Anaphase II
Anaphase II The sister
chromatids separate and move toward opposite ends of the cell
Telophase II / Cytokinesis
Telophase II / Cytokinesis Opposite of prophase occurs, cytoplasm
splits. END RESULT: four haploid (n), non-
replicated, non-identical daughter cells.
Fertilization
The haploid sperm (carrying 1 set of chromosomes) combines with the haploid egg (carrying 1 set of chromosomes). The result is a diploid zygote.
Mitosis vs. Meiosis
Diploid 2n diploid 2n 46 chromosomes 46
chromosomes Replicated
chromosomes Non-Replicated Chromosomes
Somatic Cells Creates 2 identical
cells 1 trip through PMAT
Dploid 2n haploid n 46 chromosomes 23
chromosomes Replicated
chromosomes non-replicated chromosomes
Gamete cells Creates 4 similar cells 2 trips through PMAT
Meiosis Drawing
Things to Label: pg. 276/277 Label Meiosis 1 and Meiosis 2 Label each cell as diploid (2n) or haploid (n) Label the chromosome number of each cell Circle and label a homologous pair of
chromosomes Circle and label a replicated chromosome. Circle and label a non-replicated
chromosome. 4 or 6 chromosome.
Meiosis Poster
Label each cell as diploid (2n) or haploid (n)
Circle and label a homologous pair of chromosomes
Circle and label a replicated chromosome. Circle and label a non-replicated
chromosome. Label a centromere Label a pair of centrioles Label a set of spindle fibers