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