Grab the notes sheet from back lab table…

Objectives:

Describe Meiosis and the formation of sex cells.

Compare your vocab definitions to the notes.

Chapter 9.5-9.6 Meiosis

Asexual Reproduction

- Asexual reproduction involves body cells: New organisms are produced from one parent (they are identical)

2 Types of cells in body:

Somatic cells: body cells

Gametes: sex cells – sperm & eggs

Sexual Reproduction

- a new organism is produced when sex cells from two parents combine

Sex Cells

• sperm: small w/ whiplike tails and heads that are almost all nucleus

•Egg: usually large and contain food material

Production of Sex Cells

• formed during meiosis

• During meiosis, the number of chromosomes is reduced by one-half

Example: a human body cell has 46 chromosomes or 23 pairs

-during meiosis, this number is reduced to 23 chromosomes

-An egg or a sperm cell has 23 chromosomes

Why would the sex cells only have 23 chromosomes?

Importance of Sex Cells

Diploid

-a body cell w/23 pairs of chromosomes is diploid (2n) (2 of every kind of chromosome)

-body cells receive one from mother and one from father during fertilization

Haploid

- a sex cell w/23 chromosomes (1n)

Fertilization: the joining of an egg (haploid) and a sperm (haploid)

Zygote: fertilized egg (2N)

--meiosis and fertilization keep the species number of chromosomes from changing

Describe what you see…

Meiosis

• two divisions of the nucleus (meiosis I and meiosis II)

• Before the cell enters meiosis I, the chromosomes and DNA both duplicate

Overview: Meiosis I

• Duplicated chromosomes are separated

• Each chromosome still has double the amount of DNA

• Cytoplasm divides and forms two cells

Meiosis I

1.Prophase I: DNA coils to form chromosomes

- Spindle fibers appear, nucleolus & nuclear membrane disappear

- Chromosomes begin lining up w/ their homologues

Synapsis: pairing of homologous chromosomes; doesn’t occur in mitosis

Crossing-over: portions of chromatids break off & reattach to adjacent chromatids on homologous chromosome

Tetrad: each pair of homologous chromosomes; 4 chromatids

-Orient themselves so corresponding genes are adjacent to each other

- Results in genetic recombination:

New mixture of genetic material

-source of variety in populations

Crossing-over

2.Metaphase I: tetrads line up randomly along middle of cell

- Spindle fibers from the poles attach to centromere of each homologue

- The chromosomes line up 2 by 2, unlike in mitosis

3.Anaphase I: each homologous chromosome moves toward opposite pole of cell; tetrad splits

- Each homologous chromosome consists of 2 chromatids attached at centromere

4.Telophase I & Cytokinesis I:

- Chromosomes reach poles & cytokinesis begins

- Produces 2 cells, each w/ 1 chromosome from each homologous pair

NOTE:

The two cells produced by meiosis I have chromosomes and alleles that are different from each other and from the diploid cell that entered meiosis I.

INTERKINESIS—rest period between Meiosis I and II; not all cells “rest”; some proceed immediately to Meiosis II

Overview: Meiosis II

• The amount of DNA is reduced

• Each new nucleus will contain only half the number of chromosomes w/half the DNA that was present at the start

• Cytoplasm divides resulting in 4 daughter cells

Meiosis II

- DNA does NOT copy again!

5.Prophase II: spindle fibers form & begin to move chromosomes toward the middle of cell

- Nuclear membrane will break down again if it formed after telophase I

6. Metaphase II: chromosomes in middle of cell

7.Anaphase II: chromatids separate & move toward opposite poles of cell

8. Telophase II & Cytokinesis II: nuclear membrane forms around the chromosomes

- Cytoplasm divides, resulting in 4 new cells

Metaphase II

Anaphase II

Telophase II & Cytokinesis II

Gamete FormationMales: sperm is produced

•In many female animals, only one egg results from meiosis. The other three cells, called polar bodies, are usually not involved in reproduction.

HOMEWORK:

Compare your vocab definitions to the notes.

We will be using this information tomorrow!!

Objectives for today:

Describe the vocab definitions!

Compare and Contrast Haploid to Diploid

VOCAB QUIZ TOMORROW!!

Let’s see what you remember!1. Meiosis starts out with 1 diploid cell and ends

with ________________.

2. Meiosis cuts the number of chromosomes in __________.

3. When does Crossing Over occur?

4. If an alligators haploid cell has 16 chromosomes, how many does a diploid cell have? 32

On a scratch piece of paper write if this cell is diploid or haploid and how many chromosomes it containsBone cell- Liver cell- Egg cell- Muscle cell-Sperm cell-

If a pea plant’s diploid cell has 14 chromosomes, how many does a pea plant sex cell have? If an alligators haploid cell has 16 chromosomes, how many does a diploid cell have? Frog Haploid:8 Diploid: Kangaroo Haploid: Diploid: 12

Get with a partner and using your book and notes, draw/label the steps of meiosis I and II.

Meiosis Starts

Draw this graphic organizer on a sheet of paper.Write the number of Chromosomes each cell has in each circle.Draw a vertical line showing where the first division happens.Draw a vertical line showing where the

second division happens…

46 92

46

46

23

23

23

23

Meiosis I

Meiosis II

Comparing Mitosis and Meiosis

Mitosis: produces 2 genetically identical diploid cells.

Meiosis: produces 4 genetically different haploid cells; also different from each other