MEIOSIS (making sperm and eggs…) We ’ re NOT talking about making babies, we ’ re talking about making the CELLS that make babies – eggs and sperm! Eggs.

MEIOSIS(making sperm and eggs…)

We’re NOT talking about making babies, we’re talking about making the CELLS that make babies – eggs and sperm!

Eggs and sperm are GAMETES.

• Gamete = sex cells (egg and sperm)• Each has 1 set of 23 chromosomes• When they come together, the resulting cell has 46

chromosomes

• Somatic cell = body cells (everything except egg and sperm)

23

23

Each of your body cells has 2 sets of chromosomes – one from mom, one from dad.

Any cell that has 2 complete sets of chromosomes is said to be DIPLOID (2n)

Cells with 1 set of chromosomes ( = egg/sperm, gametes) are said to be HAPLOID (n)

When 2 cells come together in fertilization to make a zygote, each can only have 1 set of chromosomes. That way, the zygote will have 46 chromosomes. (23 + 23 = 46)

(Instead of 46 +46 = 92!)

CHROMOSOME NUMBER

In humans, 2n = 46

This means we’re referring to a normal body cell that has a total of 46 chromosomes, but

with 2 sets - 23 from mom, 23 from dad)

In humans, n = 23

This means we’re talking about a cell that has only half the normal chromosome number (n) and that is only one set. This cell must be a

gamete.

What type of cells produce eggs and sperm?

Ovarian and testicular cells!

Why do meiosis?Meiosis is “reduction division”. It

reduces the chromosome number in the gametes by one half.

From diploid (2n) to haploid (n).

Why do ovary and testes cells go through meiosis?

2n 2n

egg (n) sperm (n)

fertilization

zygote

Oogonial cell Spermatogonial cell

MEIOSIS MEIOSIS

MITOSIS

You!

Meiosis is…

• The process of nuclear division that reduces the number of chromosomes by half– Start with somatic cell– End with gamete

• Making the cells that make babies! (egg and sperm)

Chromosome number questions

1. If a horse retina cell has 36 chromosomes, how many does a horse sperm cell have?

2. If the n = 4 for fruit flies, how many chromosomes does a wing cell have?

3. If the 2n number of a pine tree is 86, how many chromosomes would be found in a pine ovum?

Homologous pairs

In each body cell you have 23 homologous pairs. Two chromosomes are homologous if:

- one is from mom, one is from dad

- they are the same size and have the same genes

1. They code for the same genes

2. They are the same length

2. They have the same banding pattern

3. The position of the centromere

Homologous Chromosome Similarities:

Differences: One is from your mom, one from your dad

QUIZ TIME…1. In streptomycin fungus n = 11. What is the diploid number

for this species?

2. If a horse egg has 98 chromosomes how many does a horse sperm cell have?

3. If n = 16 for goldfish how many chromosome in a fin cell?

4. If 2n = 108 for black spruce trees then what is the haploid number?

2n = 14 for this species. Fill in the chromosome numbers for each cell.

A

BC

What is process A, B, and C?

Karyotype = picture of all chromosomes in the nucleus of 1 cell

Used to determine:

1. If there is an abnormality in number or structure of the chromosomes

(e.g. Down’s syndrome)

2. Gender

KARYOTYPE

1. Photograph a cell in metaphase.

2. Cut out the chromosomes from the picture.

3. Paste homologous pairs together according to size, banding pattern and centromere.

4. Lay pairs out largest to smallest, sex chromosomes at end.

Karyotypes can be obtained in several ways

In a developing fetus:

CVS (chorionic villus sampling), amniocentesis, blood tests

In children and adults:

Karyotypes can be obtained from any growing cells.

(Can a karyotype tell if there are any GENE mutations?)

Products of MITOSIS:

2 identical diploid

(2n) cells

Products of MEIOSIS:

4 unique haploid (n) cells

2n 2n 2n

2n n n n n

MEIOSIS is two mitotic divisions in a row, (meiosis I and II) except you will see the

homologous chromosomes coming together.

In prophase I the double homologous chromosomes come together to form a tetrad.

Crossing over occurs during Prophase I to increase the variability in gametes.

There is no DNA replication in the 2nd interphase.

MEIOSIS IPROPHASE I - TETRADS APPEAR

The homologous chromosomes come together as homolgous pairs (or tetrads)

Crossing over increases genetic variabiity – and

that’s a good thing!

Metaphase I: Tetrads (double homologous pairs) line up and lock on to spindle fibers.

 Anaphase I: Spindle fibers pull homologous chromosomes apart (separate mom’s from dad’s). Sister chromatids stay together.

Reduction Division occurs at end of Meiosis I because it is defined as the point at which mom’s info is separated from dad’s info. This occurs by the end of Meiosis I.

Telophase I: nuclear membranes form around the two new nuclei.

Cytokinesis I: cell splits in two.

Interphase between the two divisions:

NO replication - so nothing changes with the chromosomes between telophase I and

prophase II

MEIOSIS II

Prophase II: When chromosomes show up you will see sister chromatids. Each cell has only mom’s OR dad’s chromosomes.

Metaphase II: Sister chromatids line up on equator

Anaphase II: Sister chromatids are pulled apart.

Telophase II: Nuclear membranes form.

Cytokinesis: cytoplasm divides

Total product is 4 cells, each with half the normal chromosome number, or 4 haploid (n)

gametes.

Uneven cytokinesis in females: In oogenesis, what is produced is 3 non-functional polar

bodies ( who die off) and 1 big functional gamete (the egg)

First division: results in one big primary oocyte and a little polar body.

Second division: One secondary oocyte (the egg) and 3 polar bodies that are unuseable.

OOGENESIS

Spermatogenesis

Equal divisions producing four useable sperm

COMPARISONS:

1. Compare Mitosis and Meiosis

2. Compare spermatogenesis and oogenesis

3. Compare Meiosis I and Meiosis II

4. Compare haploid and diploid cells.

5. Compare a somatic (body) cell to a gamete.

Compared to the number of chromosomes contained in a body cell of a parent, how many chromosomes would normally be contained in a gamete?

A. the same number

B. twice as many

C. one-fourth as many

D. half as many

During mitosis, the chromosomes

A. are located at the cell equator during prophase

B. are located at the cell equator during telophase

C. move toward the poles of the cell during anaphase

D. move toward the poles of the cell during metaphase

Meiosis Square Dance

http://www.youtube.com/watch?v=iCL6d0OwKt8

http://www.youtube.com/watch?v=CCqAri4wtjQ

Non-disjunction

Failure of homologous chromosomes to separate, resulting in gametes with the wrong number of chromosomes

Conditions caused bynon-disjunction

Downs syndrome = Trisomy 21 - 3 copies of chromosomes 21 equalling a total of 47 chromosomes. = 2n + 1(chance of occurring in oogenesis increases with maternal age)

Turners syndrome = Monosomy X - has only one X chromosomes totalling only 45 chromosomes in her body cells. (monosomy X) = 2n-1

Klinefelter’s syndrome = XXY - male which has an extra X chromosome = 2n + 1

Down’s syndrome – trisomy 21

A simple check for the presence of a bone in the nose could more accurately test unborn babies for Down's syndrome, scientists say. Researchers say combining the nose test with existing screening methods (amniocentesis, CVS) could lead to a five-fold reduction in the number of miscarriages linked to an invasive procedure used to confirm Down's syndrome.

Klinefelter’s syndrome XXY

Normal boy who develops some female secondary sex character-istics at puberty.

-slightly lower IQ, infertile, delayed motor, speech, maturation

-treated with testosterone

KARYOTYPE

-used to diagnose chromosomal number disorders

- First 22 pairs are autosomes

- Last pair are sex chromosomes

XX = female

XY = male

How to make a KARYOTYPE

1. Photograph a cell in metaphase.

2. Cut out the chromosomes from the picture.

3. Paste homologous pairs together according to size, banding pattern and centromere position.

4. Lay pairs out largest to smallest, sex chromosomes at end.