starter in terms of chromosome number what must happen to human reproductive cells so reproduction...

Starter

• In terms of chromosome number what must happen to human reproductive cells so reproduction can take place? Why?

Meiosis

What is it?

• A type of cell division that creates gametes for sexual reproduction

– Gametes:• Egg (female) Sperm (male)

Haploid vs. Diploid

• Gametes are haploid (1N)– Contains one set of chromosomes

• Gametes come together to form a diploid (2N) cell during fertilization.– Zygote– Contains two complete sets of

chromosomes • One was from the father, one was from the

mother

Humans

• Each one of our cells has 46 chromosomes– A set of 23 from Mom, a set of 23 from

Dad

• Our haploid number is…– 23

• Our diploid number is…– 46

Homologous Pairs

• In a diploid cell, the matching pairs of chromosomes are called Homologous Pairs– For every chromosome from an egg,

there is a matching one from the sperm• Autosomes

• Which pair does not fit this rule?

The 23rd pair (in humans)– The sex pair!

Answers the question…female or male?• XX (female)• XY (male)

Sexual Reproduction

• The process by which the chromosomes of two parents combine to produce offspring– Fertilization – egg cell

and sperm cell join to produce a new individual

– Zygote – a fertilized egg• The SINGLE CELL

produced by fertilization

Why is Meiosis important?

• Two Reasons1. The production of reproductive cells containing only one set of chromosomes is vital. Why?

- For making sure that species has the same number of chromosomes from generation to generation

2. Genetic variation made possible

- Meiosis increases the amount of different characteristics you see among individuals that make up the population of a species.

- 4 Causes of Genetic Variation

Crossing Over

1. The exchange of genes (pieces of DNA) between pairs of homologous chromosomes– When done, each

chromosome in the homologous pair has a different combination of genes compared to before Meiosis began

Random Fertilization

2. Many of sperm to one egg– During reproduction a man emits

approximately 120 million sperm.• Each of these sperm contains unique

set of chromosomes

3. Random Mating

Independent Assortment

4. The homologous pairs line up independently of one another during Metaphase I.

What does this explain?• I may look similar to my family

members but none of us is exactly the same.

Except for…

• Identical twins!

• What is meiosis?• Why is meiosis so important?• What is a gamete?• Define Diploid.• What is our human diploid number?– Fruit flies have 8 chromosomes 2N = ?– Goldfish have 94 chromosomes 2N = ?

• Define Haploid– Humans 1N = ?– Fruit Flies 1N =?– Goldfish 1N =?

• Define a Homologous Pair.• What is a zygote? Diploid or Haploid?• Explain Crossing over.

The Phases

• Meiosis I – Prophase I– Metaphase I– Anaphase I– Telophase I/Cytokinesis

• Meiosis II– Prophase II– Metaphase II– Anaphase II– Telophase II/Cytokinesis

Meiosis I

• Prophase I– Chromatin condenses– Chromosomes of homologous pairs match

up and become tangled together. (4 chromatids in each chromosome pair). Crossing over occurs

– Nucleolus and nuclear membrane disappear

– Centrioles move to opposite sides of the nucleus

– Spindle fibers attach to centromeres

Draw Prophase I

Meiosis I

• Metaphase I– Centrioles move to opposite sides of the

cell– The tangled pairs of chromosomes

separate and the homologous pairs line up next to each other in the middle of the cell

Draw Metaphase I

Meiosis I

• Anaphase I– Homologous pairs of chromosomes separate from

each other– Spindle fibers shorten pulling one member from

each pair to opposite sides of the cell

Draw Anaphase I

Meiosis I

• Telophase I/Cytokinesis– Chromosomes uncoil back to chromatin– Nuclear membrane may or may not

form around both bundles of chromatin at opposite ends of the cell

– Spindle fibers disappear– Cytoplasm divides creating two,

identical daughter cells that contain one half the number of chromosome pairs as the parent cell

Draw Telophase I/Cytokinesis

• Each of the two daughter cells goes back for round 2!

Meiosis II

• Prophase II– Chromatin condenses– Nuclear membrane disappears– Centrioles move to opposite sides of the

nucleus– Spindle fibers attach to centromeres

Draw Prophase II

Meiosis II

• Metaphase II– Centrioles move to opposite sides of the

cell– Chromosomes line up in the middle of

the cell

Draw Metaphase II

Meiosis II

• Anaphase II– Centromeres that join the sister

chromatids split– Sister chromatids becoming separate

chromosomes– Spindle fibers shorten

Draw Anaphase II

Meiosis II

• Telophase II/Cytokinesis– Chromosomes uncoil becoming chromatin– Nuclear membrane forms around both bundles

of chromatin at opposite ends of the cell– Nucleolus reappears in each new daughter cell– Spindle fibers disappear– Cytoplasm divides creating 2 haploid gametes

from each daughter cell created in Meiosis I. Each gamete has half the number of chromosomes as its parent cell.

Draw Telophase II/Cytokinesis

Now you tell me…

What makes each process important?

Compare and contrast Mitosis and Meiosis using the Venn Diagram.

Let’s Compare

• Mitosis– Results in 2 daughter

cells when complete– Chromosomes identical

to parent cell– Offspring identical to

parent– Same number of

chromosomes in daughter cells as in the parent cells

• Meiosis– Results in 4 daughter

cells when complete– Chromosomes may be

different from parent cell– Offspring different from

parent– Half of the number of

chromosomes in the daughter cells as in the parent cells