meiosis (gametogenesis). 6.1 chromosomes and meiosis teks 6a, 6g key concept gametes have half the...

Meiosis (Gametogenesis)

6.1 Chromosomes and Meiosis TEKS 6A, 6G

KEY CONCEPT Gametes have half the number of chromosomes that body cells have.


You have body cells and gametes.

• Body cells are also called somatic cells.• Germ cells develop into gametes.

– Germ cells are located in the ovaries and testes.– Gametes are sex cells: egg and sperm.– Gametes have DNA that can be passed to offspring.

body cells sex cells (sperm) sex cells (egg)


• Your body cells have 23 pairs of chromosomes.– Homologous pairs of

chromosomes have the same structure.

– For each homologous pair, one chromosome comes from each parent.

• Chromosome pairs 1-22 are autosomes.

• Sex chromosomes, X and Y, determine gender in mammals.

Your cells have autosomes and sex chromosomes.

Chromosomes

• You have 23 different pairs of chromosomes, for a total of 46 chromosomes.

• One chromosome in each pair came from your mother and one from your father.

Each chromosome in a pair is said to be homologous, meaning that the chromosome from the father has a corresponding chromosome from the mother.


Body cells are diploid; gametes are haploid.

• Fertilization between egg and sperm occurs in sexual reproduction.

• Diploid (2n) cells have two copies of every chromosome.– Body cells are diploid.– Half the chromosomes come from each parent.


• Haploid (n) cells have one copy of every chromosome.

– Gametes are haploid.– Gametes have 22 autosomes and 1 sex chromosome.

• Cells that contain both sets of homologous chromosomes are called diploid (2N).

• All of your cells except the sex cells (sperm and eggs; also called gametes) are diploid (2N).

• Gametes are haploid (N), meaning they contain only one copy of each chromosome.

• When one sperm and one egg combine their DNA, there are two versions of each chromosome.

haploid haploid

diploid

We use “N” to represent the haploid number of chromosomes and “2N” to represent the diploid number of chromosomes.

1. For humans, the haploid number is 23. 2. We write this as N = 23. 3. The diploid number is 46, which we write as 2N = 46.

2.In fruit flies, N = 4 and 2N = 8.

Practice with haploid and diploid:

In fruit flies, N (______) = 4 and 2N (______) = ___8.

haploid

diploid

If elephants have a diploid number of 56, what is their haploid number?

28


• Meiosis makes haploid cells from diploid cells.

– Meiosis occurs in sex cells. – Meiosis produces gametes.

6.2 Process of Meiosis TEKS 6G

The student is expected to:

6G recognize the

significance of meiosis to sexual reproduction

Meiosis

Meiosis is the process that divides one diploid (2N) cell to form four haploid (N) cells.

This process is a reductional division because the number of chromosomes per cell are cut in half.

Meiosis is how gametes are formed.


Cells go through two rounds of division in meiosis.

• Meiosis reduces chromosome number and creates genetic diversity.


• Meiosis I and meiosis II each have four phases, similar to those in mitosis.

homologous chromosomes

sisterchromatids

sisterchromatids

– Pairs of homologous chromosomes separate inmeiosis I.

– Homologous chromosomes are similar but not identical.– Sister chromatids divide in meiosis II.– Sister chromatids are copies of the same chromosome.

There are two divisions that occur in meiosis: Meiosis I and Meiosis II. 1.Before meiosis I begins, cells go through

Interphase I. 2.This involves DNA replication, forming a

duplicate copy of each chromosome.3.Each chromosome is made of two sister

chromatids.

cell

centriolesNucleus with duplicated DNA that is not condensed.


• Meiosis I occurs after DNA has been replicated.

• Meiosis I divides homologous chromosomes in four phases.

Meiosis I is similar to mitosis.a. Prophase I:

i. Centrioles are visible on opposite sides of the nucleus

ii. Nucleolus disappearsiii. Nuclear envelope breaks down

centrioles

spindle

DNA condensed as chromosomes

iv. Homologous chromosomes pair1. When a pair of chromosomes aligns a tetrad

is formed.2. When chromosomes form a tetrad, they

exchange portions of their chromatids in a process called crossing-over.

3. Crossing-over produces new combinations of DNA.

tetrad

b. Metaphase I:i. Spindle fibers attach to the chromosomes at

the centromere

centrioles

spindle

chromosomes (DNA)

centromere

c. Anaphase I:i. Spindle fibers pull the homologous chromosomes toward opposite ends of the cell.ii. Chromosomes move and separate into two groups near the spindleiii. Anaphase ends when the chromosomes stop moving

centrioles

spindle

chromosomes (DNA)

d. Telophase Ii. Nuclear membranes form around chromosomesii. Daughter nuclei form

e. Cytokinesis i. Cytoplasm divides

centrioles

chromosomes (DNA)

Nucleus

Nucleus

Meiosis I produces two diploid (2N) daughter cells. Each chromosome is made of two sister chromatids.


• Meiosis II divides sister chromatids in four phases.

• DNA is not replicated between meiosis I andmeiosis II.

Meiosis II

After Meiosis I, cells enter Meiosis II.

No DNA replication occurs between Meiosis I and Meiosis II.

Meiosis II separates the sister chromatids.

a. Prophase II:i. Centrioles are visible on opposite sides of the nucleus

ii. Nucleolus disappears

iii. Nuclear envelope breaks down

centrioles

chromosome

b. Metaphase II: i. Chromosomes align like

they do in mitosis

ii. Chromosomes are attached to the spindle at the centromere.

centrioles

chromosome

spindle

c. Anaphase II:

Sister chromatids separate and move towards opposite ends of the cell.

centrioles

chromosome

spindle

d. Telophase II: i. Nuclear membranes

form around chromosomes

ii. Daughter nuclei form

e. Cytokinesis: i. Division of the

cytoplasm

f. Meiosis II - produces four haploid (N) daughter cells

nucleus

chromosome

centrioles

Gamete formation

• In male animals, the haploid gametes are called sperm.

• In female animals, the haploid gametes are called eggs.


• Meiosis differs from mitosis in significant ways.

– Meiosis has two cell divisions while mitosis has one.– In mitosis, homologous chromosomes never pair up.– Meiosis results in haploid cells; mitosis results in diploid

cells.

• When the male and female gametes join, this is called fertilization and a zygote is formed.


Haploid cells develop into mature gametes.

• Gametogenesis is the production of gametes.

• Gametogenesis differs between females and males.– Sperm become streamlined and

motile.– Sperm primarily contribute DNA

to an embryo.– Eggs contribute DNA, cytoplasm,

and organelles to an embryo.– During meiosis, the egg gets

most of the contents; the other cells form polar bodies.

Comparing mitosis and meiosis

• Mitosis produces two genetically identical diploid (2N) somatic (body) cells. Mitosis allows an organism’s body to grow and to replace cells. In organisms that reproduce asexually, new organisms are produced by mitosis.

• Meiosis (or gametogenesis) produces four genetically different haploid (N) cells. Meiosis produces gametes for use in sexual reproduction.

meiosis (gametogenesis). 6.1 chromosomes and meiosis teks 6a, 6g key concept gametes have half the...

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