Download Meiosis (Gametogenesis)

Meiosis (Gametogenesis)
6.1
Chromosomes and Meiosis
TEKS 6A, 6G
KEY CONCEPT
Gametes have half the number of chromosomes
that body cells have.
6.1
Chromosomes and Meiosis
TEKS 6A, 6G
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)
6.1
Chromosomes and Meiosis
TEKS 6A, 6G
Your cells have autosomes and sex chromosomes.
• 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.
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.
6.1
Chromosomes and Meiosis
TEKS 6A, 6G
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.
6.1
Chromosomes and Meiosis
TEKS 6A, 6G
• 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:
haploid = 4 and
In fruit flies, N (______)
diploid = ___8.
2N (______)
If elephants have a diploid number of
56, what is their haploid number?
28
6.1
Chromosomes and Meiosis
• Meiosis makes haploid cells from diploid cells.
– Meiosis occurs in sex cells.
– Meiosis produces gametes.
TEKS 6A, 6G
6.2
Process of Meiosis
The student is expected to:
6G recognize the
significance of meiosis to
sexual reproduction
TEKS 6G
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.
6.2
Process of Meiosis
Cells go through two rounds of division in meiosis.
• Meiosis reduces chromosome number and creates
genetic diversity.
TEKS 6G
6.2
Process of Meiosis
TEKS 6G
• Meiosis I and meiosis II each have four phases, similar to
those in mitosis.
– Pairs of homologous chromosomes separate in
meiosis I.
– Homologous chromosomes are similar but not identical.
– Sister chromatids divide in meiosis II.
– Sister chromatids are copies of the same chromosome.
homologous chromosomes
sister
chromatids
sister
chromatids
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.
centrioles
Nucleus with
duplicated DNA
that is not
condensed.
cell
6.2
Process of Meiosis
TEKS 6G
• 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.
ii.
iii.
Centrioles are visible on opposite
sides of the nucleus
Nucleolus disappears
Nuclear envelope breaks down
spindle
centrioles
DNA condensed as
chromosomes
iv.
Homologous chromosomes pair
1.
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
spindle
centromere
centrioles
chromosomes (DNA)
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 spindle
iii. Anaphase ends when the chromosomes stop moving
spindle
centrioles
chromosomes
(DNA)
d.
Telophase I
i.
ii.
e.
Nuclear membranes form around chromosomes
Daughter nuclei form
Cytokinesis
i.
Cytoplasm divides
Nucleus
centrioles
chromosomes
(DNA)
Nucleus
Meiosis I produces two diploid (2N)
daughter cells. Each chromosome is made
of two sister chromatids.
6.2
Process of Meiosis
• Meiosis II divides sister chromatids in four phases.
• DNA is not replicated between meiosis I and
meiosis II.
TEKS 6G
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
centrioles
ii. Nucleolus disappears
iii. Nuclear envelope
breaks down
chromosome
b. Metaphase II:
i. Chromosomes align like
they do in mitosis
ii. Chromosomes are
attached to the spindle
at the centromere.
centrioles
spindle
chromosome
c. Anaphase II:
Sister
chromatids
separate and
move towards
opposite ends
of the cell.
centrioles
spindle
chromosome
d. Telophase II:
i. Nuclear membranes
form around
chromosomes
ii. Daughter nuclei
form
e. Cytokinesis:
centrioles
nucleus
i. Division of the
cytoplasm
f. Meiosis II - produces
four haploid (N)
daughter cells
chromosome
Gamete formation
•
In male animals, the haploid gametes are
called sperm.
•
In female animals, the haploid gametes are
called eggs.
6.2
Process of Meiosis
TEKS 6G
• 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.
6.2
Process of Meiosis
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.
TEKS 6G
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.