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
2 Types of cells in body:
Somatic cells: body cells
Gametes: sex cells – sperm & eggs
- Asexual reproduction involves body cells:
New organisms are produced from one
parent (they are identical)
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
Tetrad: each pair of homologous
chromosomes; 4 chromatids
-Orient themselves so corresponding
genes are adjacent to each other
Crossing-over: portions
of chromatids break off
& reattach to adjacent
chromatids on
homologous
chromosome
Crossing-over
- Results in genetic recombination:
New mixture of genetic material
-source of variety in populations
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
Telophase II &
Cytokinesis II
Metaphase II
Anaphase II
Gamete Formation
Males: 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 contains
Bone cellLiver cellEgg cellMuscle cellSperm 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.
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
23
Meiosis Starts
46
23
92
23
46
Meiosis I
23
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