Download Section11.4Meiosis

Section 11.4
Meiosis
I.
Genes, Chromosomes, and Numbers
*organisms have tens of thousands of genes that
determine individual traits.
*genes are located on chromosomes; one chromosome
can have as many as 1000 genes.
A. Diploid and Haploid Cells
1. Diploid cells - a cell that contains 2 of each
kind of chromosome (2n).
a) in body cells of plants and animals, the
chromosomes are usually found in pairs.
ex) humans = 46 chromosomes
(23 pairs)
b) one chromosome in each pair
comes
from each parent
ex) one chromosome comes from
male (dad), one comes from
female (mom).
*this supports Mendel’s conclusion that organisms have two
(2) factors or alleles for each trait.
2. Haploid Cells - a cell with one of each kind of
chromosome; contains the haploid or “half” the number “n.”
a) gametes (sex cells) are haploid cells
b) egg (n) + sperm (n) = zygote (2n)
*this supports Mendel’s conclusion that each parent gives
one factor, or allele, for each trait to the offspring.
Chromosome Numbers of Some Common Organisms
Organism
Body Cell (2n)
Gamete (n)
Fruit Fly
8
4
Garden Pea
14
7
Corn
20
10
Tomato
24
12
Leopard Frog
26
13
Apple
34
17
Human
46
23
Chimpanzee
48
24
Dog
78
39
1260
630
Adder’s Tongue
Fern
B. Homologous Chromosomes
1. Homologous chromosomes - paired chromosomes with
genes for the same trait that are arranged in the same
order.
2. Gene order - on homologous chromosomes; genes are
arranged in the same order, but the alleles on one gene of
a chromosome may not be identical to the alleles on the
same gene of the other chromosome.
II. The Importance of Meiosis and Its Phases
A. Comparing Mitosis with Meiosis:
1. Mitosis - new cells have exactly the same number of
kinds of chromosomes as the original cell (2n --> 2n).
2. Meiosis - a cell division which produces gametes, or
sex cells, which contain only half the number of
chromosomes as a body cell (2n --> n).
*meiosis occurs in the sex cells to make egg and sperm.
Mitosis
Meiosis
46 (2n)
46 (2n)
46 (2n)
46 (2n)
23 (n)
23 (n)
B. Why is Meiosis so Important?
*if mitosis was the only method for cells to divide, we would
see a constant increase in the number of chromosomes with
each reproduction.
ex) mom x dad
46
46
mom x dad
46
46
child
92
child
92
184
*each offspring formed by fertilization would have 2 times
the number of chromosomes as each of the parents.
2. Meiosis keeps the number of chromosomes constant
from generation to generation.
mom x dad
46
46
mom x dad
46
46
eggs(23) sperm(23)
eggs(23) sperm(23)
child
23 + 23 = 46
child
23 + 23 = 46
child
46
C. Meiosis and the production of Sex Cells
1. Two (2) Phases of Meiosis:
a) Meiosis I - begins with one diploid (2n) cell
b) Meiosis II - ends with four haploid (n) cells
2. End Result:
at the end of Meiosis II there are four (4) haploid cells;
these cells are called sex cells or gametes.
a) sperm - male gamete (n)
b) egg - female gamete (n)
3. Zygote - the result of fertilization; the joining of an egg
and a sperm.
n + n =
2n
4. Sexual reproduction - the pattern of reproduction that
involves the production and subsequent fusion of haploid
sex cells.
III. The Phases of Meiosis
*interphase - the cell replicates its chromosomes; each cell
consists of two identical sister chromatids held together by a
centromere.
Chromosome
Sister Chromatids
Interphase & Chromosomal duplication:
A. Meiosis I
1. Prophase I
a) chromosomes coil up and spindles form
b) duplicated homologous chromosomes pair up and
form a tetrad.
*tetrad - consists of two homologous
chromosomes; each is made up of two sister
chromatids.
c) crossing over - may occur during prophase I, this
happens when non sister chromatids exchange genetic
material when partnered up.
*in prophase I of human
meiosis there are an
average of 2-3 crossovers
per pair.
2. Metaphase I
a) centromeres are attached to spindle fibers
b) spindle fibers pull tetrads into the middle of the cell
Tetrads are divided by the equatorial plate of the cell,
remember that in mitosis sister chromatids are lined
up and divided by the equatorial plate.
Metaphase I
Meiosis (2n)
Metaphase
Mitosis (2n)
3. Anaphase I
a) Anaphase I begins as the homologous chromosomes
(tetrads) are separated. Each chromosome, with its sister
connected, gets pulled toward opposite ends of the cell.
Anaphase I
meiosis (2n)
Anaphase
mitosis (2n)
b) Anaphase I ensures that each new cell will receive only
one chromosome from each tetrad or homologous pair.
4. Telophase I
*events occur in the reverse order of prophase I
a) Spindle fibers are broken down as chromosomes reach
their respective sides of the cell.
b) Chromosomes begin to uncoil.
c) Cytoplasm divides.
*at the end of telophase, each new cell has only half the
genetic info as the original cell because it has only one
chromosomes from each pair; another division is still
necessary to separate the sisters from each other,
Telophase:
Telophase I
meiosis (n)
Telophase
mitosis (2n)
Meiosis I Summary:
B. Meiosis II
*interphase period - very short; no replication takes place.
1. Prophase II
a) spindle fibers begin to
form in each of the two new
cells.
b) spindles attach to the
chromosomes at the
centromere.
2. Metaphase II
a) chromosomes composed of both sister chromatids
line up in the middle of the cell at the equatorial plate.
They can no longer pair with their partners because
their partners are in separate cells after meiosis I.
b) the plate now divides
sister chromatids, splitting
them in half.
*similar to metaphase of
mitosis
3. Anaphase II
a) centromere of each chromosome splits; now sister
chromatids are separated and free to move.
b) sister chromatids are pulled
to opposite sides of the
cell.
4. Telophase II
a) spindle fibers break down.
b) nuclei reform.
c) cytoplasm divides,
resulting in 4 haploid
(n) daughter cells.
Meiosis II
*steps of meiosis II are identical to mitosis, just happens with
only half the information.
Meiosis I & II
Can you label the phases?
IV. Meiosis Provides for Genetic Variation
*cells formed by mitosis are identical to each other and to
the original cell.
*cells formed by meiosis, however, may have chromosomes
which have been “shuffled,” causing slight variations in the
genetic information.
A. Genetic Recombination
*How many different kinds of sperm or eggs can a
human produce?
Each human cell has 23 pairs of chromosomes; and each
chromosome can line up at the equatorial plate in 2 ways,
There are 23 pairs of chromosomes in humans, and each
pair can line up in 2 ways, therefore:
2 = number of ways chromosomes can line up
23 = the number of pairs in humans
223 = 8,388,608 !!!!
This is the number of different types of sperm or
eggs that could possibly be produced!!!
*Any sperm can fertilize any egg:
223
8,388,608 x
(egg)
x
223
8,388,608 = 70,368,744,000,000
(sperm)
= 70 trillion
1. Gene combinations in the gametes vary depending on
how each pair of homologous chromosomes line up during
metaphase I - which is random.
2. These numbers increase greatly as the number of
chromosomes in the species increase.
3. Crossing over can lead to additional variations.
4. Genetic Recombination - the reassortment of
chromosomes and the genetic information they carry;
results from either crossing over, independent assortment,
or segregation.
*raw materials of evolution
B. Mistakes during Meiosis
*sometimes accidents happen in meiosis
*nondisjunction - the failure of homologous chromosomes
(partners) to separate properly during anaphase I, or the
failure of sisters to separate during anaphase II.
1.
Trisomy - occurs when a gamete with an extra
chromosome is fertilized by a normal gamete.
The result is a zygote with an extra chromosome (2n + 1)
ex) humans --> extra number 21 chromosome (three
number 21s are present, 47 total chromosomes, results in
Down Syndrome.
Pictures Of Down Syndrome:
2. Monosomy - occurs when a gamete with a missing
chromosome fuses with a normal gamete during fertilization.
*The result is a zygote that is missing a chromosome,
(2n - 1)
*most zygotes with monosomy will miscarry
ex) humans --> one condition that is not lethal is Turner
Syndrome, this happens when females only receive
one X chromosome instead of two.
Turner Syndrome:
3. Triploidy - a condition that results when a gamete with an
entire extra set of chromosomes is fertilized by a normal
haploid gamete. Thus one gamete is haploid, the other is
diploid or 2n.
*this results in an offspring that has three of each
chromosome, or is 3n:
*2n
+
n=
3n