meiosis and chromosome assortment · chromosomes in human cells •somatic cells include all cells...
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Meiosis and Chromosome Assortment
Introduction to Biology
Chromosomes in Human Cells
• Somatic cells include all cells in the human
body except sperm and eggs. • Gametes are human sperm and egg cells.
• Each human somatic cell has 23 pairs of
chromosomes, 46 total. • Each pair of chromosomes are called homologous
chromosomes.
• Each homologous chromosome carries a
copy of the same genes, either from the
father or mother.
LE 13-3
5 µm Pair of homologous
chromosomes
Sister
chromatids
Centromere
• This is called a
karyotype. All
23 pairs of
homologous
chromosomes
are lined up.
LE 13-4
Key
Maternal set of
chromosomes (n = 3) 2n = 6
Paternal set of
chromosomes (n = 3)
Two sister chromatids
of one replicated
chromosomes
Two nonsister
chromatids in
a homologous pair
Pair of homologous
chromosomes
(one from each set)
Centromere
• The sex chromosomes are called X
and Y o Human females have two X chromosomes.
o Human males have one X and one Y
chromosome
• The 22 pairs of chromosomes that do
not determine sex are called
autosomes.
Inheritance of Genes • A gene is a unit of heredity that carries the
information for a specific trait or body
function. o A gene is made of a segment of DNA.
o Each gene is located on a specific chromosome.
o Everyone has two copies of each gene (one on each
homologous chromosome).
• A cell with a full pair of each chromosome is
called diploid. o Diploid is written shorthand as 2n.
o All somatic cells are diploid (46 chromosomes).
• A cell with only one of each homologous
chromosome is called haploid. o Haploid is written shorthand as n.
o All gametes are haploid and have 23 total chromosomes.
• Gametes are haploid cells, containing only
one set of chromosomes
• For humans, this means 23 total chromosomes
(no pairs) o This includes 22 autosomes and a single sex chromosome
o In an unfertilized egg (ovum), the sex chromosome is always X
o In a sperm cell, the sex chromosome may be either X or Y
Chromosomes and the Human Sex Cycle
• At sexual maturity, the ovaries and testes begin
producing sperm and eggs through meiosis. o Gametes are the only types of human cells produced by meiosis,
rather than mitosis
• Meiosis is a form of cell division that results in one
set of chromosomes in each gamete instead of
two. o The resulting daughter cells are haploid.
• When fertilization occurs, the haploid sperm and
haploid egg fuse together to form a diploid
embryo.
Interphase • At the end of interphase, each cell has grown
into its full size, produced a full set of
organelles, and duplicated its DNA. o The cell is diploid at this point.
• The nucleus contains 23 homologous chromosome pairs.
• Each chromosome is made of two sister chromatids
(copies).
• The cells begin to divide, and the chromosomes pair
up, forming a structure called a tetrad, which contains
four chromatids.
Prophase I
Prophase I
• As homologous chromosomes pair up and form tetrads,
they undergo a process called crossing-over.
• First, the chromatids of the homologous chromosomes
overlap each other.
• Then, the crossed sections of the chromatids are
exchanged.
• Crossing-over is important because it produces new
combinations of genes in the cell.
Metaphase I
• As prophase I ends, a spindle forms and attaches to
each tetrad.
• During metaphase I of meiosis, paired homologous
chromosomes line up across the center of the cell.
Anaphase I
• During anaphase I, spindle fibers pull each
homologous chromosome pair toward opposite
ends of the cell.
• When anaphase I is complete, the separated
chromosomes cluster at opposite ends of the cell.
Telophase I and Cytokinesis
• During telophase I, a nuclear membrane forms
around each cluster of chromosomes.
• Cytokinesis follows telophase I, forming two new cells.
Summary of Meiosis I • Two new haploid cells have been produced.
• Each haploid cell contains one chromosome out of
the original pair.
• Each chromosome still contains two sister
chromatids.
Prophase II
• As the cells enter prophase
II, their chromosomes—
each consisting of two
chromatids—become
visible.
• The chromosomes do not
pair to form tetrads,
because the homologous
pairs were already
separated during meiosis I.
Metaphase II
• During metaphase of meiosis II,
chromosomes line up in the center
of each cell.
Anaphase II
• As the cell enters anaphase, the
paired chromatids separate.
Telophase II and Cytokinesis
• The two daughter cells from Meiosis I divide, resulting
in four daughter cells, each with two chromatids.
• These four daughter cells now contain the haploid
number (N)—just two chromosomes each.
Summary of Meiosis II
• A total of four cells have been produced.
• Each cell is haploid and only contains one out of the
original pairs of homologous chromosomes.
• Each chromosome only contains a single chromatid.
A Comparison of Mitosis and Meiosis
• Mitosis produces cells that are genetically identical to
the parent cell.
• Meiosis reduces the number of chromosomes sets
from two (diploid) to one (haploid).
• Meiosis allows crossing over of chromosomes. o This produces cells that are genetically different from the parents and each
other.
• Three events are unique to meiosis, and all three occur in meiosis l: o Synapsis and crossing over in prophase I:
Homologous chromosomes physically connect and exchange genetic information
o At the metaphase plate, there are paired homologous chromosomes (tetrads), instead of individual replicated chromosomes
o At anaphase I, it is homologous chromosomes, instead of sister chromatids that separate and are carried to opposite poles of the cell
LE 13-9
Propase
Duplicated chromosome
(two sister chromatids)
Chromosome
replication
2n = 6
Parent cell
(before chromosome replication)
Chromosome
replication
MITOSIS MEIOSIS
Chiasma (site of
crossing over) MEIOSIS I
Prophase I
Tetrad formed by
synapsis of homologous
chromosomes
Tetrads
positioned at the
metaphase plate
Metaphase I Chromosomes
positioned at the
metaphase plate
Metaphase
Anaphase
Telophase Homologues
separate
during
anaphase I;
sister
chromatids
remain together
Sister chromatids
separate during
anaphase
Daughter
cells of
meiosis I
Haploid
n = 3
Anaphase I
Telophase I
MEIOSIS II
Daughter cells
of mitosis
2n 2n
n
Sister chromatids separate during anaphase II
n n n
Daughter cells of meiosis II
Mitosis Meiosis
DNA
replication
During interphase
During interphase
Divisions One Two
Synapsis and
crossing over
Do not occur Form tetrads in prophase I
Daughter
cells, genetic
composition
Two diploid, identical to parent cell
Four haploid, different from parent cell and each other
Role in animal
body
Produces cells for growth and tissue repair
Produces gametes
Genetic Variation Among Offspring
• The behavior of chromosomes during meiosis
and fertilization is responsible for most of the
variation that arises in each generation
• Three mechanisms contribute to genetic
variation:
o Independent assortment of chromosomes
o Crossing over
o Random fertilization
Independent Assortment of Chromosomes
• In independent assortment, each pair of
chromosomes sorts maternal and paternal
homologous chromosomes into daughter cells
independently of the other pairs.
• Example: o One human sperm cell could contain 15 chromosomes from
his father, and 8 from his mother
o Another contains 20 from the mother, 3 from the father.
LE 13-10
Key
Maternal set of
chromosomes
Paternal set of
chromosomes
Possibility 1 Possibility 2
Combination 2 Combination 1 Combination 3 Combination 4
Daughter
cells
Metaphase II
Two equally probable arrangements of chromosomes at
metaphase I
Independent Assortment of Chromosomes
• The number of combinations possible when chromosomes assort independently into gametes is calculated by 2n, where n is the haploid number
• For humans (n = 23):
o 223 = 8,388,608 possible combinations!
Crossing Over • Crossing over produces new chromosomes with a
mixture of genes from each parent.
• Instead of a chromosome that is 100% from the person’s father or mother, it might now be 95% from the father, 5% from the mother.
LE 13-11 Prophase I
of meiosis
Tetrad
Nonsister
chromatids
Chiasma,
site of
crossing
over
Recombinant
chromosomes
Metaphase I
Metaphase II
Daughter
cells
Random Fertilization • Random fertilization adds to genetic variation
because any sperm can fuse with any egg.
Genetic Diversity • How many possible combinations of genes are there
from two parents?
• Independent assortment:
223 = 8,388,608 combinations of chromosomes in each
sperm or egg cell.
• Random assortment: 8.4 million possible sperm combinations
+ 8.4 million possible egg combinations
= 16.8 trillion possible embryos
Genetic Diversity • How many possible combinations of genes are there
from two parents?
• Crossing over
o Average of 1,000 genes in each chromosome
o At the most, about half of the chromosome can cross over to
its homologous partner.
o This results in 3.3 novemquardragintillion (1 followed by 150
zeros) gene combinations for each chromosome pair crossing
over.
Genetic Diversity • How many possible combinations of genes are there
from two parents?
• Total
3.3 novemquardragintillion possible chromosome
combinations
x 23 chromosomes
x 16.8 trillion possible sperm-egg combinations
=1.3 quinquinquagintillion (1 followed by 168 zeros)
possible different genetic combinations for two
people.