How many chromosomes do we have in body cells?
How many chromosomes do we have in sex cells?
If your parents have 46 chromosomes in their
cells, do you have 92 chromosomes in your cells?
Why?
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Hereditary information is contained
in genes, located in the
chromosomes of each cell.
An inherited trait of an individual
can be determined by one or by
many genes, and a single gene can
influence more than one trait.
A human cell contains many
thousands of different genes in its
nucleus.
Each organism must inherit a
single copy of every gene from
each “parent”
When an organism produces its
own gametes, those two sets of
genes must be separated from
each other so that each gamete
contains just one set of genes.
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Chromosomes come from both the male parent
and female parent
Homologous: each of the chromosomes coming
from one parent have corresponding
chromosomes from the other parent
Diploid: a cell that contains both sets of
homologous chromosomes = “two sets” (2N)
– Diploid cells contain two complete sets of chromosomes
and two complete sets of genes
– Human somatic (body) cells have 46 chromosomes or 23
homologous pairs
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Haploid: contain only one set of chromosomes
and one set of genes = “one set” (N)
Gametes of sexually reproducing organisms are
haploid containing one complete sets of
chromosomes and one complete sets of genes
Human gametes (sex cells) have
23 chromosomes
Sperm (23) + Egg (23) = Zygote (46)
How are haploid gamete
produced from diploid cells?
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Gametogenesis is the process in which
GAMETES are produced.
It involves MEIOTIC cell division and cell
maturation.
This process occurs in specialized organs
called GONADS.
◦ Male gonads are called TESTES perform
spermatogenesis
◦ Female gonads are called OVARIES perform
oogenesis
GAMETES
Zygote
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The process of
reduction division
in which the
number of
chromosomes per
cell is cut in half
through the
separation of
homologous
chromosomes in a
diploid cell
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Meiosis consists of
2 phases:
◦ Meiosis I
◦ Meiosis II
◦ To begin, you have
one diploid cell
◦ In the end, you have
four haploid cells
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Cells begin to divide in a way that looks similar to
mitosis
Each chromosome pairs with its corresponding
homologous chromosome to form a tetrad – there
are 4 chromatids in a tetrad
The homologous pairs may exchange portions of
their chromatids in a process called crossing-over
which results in the exchange of alleles between
homologous chromosomes and produces new
combinations of alleles
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Homologous chromosomes separate and two
new cells are formed
Each new cell has equal numbers of
chromatids but neither have the same
complete set of chromosomes or alleles as
each other or the original diploid cell
Figure 11-15 Meiosis
Section 11-4
Meiosis I
Interphase I
Prophase I
Metaphase I
Anaphase I
Cells undergo a round of
DNA replication, forming
duplicate Chromosomes.
Each chromosome pairs with its
corresponding homologous
chromosome to form a tetrad.
Spindle fibers attach to the
chromosomes.
The fibers pull the homologous
chromosomes toward the
opposite ends of the cell.
Go to
Section:
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The two cells enter a second meiotic division
Neither cell goes through chromosomal
replication, so each chromosome has 2
chromatids
Pairs of chromatids separate
Each daughter cell produced receives equal
amounts of chromatids
The four daughter cells contain haploid (N)
chromosomes
Figure 11-17 Meiosis II
Section 11-4
Meiosis II
Prophase II
Metaphase II
Meiosis I results in two haploid The chromosomes line up in a
(N) daughter cells, each with half similar way to the metaphase
the number of chromosomes as stage of mitosis.
the original.
Go to
Section:
Anaphase II
Telophase II
The sister chromatids separate
and move toward opposite ends
of the cell.
Meiosis II results in four haploid
(N) daughter cells.
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Male: sperm (4 equal sized gametes
produced)
Female: egg/polar bodies (1 egg cell
produced which contains most of the
cytoplasm, and 3 polar bodies which are
usually not involved in reproduction)
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Mitosis – produces 2 genetically identical
diploid cells
Meiosis – produces 4 genetically different
haploid cells
MITOSIS:
• THE PROCESS BY
WHICH A DIPLOID
CELL(2n) PRODUCES
TWO DIPLOID (2n)
CELLS
• INCLUDES ONE CELL
DIVISION
• ASEXUAL
REPRODUCTION
MEIOSIS:
• THE PROCESS BY
WHICH A DIPLOID
CELL (2n)
PRODUCES FOUR
HAPLOID (1n) CELLS
(GAMETES)
• INCLUDES TWO
CELL DIVISIONS
• SEXUAL
REPRODUCTION
Compare mitosis and meiosis on the graphic organizer with your seat partner…
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We know that genes on different
chromosomes are independently assorted…
But what about genes on the same
chromosome? Would they be inherited
together?
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Thomas Hunt Morgan – discovered that genes
located on the same chromosome would
generally be inherited together or “linked”
Chromosomes can be thought of as groups of
genes. The chromosomes can assort
independently, but all the genes on that
chromosomes will be linked!
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However, crossing-over sometimes separates
gene that are usually found on the same
chromosome, so genes may not be linked
together forever!
Crossing-over is soooo important because it
helps generate genetic diversity – new
combinations of allele are constantly produced
Increasing the variability of a species increases
the possibility that some individuals of that
species will be better adapted than others to
survive both short-term and long-term changes
in the environment.
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Show the relative locations of known genes
on chromosomes
Exact location on chromosomes
Chromosome 2
Comparative Scale of a Gene Map
Section 11-5
Mapping of Earth’s
Features
Mapping of Cells, Chromosomes,
and Genes
Cell
Earth
Country
Chromosome
State
Chromosome
fragment
City
People
Go to
Section:
Gene
Nucleotide
base pairs