Chromosomes and Meiosis
Section 6.1
1
Objectives
• SWBAT differentiate between body cells and
gametes.
• SWBAT compare and contrast autosomes and
sex chromosomes.
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Vocabulary
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Somatic cell
Gamete
Homologous chromosome
Autosome
Sex chromosome
Sexual reproduction
Fertilization
Diploid
Haploid
Meiosis
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Starter
• What makes sex cells different from all
others? Explain.
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Somatic Cells and Germ Cells
• Your body has two major groups of cells:
somatic cells and germ cells.
• Somatic cells – are more easily thought of as
body cells (they make up your soma – or
body).
– They make up the vast majority of your cells (ex.
spleen, eyeballs, neurons, skin cells, etc.)
– DNA in your body cells is not passed on to your
children.
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Somatic Cells and Germ Cells
• Germ Cells – are cells in your
reproductive organs.
– Germ cells are found in the ovaries and
the testes and develop into gametes.
• Gametes – are sex cells.
– They are ova, or eggs, in females.
– They are spermatozoa, sperm cells, in
males.
– DNA in your gametes can be passed on
to your children.
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Human Karyotype
What does
diploid mean?
What does
haploid mean?
Which
chromosomes
are autosomes?
Which are sex
chromosomes?
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Salmon and Field Mouse Karyotypes
Complexity of
an organism
and number of
chromosomes
do not seem to
be related.
Ferns have over
1200
chromosomes.
Each species has a characteristic number of
chromosomes found in each somatic cell.
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Homologous Chromosomes
You have 23 pairs of chromosomes – 23 from
your mother and 23 from your father.
Each chromosome pair is referred to as a
homologous pair (homologous chromosomes).
Homologous chromosomes are two
chromosomes that have the same length and
general appearance. Genetically, the two
chromosomes are not Identical!
Most importantly, the two homologous
chromosomes have copies of the same genes –
though they may be different versions of the
same gene.
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Homologous Chromosomes
For example, the gene that codes for Beta
Globin is found on chromosome 11.
Beta Globin has a recessive allele (form of
a gene) that leads to sickle cell anemia.
For an individual to have sickle cell
anemia, it is necessary for both of the
individual’s parents to have given him the
recessive, sickle cell causing, alleles
(genes) for Beta Globin.
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Autosomes
• Chromosome pairs 1-22
make up your autosomes.
– These contain genes for
characteristics not directly
related to the sex of an
organism.
– Eye color and blood group
are examples of autosomal
characteristics.
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Sex Chromosomes
• The 23rd pair of chromosomes
are sex chromosomes.
– They are XX in female and XY
for male mammals.
– Birds are different, males are
XX (homogametic) and females
are XY (heterogametic).
– The XY sex chromosomes are
not homologous.
• The X is significantly larger and
carries lots more genes than
does the small Y (which carries
few genes).
Sex Chromosomes
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Sex Chromosomes
• The Y chromosome contains genes that direct
the development of the testes and other male
traits.
– The Y chromosome is the smallest chromosome and
contains the fewest genes.
• The X chromosome is a large chromosome and
contains lots of traits that are unrelated to sexual
characteristics (traits absent from the Y
chromosome).
– This creates sex-linked traits like male pattern
baldness
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Sexual Reproduction
• Sexual reproduction involves the fusion of
two gametes that results in offspring that are
a genetic mixture of both parents.
• The actual fusion of an egg and a sperm cell is
called fertilization.
– When fertilization occurs, the nuclei of the egg
and sperm cell fuse to form one nucleus.
– The new nucleus must have the correct number of
chromosomes for a healthy new organism to
develop.
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Diploid/Haploid Cells
• The egg and sperm cell need only half the usual
number of chromosomes because they will unite
at fertilization – restoring the two pairs of each
chromosome.
– Body cells (somatic cells) are diploid - meaning a cell
has two copies of each chromosome: one copy from
the mother, and one copy from the father.
• Diploid cells can be represented as 2n.
• In humans, the diploid chromosome number is 46
(2*23=46)
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Diploid/Haploid Cells
• Gametes (eggs and sperm) are haploid cells.
– Haploid cells - means that a cell has only one copy of
each chromosome (22 autosomes and 1 sex
chromosome).
• Typically, a change in chromosome number is
harmful (remember the Down’s Syndrome
karyotype) but on occasion it can result in a new
species in plants.
– For example, there are plants that are 4n (tetraploids),
which results in very fast evolution.
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Diploid/Haploid
Haploid
Diploid
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Meiosis
• Meiosis is the process that forms gametes from
germ cells.
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It divides a diploid cell into a haploid cell.
It is part of sexual reproduction.
It is a form of, like mitosis, nuclear division.
DNA is copied once, like in mitosis, but divided twice
(only once in mitosis).
• Meiosis sometimes called a “reduction division” because it
reduces chromosome number by half.
– Meiosis makes genetically unique haploid cells from a
diploid cell.
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Meiosis – Differences Between
Meiosis and Mitosis
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