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Here, we will define what meiosis is and study Independent Assortment. A chromosome is a long DNA molecule with part or all of the genetic material of an organism. Most eukaryotic chromosomes include packaging proteins called histones which, aided by chaperone proteins, bind to and condense the DNA molecule to maintain its integrity.
1.
6.1 Chromosomes and Meiosis
KEY CONCEPT
Gametes have half the number of chromosomes
that body cells have.
2.
6.1 Chromosomes and Meiosis
• You have body cells and gametes.
• Body cells are also called somatic cells.
• Germ cells develop into gametes.
– Germ cells are located in the ovaries and testes.
– Gametes are sex cells: egg and sperm.
– Gametes have DNA that can be passed to offspring.
body cells sex cells (sperm) sex cells (egg)
3.
6.1 Chromosomes and Meiosis
• Your cells have autosomes and sex chromosomes.
• Your body cells have 23 pairs
of chromosomes.
– Homologous pairs of
chromosomes have the
same structure.
– For each homologous pair,
one chromosome comes
from each parent.
• Chromosome pairs 1-22 are
autosomes.
• Sex chromosomes, X and Y,
determine gender in mammals.
4.
6.1 Chromosomes and Meiosis
• Body cells are diploid; gametes are haploid.
• Fertilization between egg and sperm occurs in sexual
reproduction.
• Diploid (2n) cells have two copies of every
chromosome.
– Body cells are diploid.
– Half the chromosomes come from each parent.
5.
6.1 Chromosomes and Meiosis
• Haploid (n) cells have one copy of every chromosome.
– Gametes are haploid.
– Gametes have 22 autosomes and 1 sex chromosome.
6.
6.1 Chromosomes and Meiosis
• Chromosome number must be maintained in animals.
• Many plants have more than two copies of each
chromosome.
• Mitosis and meiosis are types of nuclear division that
make different types of cells.
• Mitosis makes
more diploid cells.
7.
6.1 Chromosomes and Meiosis
• Meiosis makes haploid cells from diploid cells.
– Meiosis occurs in sex cells.
– Meiosis produces gametes.
8.
6.2 Process of Meiosis
KEY CONCEPT
During meiosis, diploid cells undergo two cell
divisions that result in haploid cells.
9.
6.2 Process of Meiosis
• Cells go through two rounds of division in meiosis.
• Meiosis reduces chromosome number and creates
genetic diversity.
10.
6.2 Process of Meiosis
• Meiosis I and meiosis II each have four phases, similar to
those in mitosis.
– Pairs of homologous chromosomes separate in
meiosis I.
– Homologous chromosomes are similar but not identical.
– Sister chromatids divide in meiosis II.
– Sister chromatids are copies of the same chromosome.
homologous chromosomes
sister sister
chromatids chromatids
11.
6.2 Process of Meiosis
• Meiosis I occurs after DNA has been replicated.
• Meiosis I divides homologous chromosomes in four phases.
12.
6.2 Process of Meiosis
• Meiosis II divides sister chromatids in four phases.
• DNA is not replicated between meiosis I and
meiosis II.
13.
6.2 Process of Meiosis
• Meiosis differs from mitosis in significant ways.
– Meiosis has two cell divisions while mitosis has one.
– In mitosis, homologous chromosomes never pair up.
– Meiosis results in haploid cells; mitosis results in diploid
cells.
14.
6.2 Process of Meiosis
• Haploid cells develop into mature gametes.
• Gametogenesis is the production
of gametes.
• Gametogenesis differs between
females and males.
– Sperm become streamlined and
motile.
– Sperm primarily contribute DNA
to an embryo.
– Eggs contribute DNA, cytoplasm,
and organelles to an embryo.
– During meiosis, the egg gets
most of the contents; the other
cells form polar bodies.
15.
6.6 Meiosis and Genetic Variation
KEY CONCEPT
Independent assortment and crossing over during
meiosis result in genetic diversity.
16.
6.6 Meiosis and Genetic Variation
• Sexual reproduction creates unique combinations of
genes.
• Sexual reproduction creates unique combination of genes.
– independent assortment of chromosomes in meiosis
– random fertilization of gametes
• Unique phenotypes may give a reproductive advantage to
some organisms.
17.
6.6 Meiosis and Genetic Variation
• Crossing over during meiosis increases genetic diversity.
• Crossing over is the exchange of chromosome
segments between homologous chromosomes.
– occurs during prophase I of meiosis I
– results in new combinations of genes
18.
6.6 Meiosis and Genetic Variation
• Chromosomes contain many genes.
– The farther apart two genes are located on a
chromosome, the more likely they are to be separated
by crossing over.
– Genes located close together on a chromosome tend to
be inherited together, which is called genetic linkage.
• Genetic linkage allows the distance between two genes to
be calculated.
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