Download Meiosis - Mrs. Meadows Science

Meiosis
7.L.2.1 Explain why
offspring that result
from sexual
reproduction
(fertilization and
meiosis) have greater
variation than offspring
that result from asexual
reproduction (budding
and mitosis).
Sexual Reproduction and Genetics
Meiosis - General Overview
 Human body cells have 46 chromosomes
 Each parent contributes 23 chromosomes
 Takes place in sex cells (gametes): egg and
sperm
 One cell becomes 4 NON-IDENTICLE cells
Sexual Reproduction and Genetics
Meiosis – Homologous Chromosomes
Chromosomes and Chromosome Number
 Same length
 Same centromere position
 Carry genes that control the
same inherited traits
 One is from mother, one is
from father = a matching set
Sexual Reproduction and Genetics
Meiosis
Haploid and Diploid Cells
 An organism produces gametes to maintain the
same number of chromosomes from generation
to generation.
 Human gametes contain 23 chromosomes.
 A cell with n chromosomes is called a haploid
cell.
 A cell that contains 2n chromosomes is called a
diploid cell.
Sexual Reproduction and Genetics
Meiosis
Meiosis I
 The sexual life cycle
in animals involves
meiosis.
 Meiosis produces
gametes.
 When gametes
combine in fertilization, the number of
chromosomes is restored.
Sexual Reproduction and Genetics
Meiosis
Meiosis I
 Interphase
 Chromosomes replicate.
 Chromatin condenses.
Interphase
Sexual Reproduction and Genetics
Meiosis
Meiosis I
 Prophase I
 Pairing of homologous
chromosomes occurs.
 Each chromosome consists of two
chromatids.
 The nuclear membrane breaks down.
 Spindles form.
Prophase I
Sexual Reproduction and Genetics
Meiosis
Meiosis I
 Prophase I
 Crossing over produces exchange of genetic
information.
 Crossing over—chromosomal segments are
exchanged between a pair of homologous
chromosomes.
Sexual Reproduction and Genetics
Meiosis
Meiosis I
 Metaphase I
 Chromosome
centromeres attach
to spindle fibers.
Metaphase I
 Homologous chromosomes line up at the
equator.
Sexual Reproduction and Genetics
Meiosis
Meiosis I
 Anaphase I
 Homologous
chromosomes
separate and move
to opposite poles of the cell.
Anaphase I
Sexual Reproduction and Genetics
Meiosis
Meiosis I
 Telophase I
 The spindles
break down.
Telophase I
 Chromosomes uncoil and form two nuclei.
 The cell divides.
Sexual Reproduction and Genetics
Meiosis
Meiosis II
 Prophase II
 A second set of
Prophase II
phases begins
as the spindle apparatus forms and the
chromosomes condense.
Sexual Reproduction and Genetics
Meiosis
Meiosis II
 Metaphase II
 A haploid number
of chromosomes
line up at the equator.
Metaphase II
Sexual Reproduction and Genetics
Meiosis
Meiosis II
 Anaphase II
 The sister
Anaphase II
chromatids are
pulled apart at the centromere by spindle
fibers and move toward the opposite poles
of the cell.
Sexual Reproduction and Genetics
Meiosis
Meiosis II
 Telophase II
 The chromosomes
Telophase II
reach the poles, and
the nuclear membrane and nuclei reform.
Sexual Reproduction and Genetics
Meiosis
Meiosis II
 Cytokinesis results in
four haploid cells,
each with n number of
chromosomes.
Cytokinesis
Sexual Reproduction and Genetics
Meiosis
The Importance of Meiosis
 Meiosis consists of two sets of divisions
 Produces four haploid daughter cells that are
not identical
 Results in genetic variation
Sexual Reproduction and Genetics
Meiosis
Meiosis Provides Variation
 Depending on how the
chromosomes line up at the
equator, four gametes with
four different combinations
of chromosomes can result.
 Genetic variation also is
produced during crossing
over and during fertilization,
when gametes randomly
combine.
Mitosis vs Meiosis