Download Meiosis I

11.4
Meiosis
Lesson Overview
Meiosis
Chromosome Number
• Chromosomes carry
genes.
• Genes are located in
specific
positions on
chromosomes.
Lesson Overview
Meiosis
Diploid Cells
• Fruit flies have eight
chromosomes.
1 2 2
1
3 3
• Four come from the father, four
come from the mother.
4
• Homologous pair –
The two sets of chromosomes
that match.
Always one from each parent.
4
Lesson Overview
Meiosis
Diploid Cells
• Diploid cell –
Cell containing both sets
of homologous chromosomes.
• Represented by 2N.
• For the fruit fly, the diploid number
is 8, which can be written as 2N = 8
• All cells are diploid
except the sperm and egg.
Lesson Overview
Meiosis
Haploid Cells
Haploid (N) - cells containing
a single set of chromosomes,
and therefore a single set of genes.
Gametes (sperm and egg) are haploid.
Fruit flies are 2N with 8 chromosomes. How
many chromosomes do the gametes have?
Four
Lesson Overview
Meiosis
Phases of Meiosis
• Meiosis - process in which
the number of chromosomes in a diploid cell
is cut in half.
• Made up of meiosis I and meiosis II.
• By the end of meiosis II, the diploid cell
becomes four haploid cells.
Meiosis I
Interphase
Prophase I
Metaphase I
Anaphase I
Telophase I
Cytokinesis
Prophase II
Metaphase II
Anaphase II
Telophase II
Cytokinesis
Meiosis II
Lesson Overview
Meiosis
Interphase
Chromosomes
replicate
Example cell: 2N
(4 chromosomes) 
Meiosis I - Prophase I
• Homologous
chromosomes pair up.
• Form tetrads –
four chromatids
• Go through
crossing-over.
Lesson Overview
Meiosis
Prophase I – Crossing-Over
• First, homologous chromosomes cross over
one another.
• Crossed sections of the chromatids are
exchanged.
• Important because it produces new
combinations of alleles (genes).
Lesson Overview
Meiosis
Metaphase I
• Homologous
chromosomes line
up across the
middle
of the cell.
Anaphase I
• Homologous
chromosomes are
pulled apart
and move towards
opposite
ends of the cell.
Lesson Overview
Meiosis
Telophase I
• Nuclear membrane
reforms
around each
cluster of
chromosomes.
Cytokinesis
• Cells splits, forming
two new cells.
Lesson Overview
Meiosis
Meiosis I Concludes
• Results in
two haploid (N)
daughter cells
• Each has 2
chromosomes,
4 chromatids.
• Each cell has different
chromosomes than when
it started because of
crossing-over.
Lesson Overview
Meiosis
Meiosis II
• After meiosis I
comes meiosis II.
• No interphase
• The final four phases of
meiosis II are similar to those
in meiosis I. However, the
result is four haploid
daughter cells.
Lesson Overview
Meiosis
Prophase II
• Chromosomes
become visible.
• No tetrads
Metaphase II
• Chromosomes line
up in the
middle
of each cell.
Lesson Overview
Meiosis
Anaphase II
• Paired chromatids
separate.
Telophase II,
and Cytokinesis
• Splits into
four haploid (N)
daughter cells
• Each has 2
chromosomes, 2
chromatids.
Lesson Overview
Meiosis
Interphase
Prophase I
Meiosis I Metaphase I
Anaphase I
Telophase I
Prophase II
Metaphase II Meiosis II
Anaphase II
Telophase II
Cytokinesis
Cytokinesis
2 cells
2 chromosomes
4 chromatids
4 cells
2 chromosomes
2 chromatids
Lesson Overview
Meiosis
Gametes to Zygotes
• The haploid cells produced by meiosis II are
gametes.
• In males, gametes are
called sperm.
• In females, gametes are
called eggs.
Lesson Overview
Meiosis
Gametes to Zygotes
• Fertilization — the fusion of
the male and female gametes
• Makes a zygote.
• The zygote undergoes mitosis
and eventually forms a new organism.
Lesson Overview
Meiosis
Comparing Meiosis and Mitosis
Mitosis
Meiosis
One division
Two divisions
Daughter cells have the
same number of
chromosomes as first
cell
Results in two
genetically identical
diploid cells
Daughter cells have
half the number of
chromosomes as first
cell
Results in four
genetically different
haploid cells
Lesson Overview
Meiosis
Lesson Overview
Meiosis
Meiosis I
23
Meiosis II
Lesson Overview
Meiosis
Gene Linkage and Gene Maps
• How can two alleles from different genes be
inherited together?
• Alleles from different genes tend to be
inherited together when
those genes are located
on the same chromosome.
Lesson Overview
Meiosis
Gene Linkage
• A scientist used a fly with reddish-orange eyes
and miniature wings in a series of test crosses.
• His results showed that the genes for those
two traits were almost
always inherited together.
Lesson Overview
Meiosis
Gene Linkage
• Findings led to two conclusions:
• First, each chromosome has groups
of linked genes.
• Second, it is the chromosomes
that assort independently, not individual
genes.
• Alleles of different genes tend to be
inherited together
when those genes are located on
the same chromosome.
Lesson Overview
Meiosis
Gene Mapping
• Sturtevant wondered could gene linkage be
a clue to the genes’ locations?
• Sturtevant reasoned if two genes are
close together, then crossovers
between them should be rare.
• If two genes are far apart,
then crossovers between them
should be common.
Lesson Overview
Meiosis
Gene Mapping
• Used the frequency
of cross-overs between genes to
determine their distances
from each other.
• Method still used today.