Download meiosis I - Nicholas County Schools

Meiosis
Section 10 - 1
• Learning Targets:
• I can…
– Describe the basic process of meiosis
– Compare and contrast mitosis and meiosis
Chromosomes
• Hair color, eye color, height, etc. are all
characteristics that are passed on from a
parent to child
– Each of these characteristics are called traits
– Instructions for each trait are located on
chromosomes found in the nucleus of cells
Chromosomes
– DNA on chromosomes are arranged in
segments that control the production of
proteins
• The DNA segments are called genes
– Each chromosome consists of hundreds of
genes
• Each gene plays an important role in
determining the characteristics and functions of
the cell
Homologous Chromosomes
• Human body cells have 46
chromosomes
– Each parent contributes 23 chromosomes,
resulting in 23 pairs of chromosomes
– Each pair (one chromosome from each
parent) are called homologous
chromosomes
• Each homologous chromosome will be
the same size/shape and the centromere
will be in the same position
Homologous Chromosomes
• Each homologous chromosome carries
genes that control the same inherited
traits
– Ex: the gene for earlobe type will be
located at the same position on both
homologous chromosomes
• Though both genes will code for earlobe type,
they will not necessarily code for the same
earlobe type
Homologous Chromosomes
Haploid Cells
• In order to maintain the same
chromosome number from generation
to generation, an organism produces
gametes
– Gametes: sex cells that have half the
number of chromosomes
• Each human gamete contains 23 chromosomes
– Gametes are haploid cells
• Haploid: cells with a single (n) set of
chromosomes
Diploid Cells
• When one haploid gamete combines
with another haploid gamete fertilization
occurs
– The cell created will have 2n chromosomes
(n from mother and n from father)
• This is called a diploid cell
• In humans diploid cells have 46 chromosomes
or 23 homologous chromosomes
Meiosis
• Gametes are formed during a process
called Meiosis
– Meiosis is a type of cell division that
reduces the number of chromosomes
– Occurs in the reproductive structures of
organisms that reproduce sexually
– Meiosis involves 2 consecutive cell
divisions called meiosis I and meiosis II
Meiosis I
• Steps of Meiosis I
1. Interphase: Undergo interphase like
mitosis
–
Cells carry out various metabolic processes
and replicate DNA
Meiosis I
2. Prophase I
–
Replicated chromosomes become visible
• Consist of 2 sister chromatids
–
–
Homologous chromosomes begin to form
pairs as they condense in a process called
synapsis
Homologous chromosomes are held
tightly together
Meiosis I
• Prophase I cont…
– Crossing Over: process during which
chromosomal segments are exchanged
between a pair of homologous
chromosomes
– Centrioles move to the cell’s opposite
poles, spindle fibers form and bind to the
sister chromatids at the centromere
Meiosis I
• Prophase I
Crossing Over
Meiosis I
3. Metaphase I
• Pairs of homologous chromosomes line
up at the equator
• Spindle fibers attach to each homologous
chromosome
Meiosis I
4. Anaphase I
• Homologous chromosomes separate
• Each member of the pair is guided by
spindle fibers and moves toward
opposite poles
• Unlike mitosis, when homologous
chromosomes split in anaphase, each still
has 2 sister chromatids
Meiosis I
• Anaphase I
Meiosis I
5. Telophase I
• Homologous chromosomes reach the
cell’s opposite poles
• Each pole only contains one member of the
original pair of homologous chromosomes
• Note that each chromosome may not be identical
because of crossover that occurred during synapsis
• Cytokinesis also happens at this point
Meiosis II
• Steps of Meiosis II
1. Prophase II
–
Spindle apparatus forms in each new cell
from Meiosis I
– Chromosomes condense
– Spindle fibers attach to each chromosome
Meiosis II
2. Metaphase II
• Centromeres of chromosomes line up
randomly at the equator of each cell
3. Anaphase II
• Sister chromatids are pulled apart at the
centromere by the spindle fibers
• Sister chromatids move toward opposite
poles
Meiosis II
4. Telophase II
• Sister chromatids reach opposite poles
• Nuclear membrane and nuclei reform
• At the end of Meiosis II, cytokinesis
occurs
• Results in 4 haploid cells
Meiosis
Importance of Meiosis
• Meiosis provides variation among
offspring
– This occurs when homologous
chromosomes line up at the equator during
metaphase I randomly
• Since crossing over may have occurred, the
chromosomes are not identical, therefore each
of the 4 haploid cells may not be identical
Asexual v. Sexual Reproduction
• Asexual Reproduction – organisms
inherit all of its chromosomes from a
single parent
– Therefore, the new individual is genetically
identical to its parent
• Some organisms use both asexual and
sexual reproduction
– Considered less advance because it has
been shown that beneficial genes multiply
faster with sexual reproduction