Download meiosis_1

Meiosis
All the cells except the sex cells (sperm
and eggs) of organisms that reproduce
sexually contain 2 sets of chromosomes,
one from the mother, one from the father.
In a eukaryote, a cell other a gamete, gametocyte,
germ cell or stem cell, is a somatic cell.
It is said to be diploid because it has 2 sets of
chromosomes (one from the father, one from the
mother). Diploid can be represented by the symbol
2N (2 sets).
Diploid cells contain 2 complete sets of
chromosomes therefore 2 copies of each gene
(except the X and Y chromosomes which each
have genes for different things.)
The gametes, or sex cells of sexually-reproducing
organisms, contain only a single set of
chromosomes, therefore only a single set of genes.
These cells are called haploid cells, (means one
set).
Haploid is represented by N.
For example, the egg of a Drosophila (fruit
fly) has 4 chromosomes. So does a sperm.
This can be written as:
N=4
The haploid number is 4.
How many chromosomes are there in a
somatic cell of a fruit fly?
So, how does an organism make egg and
sperm cells, with only half the number of
chromosomes of regular somatic cells?
By a process called meiosis
Meiosis is said to be a process of
reduction division
The division of the cells which are
involved in sexual reproduction so that
the number of chromosomes is reduced
by half. This results in either sperm or egg.
sperm +
n
+
egg
n
=
=
zygote
2n
Meiosis
Meiosis is a process of reduction division. In
a human, at the beginning of meiosis, there
is a cell with 46 chromosomes; at the end, a
cell with half that.
Through meiosis, one diploid cell becomes 4
haploid cells. In animals, these haploid
cells are sperm or eggs.
Where do sperm cells come
from?
 They
are created from diploid cells called
spermatocytes in the testes.
 The
sperm cell itself is a haploid cell.
Egg cells
 Egg
cells come from diploid cells call
oocytes located in the ovaries.
 The
egg cell itself is haploid.
The process of meiosis involves 2 distinct
divisions called meiosis I and meiosis II.
The phases have the same names as the
phases in mitosis except the name is
followed by “I” or “II”, depending on
whether the phase is in meiosis I or meiosis II.
Before you go any further, look at page 277 of your
text book and watch the Crash Course on meiosis
using the link below.
http://www.youtube.com/watch?v=qCLmR
9-YY7o
Understanding the
vocabulary of meiosis
Homologous chromosomes
In each diploid cell there are two
chromosomes that have copies of genes
on them that code for the same things.
One of the pair came from the mother;
the other chromosome in the pair came
from the father. These pairs of
chromosomes that code for the same
thing are called homologous pairs.
Each chromosome in a homologous pair is:
 the same size
 the same shape
 the same gene arrangement (generally
genes that code for the same trait)
but
 they are not identical because the actual
code is different. (have different alleles)
For example, both chromosomes in a
homologous pair might code for “curling of
your tongue” but one might let you curl
your tongue and the other might not.
Homologous pairs of
chromosomes
 Remember
that DNA is replicated during
the S phase of interphase. This replication
also occurs in the cells that will ultimately
produce sperm and egg cells.
So, after replication, each homologous pair
has a matching homologous pair.
Meiosis I
 In
Prophase I of Meiosis I, each homologous
chromosome pair joins with its corresponding
homologous pair to form a tetrad.
 See
Figure 11-16 page 276 in your book!
 Synapsis
is the process in which homologous
pairs of chromosomes pair up during
Prophase I to form what is called a tetrad.
 During
Prophase I , genetic material is
exchanged between the maternal and
paternal chromosomes. This process is
called crossing over. The result of crossing
over is genetic recombination.
 The
point at which the
chromosomes cross over is called
the chiasma.
 During
synapsis the chromosomes within a
homologous pair may twist around the
chromosomes of the other homologous
pair in the tetrad, break off and attach to
the other chromosomes in a process
called crossing over.
Tetrad
 Remember,
these are the
homologous chromosome pairs,
together.
 There
are 4 sister chromatids in a
tetrad.
Prophase I of Meiosis I is where crossing over takes place!
 Make
sure that you know what happens
in each phase of meiosis by watching the
Crash Course video, reading your text
and looking at the following slides.
Meiosis I
Interphase I
Cells undergo a round of
DNA replication, forming
duplicate Chromosomes.
Go to
Section:
Prophase I
Each chromosome pairs with
its corresponding
homologous chromosome to
form a tetrad.
Metaphase I
Spindle fibers attach to the
chromosomes.
Anaphase I
The fibers pull the
homologous chromosomes
toward the opposite ends of
the cell.
Meiosis II
 The
2 cells produced in Meiosis 1 then go through
another meiotic division.
 There
is no DNA replication between Meiosis I and
Meiosis II.
 The
end result of Meiosis II is 4 cells with 2 sister
chromatids in them.
Meiosis II
Prophase II
Meiosis I results in two
haploid (N) daughter cells,
each with half the number of
Go to
chromosomes as the
Section:
original.
Metaphase II
The chromosomes line
up in a similar way to the
metaphase stage of
mitosis.
Anaphase II
The sister chromatids
separate and move
toward opposite ends
of the cell.
Telophase II
Meiosis II results in
four haploid (N)
daughter cells.
Law of Independent
Assortment
 During
metaphase I, as the tetrads
line up, the orientation of maternal
and paternal chromosomes is
random
Not all maternal go to one side,
and paternal the other….it is mixed.
Law of independent assortment
As the tetrads are separated in
anaphase I, the maternal and
paternal chromosomes have random
separation which results in genetic
variation.
This random alignment and
separation occurs again in
metaphase II and anaphase II,
except pairs are separated into
individual chromosomes.
Formation of Gametes
 Meiosis
only occurs in the reproductive
cells…all other cells that divide undergo
mitosis.
 Ending products of meiosis are haploid
gametes


Eggs in female - oogenesis
Sperm in male - spermatogenesis
 Fusion
of sperm and egg results in a
diploid cell
Review – If you understand
this, you understand
meiosis.
 http://www.youtube.com/watch?v=iCL6d
0OwKt8
Chromosomal Disorders
Non-disjunction: when this
occurs, abnormal numbers of
chromosomes may find their way into
gametes, and a disorder of
chromosome numbers may result.
Nondisjunction
Section 14-2
Homologous
chromosomes
fail to separate
Go to
Section:
Meiosis I:
Nondisjunction
Meiosis II
Karyotype
DNA v. Gene