Meiosis
Meiosis
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Meiosis is cell division that produces gametes.
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Gametes are sex cells (sperm and eggs).

During meiosis, a cell divides twice after
copying its DNA.
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This reduces the number of chromosomes in
each ‘daughter’ cell to half the original.
Meiosis is a part of
Sexual Reproduction

Meiosis produces gametes.

In males, 4 sperm cells are produced.

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In females, 1 egg cell & 3 ‘polar bodies’ are
produced.
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They are equally small in size.
The egg is larger in size.
Human gametes have 23 chromosomes.
Oogenesis: the
making of an egg
egg cell
 polar bodies
Spermatogenesis:
the making of
sperm cells
The phases of meiosis

Meiosis is a “reduction-division” process.

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1 ‘Diploid’ ends up as 4 ‘Haploid’
The number of chromosomes per cell is cut in
half.
Homologous chromosomes separate.
There are two parts to meiosis:


Meiosis I (similar to Mitosis)
Meiosis II
Remember…

Cell division is a
very small part of a
cell’s life cycle
Copy the text part of this
diagram onto the drawing
found on the back of your
worksheet.
Remember… Why cells are small

Diffusion limits cell size


Essential substances (nutrients, water, enzymes,
wastes, etc.) must diffuse into and out of the cell.
Surface area to volume ratio
The volume increases faster than the surface area
 If size of the cell doubles, the cell would require 8 times
more nutrients


A small cell size insures that materials can
get in and out quickly.
Remember…
DNA is first copied during
Interphase before either
Mitosis or Meiosis occurs.
Remember… Complete the drawing on
your worksheet with the terms below
centrioles
nuclear envelope
Daughter
cells
spindle fibers
Prophase
chromatin centromere
sister chromatids
Mitosis
Telophase
nuclear envelope
spindle fibers
centrioles
sister
chromatids
Anaphase
centrioles
Metaphase
Meiosis I

Meiosis I has four phases. They are:
1.
2.
3.
4.
Prophase I
Metaphase I
Anaphase I
Telophase I and cytokinesis
Prophase I

Each chromosome pairs with its
corresponding homologous chromosome to
form a tetrad (4 chromosomes).
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This is the phase when crossing over occurs.
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Crossing over is when tetrads exchange portions
of their chromatids.
This creates new, unique chromosome
combinations
Remember…
Chromosomes are made up of sequences of
genes
 Homologous chromosomes have the ‘same’ gene
sequences from ‘mom’ & ‘dad’

Remember…

These ‘same’ genes are called alleles
Some may be
chemically stronger
and are called
‘dominant’.
 Those not as strong
are called
‘recessive’

Crossing over during Prophase I:
Complete the drawing on your worksheet using the labels
shown below.
homologous
chromosomes
in a tetrad
crossing over
(gene jumping)
later separation
into gamete cells
2 sets of
sister
chromosomes
from DNA
replication
from from
‘mom’ ‘dad’
a little bit of ‘mom’ &
‘dad’ end up in all cells
Metaphase I

During metaphase I, the spindle fibers
attach to the chromosomes.
Prophase I
Metaphase I
Anaphase I

During anaphase I,
the fibers pull the
homologous
chromosomes
toward opposite
ends of the cell.
Telophase I and Cytokinesis
During this phase, the
nuclear membranes
reform and the cell
divides.
 After cytokinesis,
meiosis II occurs.

Result of Meiosis I
Two daughter cells with ‘half’ the
number of chromosomes (‘N’) as the
original cell (‘2N’).
 Homologous chromosomes, with their
‘crossed over’ genes in each cell
 Each homologous chromosome still has
the ‘sister’ formed when DNA was
replicated (also possibly ‘crossed over’).

Meiosis II
The four phases in Meiosis II are:
1.
2.
3.
4.
Prophase II
Metaphase II
Anaphase II
Telophase II
Prophase II
After a brief resting
period, the nuclear
envelope breaks
down
 chromatin again
daughter
condenses into
cells from
meiosis I
chromosomes
 Spindle fibers reform

Metaphase II
The chromosomes line up
along the middle (equator)
of the cell
 Spindles attach to sister
chromatids

daughter
cells from
meiosis I
Anaphase II

The sister chromatids
separate and are pulled
toward opposite ends of
the cell
daughter
cells from
meiosis I
Telophase II and cytokinesis
During this phase,
the nuclear
membranes reform
and the cell divides.
 After cytokinesis,
there are 4 haploid
sex cells (gametes)

The difference between
mitosis and meiosis
Mitosis results in two genetically
identical diploid cells (2N)
 Meiosis produces four genetically
different haploid (N) cells.

The difference between
mitosis and meiosis

Mitosis creates ‘clones’
necessary for growth and repair of tissues
in multicellular creatures
 Prokaryotes (e.g. bacteria) reproduce this
way

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Meiosis produces ‘gametes’

leads to greater diversity and chance for
favorable mutations
Complete
the
drawing
on your
worksheet
by
copying
the
circled
terms
shown
here
Diploid cells (2N)
Haploid cells (N)
Fertilization
Fertilization is the union of 2 gametes
(sperm cell + egg cell).
 ‘Haploid’ + ‘Haploid’ = ‘Diploid’
 The zygote is the fertilized egg.
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It is the first cell of a new organism.
In humans, 23 + 23 = 46 chromosomes

homologous chromosomes come from each
parent
Fertilization
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Sperm (N) + Egg (N)  Zygote (2N)
Finire! (Finished!)
Double-check all blanks are filled in on your
worksheet
 Double-check all diagrams are labeled and
color-coded
 Double-check you answered the Essential
Question (2-word answer from notes)
 Place worksheet in notebook with “Genetics &
Heredity” notes. Your next test will be over
these concepts.
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