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Cell Reproduction
Need For Reproduction
Reproduction is the life process in which living things produce other living things of the same species.
Reproduction is not necessary for the life of one individual organism.
It is necessary for the continued survival of a particular group of organisms.
Cell Division All cells arise from other cells by cell division. When cells grow to a certain size they must
either divide or die. If a cell continued to grow without dividing,
the surface area of the cell would become too small to hold the cell’s contents. (Problem of Surface Area/Volume Ratio)
Mitosis Cell Division
Cell division is a complex series of changes in the nucleus of a cell that leads to the production of two new cells.
The new cells are called daughter cells. The nuclei of the daughter cells are identical to
each other and to that of the parent cell. The daughter cells grow and increase in size until
they divide and produce two more daughter cells. This process continues, resulting in organism
growth and reproduction.
Mitosis All cells in the body (except sex cells) are produced
by the process of mitotic cell division. Mitosis involves a complex series of changes in
the nuclei of body cells that produce identical (same) daughter cells.
They have the same number and type of chromosomes as the parent cells. Primary purpose is to increase the number of cells
Daughter cells are genetically identical to the parents
Occurs during growth and asexual reproduction.
The Cell Cycle – Stages of Mitosis
Although the events of mitosis are an ongoing process, they are generally described in terms of separate phases, or stages. Interphase Prophase Metaphase Anaphase Telophase
Mitosis
Interphase Interphase is the period
between cell divisions. During interphase, the
single-stranded chromosomes replicate (make an extra copy).
The chromosomes can not be seen during interphase.
Nucleus
Chromosomes
Interphase
Interphase Sometimes called the
“resting stage” between divisions
BUT cells are metabolically very active
The amount of DNA in the nucleus doubles
New organelles such as mitochondria are made.
Prophase In prophase, the double-
stranded chromosomes become visible, and the nuclear membrane disappears.
A spindle apparatus, consisting of fibers, forms between opposite poles of the cell.
Chromosomes
Prophase
Prophase Protein microtubules
develop from each centriole forming spindle fibres.
(In plant cells there are no centrioles and the spindle forms independently)
Prophase
Prophase Towards the end of
prophase each chromosome can be seen to consist of two chromatids held together by a centromere.
Prophase
Prophase At the end of prophase
the nucleolus disappears and the nuclear envelope breaks down.
Metaphase During metaphase, the chromosomes move toward
the middle of the cell and line up at the cell equator (midline).
Chromosomes
Centrioles
Metaphase
Metaphase Chromosomes line up
on the equator of the spindle.
They attach themselves to the spindle by their centromere
Anaphase During anaphase, the double-stranded chromosome
separates and move to opposite poles of the cell. There is a complete set of chromosomes at each pole
of the cell.
ChromosomesCentrioles
MITOSIS
Anaphase This movement results
from the contraction of the spindle fibres.
As they shorten they pull the chromatids apart.
Anaphase
Anaphase The centromeres divide The free chromatids
move to the poles
Telophase and Cytoplasmic Division
The last stage of mitosis is telophase. In telophase, a nuclear membrane forms around each
set of chromosomes, forming two identical nuclei. At the end of mitosis,
the cytoplasm divides, forming two new identical daughter cells.
Telophase
Telophase The chromatids have
reached the poles and are now regarded as distinct chromosomes again.
A nuclear envelope forms around each group of chromosomes
Telophase
Telophase The chromosomes
uncoil returning to chromatin
The cytoplasm divides by cytokinesis.
CYTOKINESIS
Animal cells The centre of the cell
‘pinches in’ to form a division furrow.
As the division deepens, the cell surface membrane on each side joins up.
Two separate cells result.
CYTOKINESIS
Plant cells Vesicles produced by the Golgi body collect
on equator of cell These vesicles fuse to form a cell plate. The cell plate eventually stretches right
across the cell forming the middle lamella. Cellulose builds up on lamella to form cell
walls.
CYTOKINESIS
The Cell Cycle
Meiosis The process of meiosis involves two cell
divisions and produces cells that are different from the parent cell. Meiosis produces cells that have one-half the
number of chromosomes as the parent cells. If meiosis did not take place, the fertilized egg would
have double the amount of chromosomes it needs. Meiosis takes place during sexual reproduction
when sex cells, called the egg and sperm, are produced in sex organs.
Meiosis One-half the number of chromosomes is called
the hapolid, (or n) number. In humans, the haploid number (n) is 23.
When the egg and sperm unite during fertilization, the species normal chromosome number called the diploid or (2n) number is restored (brought back). The diploid number in humans is 46. n (sperm) + n (egg) = 2n
Meiosis
Meiosis
Cell division that is vital for sexual reproduction
Takes place in reproductive organs
Results in formation of haploid gametes
The four daughter cells are not genetically identical
Meiosis and genetic variation
There are two main ways in which genetic
variation occurs at meiosis:
1. Random segregation of chromosomes
2. Crossing over
Meiosis and genetic variation
Random segregation During metaphase 1
homologous chromosomes arrive at the equator
They arrange themselves in a random order on the equator
Meiosis and genetic variation
Random segregation In a cell with just two
pairs of chromosomes there are four different types of outcomes for the way the chromosomes can separate.
Meiosis and genetic variation
Random segregation In human cells with 23
chromosomes there are 223 different possible ways the chromosomes can segregate!
That is over 8 000 000 different types of gamete
Meiosis and genetic variation
During fertilisation any male gamete can join with any female gametes.
So thousands of millions of new genetic combinations are possible.
Meiosis and genetic variation
Crossing over During prophase 1 of
meiosis homologous chromosomes come together in pairs
Meiosis and genetic variation
Crossing over Each chromosome is divided into two
chromatids The homologous chromosomes twist around
each other This creates tension, which may cause
breaks to occur along the length of the chromatids
Meiosis and genetic variation
Crossing over During cross over, corresponding fragments
may get swapped over. This “cutting and sticking” means that genetic
material is exchanged. This creates new genetic combinations and
variation in the gametes is increased.
Differences between mitosis and meiosis
Mitosis Meiosis
one division
Differences between mitosis and meiosis
Mitosis Meiosis
one division two divisions
Differences between mitosis and meiosis
Mitosis Meiosisone division two divisions
the number of chromosomes
remains the same
Differences between mitosis and meiosis
Mitosis Meiosisone division two divisions
the number of chromosomes the number of
remains the same chromosomes is halved
Differences between mitosis and meiosis
Mitosis Meiosisone division two divisions
the number of chromosomes the number of
remains the same chromosomes is halved
homologous chromosomes
do not pair up
Differences between mitosis and meiosis
Mitosis Meiosisone division two divisions
the number of chromosomes the number of
remains the same chromosomes is halved
homologous chromosomes homologous chromosomes
do not pair up pair up to form bivalents
Differences between mitosis and meiosis
Mitosis Meiosisone division two divisions
the number of chromosomes the number of
remains the same chromosomes is halved
homologous chromosomes homologous chromosomes
do not pair up pair up to form bivalents
chiasmata do not form and
cross over never occurs
Differences between mitosis and meiosis
Mitosis Meiosisone division two divisions
the number of chromosomes the number of
remains the same chromosomes is halved
homologous chromosomes homologous chromosomes
do not pair up pair up to form bivalents
chiasmata do not form and chiasmata form and cross over
cross over never occurs occurs
Differences between mitosis and meiosis
Mitosis Meiosisone division two divisions
the number of chromosomes the number of
remains the same chromosomes is halved
homologous chromosomes homologous chromosomes
do not pair up pair up to form bivalents
chiasmata do not form and chiasmata form and cross over
cross over never occurs occurs
daughter cells are genetically
identical
Differences between mitosis and meiosis
Mitosis Meiosisone division two divisions
the number of chromosomes the number of
remains the same chromosomes is halved
homologous chromosomes homologous chromosomes
do not pair up pair up to form bivalents
chiasmata do not form and chiasmata form and cross over
cross over never occurs occurs
daughter cells are genetically daughter cells are genetically
identical different from the parent cells
Differences between mitosis and meiosis
Mitosis Meiosisone division two divisionsthe number of chromosomes the number ofremains the same chromosomes is halvedhomologous chromosomes homologous chromosomesdo not pair up pair up to form bivalentschiasmata do not form and chiasmata form and cross overcross over never occurs occursdaughter cells are genetically daughter cells are geneticallyidentical different from the parent cellstwo daughter cells are formed
Differences between mitosis and meiosis
Mitosis Meiosisone division two divisionsthe number of chromosomes the number ofremains the same chromosomes is halvedhomologous chromosomes homologous chromosomesdo not pair up pair up to form bivalentschiasmata do not form and chiasmata form and cross overcross over never occurs occursdaughter cells are genetically daughter cells are geneticallyidentical different from the parent cellstwo daughter cells are formed four daughter cells are
formed
Comparison of Mitosis and Meiosis
Characteristic Mitosis MeiosisNumber of daughter cells 2 4
Number of cell division 1 2
Daughter cells are diploid or haploid
Diploid Haploid
Daughter cells are identical or different
Identical Different
Parent cell is diploid or haploid
Diploid Diploid