Cells divide through the processes of mitosis and meiosis. Mitosis produces two identical daughter cells and is used for growth and repair of the body. Meiosis produces four non-identical haploid cells through two divisions and crossing over, which introduces genetic variation important for sexual reproduction. The cell cycle controls cell division, and errors can lead to diseases like cancer.
The ability of organisms to produce more of their own kind is the one characteristic that best distinguishes living things from nonliving matter. This unique capacity to procreate, like all biological functions, has a cellular basis. The continuity of life is based on the reproduction of cells, or cell division. Cell division plays several important roles which are giving rise to a new organism, enabling development, renewing and repairing and also replacing damaged tissue is multicellular organisms.
توانایی موجودات زنده در تولید بیشتر از نوع خود یکی از ویژگیهایی است که موجودات زنده را از مواد غیر زنده به بهترین وجه متمایز می کند. این ظرفیت منحصر به فرد برای تولید مثل، مانند همه عملکردهای بیولوژیک، دارای پایه سلولی است. تداوم زندگی براساس تولید مثل سلولها یا تقسیم سلولی است. تقسیم سلولی چندین نقش مهم را ایفا میکند که نه تنها باعث ایجاد یک ارگانیسم جدید میشود، همچنین امکان رشد، تجدید و ترمیم و همچنین جایگزینی بافت آسیب دیده موجودات چند سلولی را نیز فراهم می کند.
1. Why cells divide
• Cells divide because:
– Demands the cell places on its DNA
– Moving enough nutrients and wastes across
the cell membrane
• Cell division – process forming two
“daughter” cell
2. Cell Division
• 1st stage – mitosis (division of the nucleus)
• 2nd stage – cytokinesis – division of
cytoplasm
• Chromosomes – contain DNA and
proteins
• Before cell division, each chromosome
replicates and consists of two “sister”
chromatids
3. Cont.
• Each pair of chromatids is attached at the
centromere, which will separate during cell
division and each new cell will have
identical copies of the old cell
• Cell cycle:
– Cell grows
– Prepares for division (replicates DNA)
– Divides into two daughter cells
– Cycle begins again
4.
5. Mitosis
• 4 phases:
• Prophase – chromatin condense into
chromosomes, centriole separates and a
spindle begins to form, the nuclear
envelope breaks down
• Metaphase – chromosomes line up
across center of cell, connected to spindle
fiber (helps separate chromosomes) at
centromere
6. Cont.
• Anaphase – mitosis: centromeres that join
the sister chromatids split, chromatids
separate and become individual
chromosomes (two groups)
• Telophase – chromosomes disperse into
tangle of dense material, nuclear envelope
re-forms around chromosome clusters,
spindle breaks apart, nucleolus is visible in
daughter nucleus.
7.
8. Cont.
• Cytokinesis – division of cytoplasm in the
M phase, cell membrane folds into cell
pinching cell into two equal parts
containing nucleus and cytoplasmic
organelles
9.
10. Cell regulating
• Cyclin – (protein) regulates the timing of
the cell cycle in eukaryotic cells
• Regulation – internal (replication) and
external (growth) are controlled by
proteins
• Cancer – uncontrolled cell growth (disease
of the cell cycle)
11. Meiosis
• Homologous – two sets of chromosomes
(set from female, set from male), two
complete sets of chromosomes and two
complete sets of genes
• Diploid – “two sets” cell that contains
homologous chromosomes (2N=8)
• Haploid – “one set” (N=4)
12. Phases of Meiosis
• Meiosis – process of reduction division in
which the number of chromosomes per
cell is cut in half through the separation of
homologous chromosomes in a diploid cell
• Divided into two distinct phases
• Meiosis I and II
13. Meiosis I
• Prophase I – each chromosome pairs with
its corresponding homologous
chromosome to form a tetrad (4
chromatids)
• Metaphase I – spindle fibers attach to the
chromosomes
• Anaphase I – fibers pull the homologous
chromosomes toward opposite ends of the
cells
14. Cont.
• Telophase I and cytokinesis – nuclear
membrane form and the cell separates
into two haploid (N) daughter cells, each
with half the number of chromosomes as
the original cells
• Crossing-over – exchange of chromatids
which results in the exchange of alleles
between homologous chromosomes and
produces new combinations of alleles
15.
16. Meiosis II
• Prophase II - two haploid (N) daughter
cells, each with half the number of
chromosomes as the original cells
• Metaphase II – chromosomes line up in a
similar way to the metaphase stage of
mitosis
• Anaphase – sister chromatids separate
and move toward opposite ends of the cell
17. Cont.
• Telophase and cytokinesis – Meiosis II
results in 4 haploid (N) daughter cells (2
chromosomes each)
18.
19. Gamete Formation
• Males – haploid gamete is the sperm
• Females – only one haploid gamete, egg
• Comparing mitosis and meiosis:
– Mitosis produces 2 genetically identical diploid
(2N) daughter cells: allows the body to grow
and replace cells
– Meiosis produces four genetically different
haploid cells: used in sexual reproduction
20. Mitosis Meiosis
Asexual Reproduction Sexual Reproduction
Produces two identical cells Produces four different cells
New cell contains a full set New cells contain half the
(diploid) of chromosomes number of chromosomes
(haploid)
DNA replication once during DNA replication once during
interphase interphase before Meiosis 1
One nuclear division Two nuclear divisions
No crossing over – no genetic Crossing over during prophase
variation 1 to ensure genetic variation
Sister chromatids pulled to Homologous chromosomes
opposite poles during move to opposite poles during
anaphase resulting in 2 anaphase 1. Centromeres hold
identical cells. The centromere sister chromatids together
splits ensuring that identical resulting in non-identical cells
chromatids are pulled to the
opposite poles
Occurs in all types of body Occurs only in sex cells
cells (egg & sperm)
Purpose: cell growth and Purpose: produce sex cells
repair