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Cell Division
A. Purpose of Cell Division
 To create new cells or offspring
 Copy all of the chromosomes/genes exactly to create body cells
(mitosis) or vary the gene combinations and pass on half of the
chromosomes to form sex cells (meiosis)
B. Mitosis and Types of Asexual Reproduction in Organisms
 Also known as asexual reproduction or cloning
 Reproductive process involving only one parent resulting in
two offspring that are genetically identical
1. Vegetative Propogation/Reproduction
 Occurs in plants
 New plant is produced from an existing plant
structure
 Cutting off a branch will develop into a new plant
2. Sporulation:
 Spores (reproductive cells) are released from
mold and develop into new mold cells
3. Budding:
 Yeast cell produce offspring by forming a little
“bud” that pinches off
4. Regeneration:
 When a portion of the organism is lost but is
replaced with an identical structure
 Ex. Starfish, earthworm, lizard
5. Binary Fission:
 Single-celled organisms copy its chromosomes
and then divide producing 2 smaller offspring
C. Phases of Mitosis
Diploid (2n):
Normal number of chromosomes for a particular species
Chromosomes are paired = homologous (have the same genes)
Human diploid number = 46 (23 pairs)
Ex.
A cell contains 4 chromosomes
1. Interphase:
period between cell divisions
growth and preparation for the next mitosis
a. G1 phase: cell finished mitosis and is now growing
b. S phase: chromosomes replicate (copy) [46 to 92]
c. G2 phase: centrioles (animal cells only) and spindles
form
G1 phase:
Growth
S phase:
Chromosomes double (4 to 8)
G2 phase:
Centrioles and spindles form
2. M phase (Mitosis) = PMAT
 4 phases
a. Prophase:
 The replicated DNA or chromatin coils up to form
chromosomes
 The chromosomes find their identical match and are
joined together by a centromere
 Each chromosome in this new pair is called a sister
chromatid
 Centrioles move to opposite sides of the cell (animal
only)
 Spindles attach the centriole to each centromere and
will help separate the chromatid pairs in the next few
phases
b. Metaphase:
 Chromosomes (chromatid pairs) line up in the middle of
the cell
 Each pair lines up under each other
 Each chromatid is attached to its own spindle fiber
c. Anaphase:
 Sister chromatids separate
 Each individual chromosome moves to the opposite side
of the cell
d. Telophase:
 Reverse prophase
 Chromosomes uncoil to form chromatin
 The cell membrane pinches in
 Nucleus with nucleolus reappears
 Cytokinesis: separation of cytoplasm to form 2
new cell
i. animal cells: cell membrane pinches in
ii. plant cells: cell plate forms in the middle to
separate the 2 nuclei and cytoplasm eventually
becoming a new cell wall
C. Regulating the Cell Cycle
 Proteins/Enzymes: control cell division
 Cancer: uncontrolled cell division (mitosis)
 Caused by mutations: changes in DNA causing a
change in the protein that starts or stops cell division
 Due to: smoking, radiation (x-rays, sun), infections
(bacteria, virus), chemicals (pollutants)
D. Summary of Mitosis
 1 cell forms 2 cells
 Genetically identical, clones, asexual reproduction, mitosis
 1 diploid (2n) cell forms 2 diploid cells
 Occurs in all body cells except sex cells (gametes:
egg/sperm)
E. Meiosis/Sexual Reproduction
Homologous Chromosomes:
 Paired chromosomes (have genes that code for the same trait)
 One from the female parent and one from the male parent
Diploid Cells:
 Number in humans = 46 (23 pairs) half of from each parent
 Contain both homologous chromosomes
 These cells contain a complete set of genes to create traits
 All body cells are diploid except gametes
Haploid/Monoploid Cells (n):
 Cells that only contain one of the homologous chromosomes
 Cells have half the pair
 Only gametes (sex cells= egg/sperm, oocyte or ovum/spermatocyte)
 Haploid (n) number in humans = 23 (unpaired chromosomes)
1. Meiosis:
 a 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 to become haploid
 2 cell divisions
 Formation of sex cells/gametes
 Creates 4 haploid cells from 1 diploid cell
 Creates variation in gametes
 Ex. Cell with 2 chromosomes 2n = 2, n = 1
a. Meiosis I:
1. Interphase: DNA is copied
2. Prophase I: homologous chromosomes pair up to form a
tetrad
 Tetrad: 2 sets of homologous chromosomes or 4
chromatids
All have the same genes in the same position
 Crossing Over/Genetic Recombination:
homologous chromosomes exchange portions of
genetic information when in tetrad formation
o Creates 4 different chromatids
o Chromosomes #2 and #3 switch portions
3. Metaphase I: tetrads line up in the middle, not underneath
4. Anaphase I: tetrad separates and homologous
chromosomes (now varied) move to opposite ends
5. Telophase I: 2 diploid cells are formed but they are not
identical as the original diploid cell; variation in
chromosomes due to crossing-over
b. Meiosis II: second cell division to form 4 cells
1. Prophase II: no replication of chromosomes, spindle
forms, chromosomes stay exactly as they were at the end
of telophase I
2. Metaphase II: chromosomes line up in homologous pairs
the same way as in mitosis (in the middle)
3. Anaphase II: the chromatids separate to opposite sides
4. Telophase II: 4 haploid cells are formed
 Not identical to each other because of crossing over
 Ensures that no 2 sperm or egg are identical
 Cytokinesis
2. Meiosis Explains Mendel’s Results
 When Mendel crossed 2 hybrid plants it resulted in 25% of the
offspring inheriting the recessive trait
 Anaphase I explains how the segregation of alleles for the recessive
trait at random can produce recessive offspring
3. Mistakes in Meiosis:
 Can be beneficial to plants creating new varieties or desired traits
beneficial to farmers
 Most of the time it is harmful to humans
a. Nondisjuction: failure for homologous chromosome
pairs to separate properly during anaphase I
 Problems separating the tetrads
 Results in gametes with too many or too few
chromosomes
i.
Trisomy 21 = Down Syndrome:
 Extra chromosome #21 = cell with 47
ii.
Monosomy = Turner Syndrome:
 One less chromosome = cell with 45
4. Meiosis Summary:
4 haploid cell
Variations, not identical
4 sperm, 1 egg + 3 polar bodies
Diploid
Diploid
Variations
sperm
egg
sperm
polar body
All
Haploid
sperm
polar body
sperm
polar body
Comparing Mitosis to Meiosis
Mitosis
Meiosis
Asexual Reproduction
Sexual Reproduction
Cloning
Gametogenesis
Identical Cells
Cells with variation –
crossing over
All body cells
Tissue repair
Gametes Only
Sex cells (egg/sperm)
1 Diploid = 2 Diploid
1 Diploid = 4 Haploid
2n = 46
2n = 46
2n = 46
1 Cell Division
n = 23
2 Cell Divisions