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Cell Division & Its Regulation: (Mitosis & Meiosis) (Outline) • Why do cells divide? • New Terms: gamete, zygote, somatic cells, germ cells, haploid, and diploid. • Types of eukaryotic cell division- Mitosis & Meiosis • Cell division of somatic cells and germ line cells. • Comparison of Mitosis & Meiosis in terms of # of cell divisions, # of daughter
cells, DNA content in comparison to mother cell, haploid/diploid state of daughter cells, genetic sameness or variability of daughter and mother cells.
• Cell cycle and its phases; Interphase and the Mitotic phase and events and outcomes of subphases of Interphase: G1, S, G2
• State of chromatin in non-dividing and dividing cells: looses chromatin and condensed chromosome.
• The Mitotic phases: (a) Mitosis: Prophase, Metaphase, Anaphase, Telophase, and (b) Cytokinesis.
• Cytokinesis in animal and plant cells. • Cell Cycle control checkpoints: G1, G2, & M. Cancer cells. • What is a karyotype and when is it used? Lab • Meiosis and events and outcomes of its two divisions. • Sources of variability in sexually reproducing organisms
Sperm cell (haploid)
Nuclei containing DNA
Egg cell (haploid)
Fertilized egg with DNA from both parents
Embryo cells with copies of same inherited
DNA Offspring with traits inherited from both parents
Zygote (diploid)
Eukaryotic multi-cellular organisms • From gametes -sperm or egg (haploid) • Fertilization of sperm and egg produces zygote
(diploid) • Somatic cells- Body cells of multi-cellular
organism (diploid)
Cell Division & Its Regulation
Key Roles of Cell Division • . • . Purpose: distribution of genetic material to
daughter cells
Types of cell division
Two types in eukaryotic cells: Mitosis produces genetic sameness (asexual reproduction) Meiosis produces genetic variability
(sexual reproduction)
Eukaryotic Cell division 1. Asexual reproduction (Identical cells) - Unicellular/ Amoeba - Some multi-cellular
eukaryotes plants and some animals like hydra, by budding cells
Eukaryotic Cell division ( cont’d) 2. Sexual reproduction (gametes, non-identical
cells- for genetic variety of offspring)
Most multi-cellular organisms have both asexual and sexual reproduction
Cell Reproduction in Humans Somatic cells (sameness)
Germ cells of the gonads (variability)
Mitosis produces 2 genetically identical cells
Meiosis produces 4 genetically non-identical cells each with ½ the number of chromosomes Mitosis and
development
Multicellular diploid adults
(2n = 46)
Diploid zygote
(2n = 46) 2n
Meiosis Fertilization
Egg cell
Sperm cell
n
Haploid gametes (n = 23)
n
Cell Division • One mother cell divides into two identical
cells following an ordered sequence of events (Cell Cycle)
• Summary of event of dividing cells • Replicate the genetic material • Manufacture additional cellular content • Divide the nucleus • Separate the cytoplasm
Interphase with gaps for growth Mitosis- division of the nucleus Cytokinesis- division of the cytoplasm www.cellsalive.com
Cell Cycle Phases
Phases of cell Cycle
Interphase Chromosomes duplicate and cell parts are made Mitosis Duplicated chromosomes are evenly distributed
into two daughter nuclei
Overall Comparison of Mitosis and Meiosis
State of DNA inside a living cell In a non-dividing cell- DNA (2-3 m) is coiled as
Chromatin (DNA + proteins/histones)
In a dividing cell- chromatin condenses to form chromosomes (Chromatin + scaffold proteins)
State of DNA inside a living cell
http://www.biostudio.com/demo_freeman_dna_coiling.htm
In a non dividing and in a dividing cell • Packaging of long strands of DNA in small
nucleus (loose chromatin: non-dividing). • Condensed chromosomes in a dividing cell.
• Chromosomes are visible only when the cell is in
the process of dividing
Figure 2.3
Replication of Chromosomes
Chromosomes are replicated during S phase prior to mitosis
The result is two sister
chromatids held together at the centromere
The Mitotic Spindle
An apparatus of microtubules that controls chromosome movement during mitosis
Arises from the centrosome
Figure 2.3
Mitosis Used for growth, repair, and replacement
Consists of a single division that produces two identical daughter cells
A continuous process divided into 4 sub-phases
- Prophase - Metaphase - Anaphase - Telophase
- Centrosome- a pair of centrioles, microtubule organizing center (MOC).
- Spindle fibers- mirotubules (tubulin) - Nuclear membrane - Nucleolus - Loose chromatin - Condensed chromosome –two sister
chromatids held by centromere
www.cellsalive.com /mitosis
http://www.biology.arizona.edu/cell_bio/tutorials/cell_cycle/cells3.html
Mitosis in a fish blastula
Mitosis in an onion root
Cytokinesis in animal and plant cells
Cleavage furrow
SEM
140
×
Daughter cells
Cleavage furrow Contracting ring of microfilaments
TEM
7,
500 ×
Cell plate forming
Wall of parent cell
Daughter nucleus
Cell wall New cell wall
Vesicles containing cell wall material
Cell plate Daughter cells
Cytokinesis Animal cells Plant Cells
Cleavage furrow Cell plate
Microfilament (actin) and myosin contracting ring
Golgi-derived vesicles
Frequency of cell division varies with the type of cell
• Very often Skin cells Bone marrow Lining of stomach and intestines • Sometimes Liver cells • Do not divide in mature animal Nerve cells
The Cell Cycle Control System The sequential events of the cell cycle
are directed by a distinct cell cycle control system of regulatory proteins
G2 checkpoint
Control system
M checkpoint
G1 checkpoint
G1
S
G2 M
Three major checkpoints are found in the G1, G2, and M phases
www.cellsalive.com
The Cell Cycle Control
• Progression through cell cycle is controlled by regulatory proteins
Density-dependent inhibition of cell division
Anchorage dependence of cells
Mortality of cells- limited numbers of cell divisions
Behavior of normal and cancer cells in cell
culture
Tumor
Glandular tissue
A tumor grows from a single cancer cell.
Cancer cells invade neighboring tissue.
Cancer cells spread through lymph and blood vessels to other parts of the body.
Lymph vessels
Blood vessel
Figure 8.10
Cancer cells - grow out of cell cycle control
- form masses called tumors - malignant tumors spread and invade other tissues
MEIOSIS AND CROSSING OVER Genetic Variability
Chromosomes are matched in homologous pairs
– The somatic (body) cells of each species contain a specific number of chromosomes
– For example human cells have 46 chromosomes making up 23 pairs of homologous chromosomes
Chromosomes are matched in homologous pairs which carry genes for the same characteristics at the same place, or locus
Chromosomes
Centromere
Sister chromatids
Figure 8.12
Genetic Variability
Offspring acquire genes from parents by inheriting chromosomes
Meiosis
• Two consecutive cell divisions, meiosis I and meiosis II
• Results in four daughter cells • Each final daughter cell has only half as
many chromosomes as the parent cell
Meiosis animation
• http://www.sumanasinc.com/webcontent/anisamples/majorsbiology/meiosis.html
• www.cellsalive.com / Meiosis
• Stages of Meiosis http://highered.mcgraw-
hill.com/sites/0072437316/student_view0/chapter12/animations.html#
The results of crossing over during meiosis
Sources of variation in sexually reproducing organisms
1. Independent assortment of chromosomes during meiosis
2. Crossing over during meiosis 3. Random fertilization
The results of independent assortment of homologous chromosome pairs at the metaphase plate in meiosis I
Chromosome Abnormalities involving chromosome numbers are caused by non-disjunction of
- Homologous chromosomes during Meiosis I - Sister chromatids during Meiosis II
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