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Cellular Reproductionand the Cell Cycle
Mitosis
Why do cells divide?
GrowthDevelopmentRepair.
Cell division
All complex organisms originated from a single fertilised egg
Every cell in your body started here, through cell division the numbers are increased
Cells then specialise and change into their various roles.
Essential Features of Cell Division
Transmit a complete copy of genetic information (DNA) Transmit materials necessary for
cell to survive and use genetic information.
Two Fundamental Types of Cells(Organisms)
Prokaryotes Eukaryotes.
Prokaryotic Cell
No nucleus – genetic material (DNA) in cytoplasm
No membrane-bound organelles
Cell division is called binary fission
Example: bacteria.
Prokaryotic Cell
Rod-Shaped Bacterium, E. coli, dividing by binary fission
Prokaryotic Cell
Rod-Shaped Bacterium, hemorrhagic E. coli, strain 0157:H7
Prokaryotic Cell
Eukaryotic Cell
Membrane-bound organelles, including a nucleus
Genetic material (DNA) contained within the nucleus
Cell division of somatic cells called mitotic cell division
Examples: fungi, protists, plants, animals.
Eukaryotic Cell
What is Mitotic Cell Division?
Division of somatic cells (non reproductive cells) in eukaryotic organisms
A single cell divides into two identical daughter cells◦ Maintains chromosome ploidy of cell.
Ploidy Level
Ploidy – refers to the number of pairs of chromosomes in cells
haploid – one copy of each chromosome– designated as “n”
diploid – two copies (=pair) of each chromosome
– designated as “2n”
triploid – three copies of each chromosome – designated as “3n”
Number of Chromosomes
Each species has a characteristic number of chromosomes:
◦ Prokaryotes = one chromosome
◦ Crayfish (2n) = 200 chromosomes
◦ Fruit fly (2n) = 8 chromosomes
◦ Human (2n) = 46 chromosomes
◦ Wheat (6n) = 42 chromosomes
◦ Potato (4n) = 48 chromosomes.
Number of Chromosomes
Diploid organisms receive
◦ one set of chromosomes from female parent (= maternal)
◦ one set of chromosomes from male parent (= paternal)
A “matched” pair of maternal and paternal chromosomes are called homologues
gamete (n)gamete (n)
zygote (2n)
fertilization
unreplicated chromosome
arm arm
centromere
Structure of a eukaryotic chromosome
Structure of a eukaryotic chromosome Prior to cell division:
chromosomes (DNA) are replicated (duplicated)
duplicated chromosome
◦ attached at their centromeres
◦ as long as attached, known as sister chromatids
duplicated
chromosome
sister
chromatids
Structure of a eukaryotic chromosome
metacentric submetacentric acrocentric telocentric
Structure of a eukaryotic chromosome
daughter
chromosomes
sister
chromatids
Structure of a eukaryotic chromosome
The Cell Lifecycle
The cell lifecycle is well defined and can be divided into four stages:◦ Gap 1 (G1) - The growth phase in which
most cells are found most of the time◦ Synthesis (S) - During which new DNA
is synthesized◦ Gap 2 (G2) - The period during which
no transcription or translation occurs and final preparations for division are made
◦ Mitosis - Cell division.
G1
M
G2
S
The Cell LifecycleGap 1 - Doubling
of cell size.
Regular cellular
activities.
transcription and
translation etc.
Synthesis of DNA -
Regular cell
activities cease
and a copy of all
nuclear DNA is
made
Gap 2 - Final
preparation for
division
Mitosis - Cell
division
MitosisCell Division
Mitosis
Mitosis is the process by which new body cell are produced for:◦ Growth◦ Replacing damaged or old cells
This is a complex process requiring different stages.
The Cell Lifecycle
G1 S
G2M
Stages of Mitosis
During mitosis an exact copy of the genetic material in the “mother” cell must be distributed to each “daughter” cell
Each stage of mitosis is designed to achieve equal and exact distribution of the genetic material which has been copied during the S phase of the cell cycle.
Stages of Mitosis
Interphase - The ‘in between’ stage - this is the stage most cells spend their time in doing the things that cells do and, if they are preparing to divide, growing and replicating their DNA
G1
M
G2
S
Interphase
Mitosis
Prophase
Metaphase
Anaphase
Telophase.
Prophase
The chromosomes condense
The nuclear envelope and nucleolus disappear
The centrosomes move to opposite poles
The spindle starts to form, growing out of the centrosomes towards the chromosomes.
Metaphase
Metaphase is a short resting period
the chromosomes are lined up on the equator of the cell
with the centrosomes at opposite ends and the spindle fibers attached to the centromeres
Everything is aligned for the rest of the division process to occur.
Anaphase
In anaphase, the centromeres divide at this point, each individual chromosome goes from:◦ 1 chromosome with 2
chromatids
◦ to:
◦ 2 chromosomes with one chromatid each
Then the spindle fibers contract, and the chromosomes are pulled to opposite poles, towards the centrosomes.
Telophase
In telophase the cell actually divides
The chromosomes are at the poles of the spindle
The spindle disintegrates
The nuclear envelope re-forms around the two sets of chromosomes
The cytoplasm is divided into 2 separate cells, the process of cytokinesis.
Cytokinesis
The organelles get divided up into the 2 daughter cells passively: they go with whichever cell they find themselves in
In plant cells, a new cell wall made of cellulose forms between the 2 new nuclei, about where the chromosomes lined up in metaphase◦ Cell membranes form along the surfaces of this
wall
In animal cells, a ring of actin fibers forms around the cell equator and contacts, pinching the cell in half.
Cytokinesis
Summary of Mitosis Prophase:
◦ Chromosomes condense◦ Nuclear envelope disappears◦ centrosomes move to opposite sides of the cell◦ Spindle forms and attaches to centromeres on the chromosomes
Metaphase◦ Chromosomes lined up on equator of spindle◦ centrosomes at opposite ends of cell
Anaphase◦ Centromeres divide: each 2-chromatid chromosome
becomes two 1-chromatid chromosomes◦ Chromosomes pulled to opposite poles by the spindle
Telophase◦ Chromosomes de-condense◦ Nuclear envelope reappears
Cytokinesis: the cytoplasm is divided into 2 cells.
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