mitosis and meiosis

©2000 Timothy G. Standish

Isaiah 61:1-31 The Spirit of the Lord GOD is upon me; because the LORD hath anointed

me to preach good tidings unto the meek; he hath sent me to bind up the brokenhearted, to proclaim liberty to the captives, and the opening of the prison to them that are bound;

2 To proclaim the acceptable year of the LORD, and the day of vengeance of our God; to comfort all that mourn;

3 To appoint unto them that mourn in Zion, to give unto them beauty for ashes, the oil of joy for mourning, the garment of praise for the spirit of heaviness; that they might be called trees of righteousness, the planting of the LORD, that he might be glorified.


MitosisMitosisand Meiosisand Meiosis

Timothy G. Standish, Ph. D.


Mitosis: In The Beginning OneMitosis: In The Beginning One Most of the organisms we see started out as one cell Humans start out as a single cell, the zygote, formed by

uniting a sperm and egg The zygote divides to make approximately one trillion

cells During the process of dividing, cells become

specialized to function in the various tissues and organs of the body

Mitosis is the process of cell division in eukaryotic cells


Egg1n

Haploidnucleus

Fertilization Results In A Fertilization Results In A Diploid ZygoteDiploid Zygote

Sperm1nHaploid

nucleus


Sperm1n

Fertilization Results In A Fertilization Results In A Diploid ZygoteDiploid Zygote

Egg1n

Haploidnucleus

Haploidnucleus


From Zygote to EmbryoFrom Zygote to Embryo

Zygote2n

Zygote

2n


Cleavage



Morula



Why Cells Must DivideWhy Cells Must Divide In multi-celled organisms (like humans) cells

specialize for specific functions thus the original cells must divide to produce different kinds of cells

Cells can only take in nutrients and excrete waste products over the surface of the membrane that surrounds them. The surface to volume ratio decreases with the square of the volume (unless special accommodations are made)

2 cmSurface 24 cm2/volume 8 cm3 = 3

1 cm

Surface 6 cm2/volume 1cm3= 6


The Cell LifecycleThe 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


G1G1

MM

G2G2

SS

The Cell Life CycleThe Cell Life CycleGap 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


Stages Of MitosisStages Of MitosisDuring 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 InterphaseStages Of Interphase Interphase - The in between stage - Originally

interphase was thought to be a resting stage. Now we know that this is the stage most cells spend their time in as they do the things cells do including, if they are preparing to divide, growing and replicating their DNA

G1G1

MM

G2G2

SS

Interphase


Stages Of MitosisStages Of Mitosis Prophase - The beginning phase - DNA which was

unraveled and spread all over the nucleus is condensed and packaged

Metaphase - Middle stage - Condensed chromosomes line up along the equator of the cell

Anaphase - One copy of each chromosome moves to each pole of the cell

Telophase - End stage - New nuclear membranes are formed around the chromosomes and cytokinesis (cytoplasm division) occurs resulting in two daughter cells


Stages Of MitosisStages Of Mitosis

Interphase

Anaphase

Telophase

Metaphase Mitotic spindle

Prophase

Nucleus with un-condensed chromosomes Equator

of the cell

Condensed chromosomes

Disappearing nuclear membrane

Poles of the cell

Mother cell

Two daughter

cells

Metaphase plate


A TT AG CC

G

G C

TA

T

AG

C

C G

G C

T A

A T

Packaging DNAPackaging DNA

Histone proteins

Histoneoctomer

B DNA Helix 2 nm


A TT AG CC

G

G C

TA

T

AG

C

C G

G C

T A

A T


Histone proteins

B DNA Helix

Histoneoctomer

2 nm


A TT AG CC

G

G C

TA

T

AG

C

C G

G C

T A

A T


Histone proteins

Histoneoctomer

Nucleosome

11 nm

B DNA Helix 2 nm



A TT AC G

C G

G C

T A

A T



A TT AC G

C G

G C

T A

A T

Protein scaffold

11 nm“Beads on a string”

30 nm

Tight helical fiber

Looped Domains200 nm



G

C

A

T

Protein scaffold

Metaphase Chromosome

700 nm

11 nm

30 nm200 nm

2 nm

Looped Domains

Nucleosomes

B DNA Helix

Tight helical fiber


Replication

Chromosomes, Chromatids Chromosomes, Chromatids and Centromeresand Centromeres

Centromere

Chromosome arm

Chromosome arm

Identicalchromatid

Chromatid

Anaphase

A packaged chromosome

Two identical chromosomes


Chromosome MorphologyChromosome Morphology

Chromosome arm

Chromosome arm

Centromere

Submetacentric Acrocentric TelocentricMetacentric

q arm

p armpetite

Chromosomes can be distinguished on the basis of size and the relative location of centromeres.


Controlling The Cell CycleControlling The Cell Cycle CDC Mutants - Cell Division Cycle mutants helped elucidate

genetic control points of the cell cycle Three major checkpoints controlled by Cyclin dependant kinase

(Cdk) proteins which add phosphates to cyclin proteins changing their activity:

1 G1S - Monitors cell size and checks for DNA damage2 G2M - Ensures physiological conditions are right for division

including completion of DNA replication and any necessary repair3 M - Checks for successful formation of the mitotic spindle and

attachment to the kinetochores


p53p53


Meiosis: In The Beginning TwoMeiosis: In The Beginning Two Humans and many other complex multi-celled

organisms incorporate genetic recombination in their reproduction

Reproduction in which there is a re-mixing of the genetic material is called sexual reproduction

Two cells, a sperm and an egg, unite to form a zygote, the single cell from which the organism develops

Meiosis is the process of producing sperm and eggs (gametes)


Gametes Are HaploidGametes Are Haploid Gametes must have half the genetic material of a normal cell If the genetic material in the gametes was not halved, when they

combined the zygote would have more genetic material than the parents

Meiosis is specialized cell division resulting in cells with half the genetic material of the parents

Gametes have exactly one set of chromosomes, this state is called haploid (1n)

Regular cells have two sets of chromosomes, this state is called diploid (2n)


Stages Of MeiosisStages Of Meiosis Meiosis resembles mitosis except that it is

actually two divisions not one These divisions are called Meiosis I and Meiosis

II Meiosis I results in haploid cells with

chromosomes made up of two chromotids Meiosis II is essentially mitosis on haploid cells Stages of meiosis resemble mitosis with two

critical differences: the first in prophase I and the second in Metaphase I


Stages Of Meiosis - Meiosis IStages Of Meiosis - Meiosis I Prophase I - The beginning phase -

– DNA which was unraveled and spread all over the nucleus is condensed and packaged

– Homologous chromosomes (each made of two identical chromatids) come together and form tetrads (4 chromatids)

– Crossing over, in which chromatids within tetrads exchange genetic material, occurs

Metaphase I - Middle stage - Tetrads line up along the equator of the cell


Stages Of Meiosis - Meiosis IStages Of Meiosis - Meiosis I Anaphase I - One copy of each

chromosome still composed of two chromatids moves to each pole of the cell

Telophase I - End stage - New nuclear membranes are formed around the chromosomes and cytokinesis (cytoplasm division) occurs resulting in two haploid daughter cells


Stages Of Meiosis - Meiosis IIStages Of Meiosis - Meiosis II Prophase II - Cells do not typically go into

interphase between meiosis I and II, thus chromosomes are already condensed

Metaphase II - Chromosomes line up at the equator of the two haploid cells produced in meiosis I

Anaphase II - Chromosomes made up of two chromatids split to make chromosomes with one chromatid which migrate to the poles of the cells

Telophase II - Cytokinesis and reformation of the nuclear membrane in haploid cells each with one set of chromosomes made of one chromatid


InterphaseInterphase

Mother cell Stages Of Meiosis: Stages Of Meiosis: Meiosis IMeiosis I

Meiosis IIMeiosis II

Prophase I:Tetrad formation/

crossing over


crossing overMetaphase I Metaphase I

Telophase ITelophase I

Prophase I:Condensing

Chromosomes

Prophase I:Condensing

Chromosomes

Anaphase I Anaphase I

Telophase ITelophase I

Stages Of Meiosis: Stages Of Meiosis: Meiosis IIMeiosis II

Metaphase II Metaphase II

Anaphase II Anaphase II

Telophase II Telophase II

The products of mitosis are 2 diploid cells with identical chromosomes.

The products of meiosis are 4 haploid cells each with a unique set of chromosomes.

Prophase IIProphase II



crossing over


crossing over

Crossing OverCrossing Over

Anaphase I Anaphase I

Telophase II Telophase II

Metaphase I Metaphase I

Telophase ITelophase IBecause of crossing over, every gamete receives a unique set of genetic information.

mitosis and meiosis

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g c c g c g g c t

g c c g g c t

n zygote

t protein scaffold

mitosis cell division

cell anaphase

cell humans

cell cycle