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Cell Division & Cell Cycle. Comparing Mitosis and Meiosis. Cell Division. Cell Division - when a parent cell divides into two daughter cells. All body cells are somatic cells and divide by mitosis Sperm and egg, which are gametes (sex cells) divide by meiosis. Cell Division.

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Cell division cell cycle

Cell Division & Cell Cycle

Comparing Mitosis and Meiosis

Cell division
Cell Division

  • Cell Division-when a parent cell divides into two daughter cells.

  • All body cells are somaticcells and divide by mitosis

  • Sperm and egg, which are gametes (sex cells) divide by meiosis

Cell division1
Cell Division

  • Mitosis-the nucleus of the cell divides equally

Parent Cell

Two identical daughter cells

Eukaryotic cell cycle
Eukaryotic Cell Cycle

Period between cell division

DNA is Copied

Organelles form&Cells prepares to Divide

Synthesis phase

Cells Mature

Gap 1 phase

Gap 2 phase

Daughter Cells

Cell nucleus divides equally

Cytoplasm Divides

Cell cycle
Cell Cycle

Remember: I P M A T C

  • I = Interphase (G1, S, G2)

  • P = Prophase

  • M= Metaphase

  • A= Anaphase

  • T= Telophase

  • C= Cytokinesis

Interphase part 1 of cell cycle
Interphase: Part 1 of Cell Cycle

  • Interphase-cell prepares for cell division.

  • Genetic Material (DNA)-is in the form of chromatin-uncoiled chromosomes

  • Has 3 phases: G1, S, G2

Eukaryotic chromosomes
Eukaryotic Chromosomes

  • Eukaryotecells store genetic info.in tightly coiled DNA segments orchromosomes

  • Human body cells =46 chromosomes Chromosomes can’t be seen when cells aren’t dividingand are called chromatin

  • Chromatin-uncoiled chromosomes

Chromosome pictures
Chromosome Pictures

  • Duplicated chromosomes are called sisterchromatids & are held together by the centromere-found at the center of the chromosome

Sister Chromatids

Mitosis part 2 of cell cycle
Mitosis: Part 2 Of Cell Cycle

  • Mitosis-is equal division of the nucleus of eukaryote cells

    Has four stages:

  • P rophase

  • Metaphase

  • Anaphase

  • Telophase


  • Prophase-first and longest phase of mitosis

  • Nucleolus and nuclear envelope will disappear

  • Chromosomes are now visible and are attached to the spindle

  • The Spindle-(a fan-like structure that pulls the chromosomes apart) appears.

  • Centrioles-(two tiny structures in the cytoplasm that travel toward opposite sides of the nucleus) in animal cells help to form the spindle


  • Second, shortest phase of mitosis; lasts a few minutes

  • Chromosomes are now lined up at the equator or center of the cell

Equator of Cell

Pole of the Cell


Spindle is pulling the sister chromatids apart


sister chromatids

Sister Chromatids being separated


  • Sister chromatids are at opposite poles

  • Spindle breaks apart

  • Nuclear envelope forms around each set of sister chromatids

  • Nucleolus reappears

  • Chromosomes reappear as chromatin


  • Means division of the cytoplasm

  • Division of cell into two, identical halves called daughter cells

  • In plant cells, cell plateforms at the equator to divide cell

  • In animal cells, cleavage furrow forms to split cell

Identical daughter cells
Identical Daughter Cells

Mitosis-goes from diploid to diploid

What is the 2n or diploid chromosomenumber in humans?





Chromosome number is maintained or is the same (Diploid), but cells smaller than parent cell

Meiosis formation of gametes eggs sperm
MeiosisFormation of Gametes(Eggs & Sperm)

Facts about meiosis
Facts About Meiosis

  • Interphase occurs before meiosis;chromosome replicationoccurs

  • Twomeiotic divisions-Meiosis I and Meiosis II

  • Called Reduction Division-chromosome # is reduced by ½ (4623)

  • Original cell is diploid (2n)=46

  • Four daughter cellsproduced at the end of meiosis are monoploid (n) or haploid (chromosomes reduced by½)

Facts about meiosis1
Facts About Meiosis

  • Daughter cellscontain half (23) the number of chromosomesas the original cell (46)

  • Meiosis producesgametes(eggs & sperm)

  • Occurs in the testes in males (Spermatogenesis)

  • Occurs in the ovaries in females (Oogenesis)

Why do we need meiosis
Why Do we Need Meiosis?

  • It is the fundamental basis of sexual reproduction

  • Two haploid (n) gametes are brought together through fertilization to form a diploid (2n) zygote

Meiosis forms haploid gametes

from mom

from dad




meiosis reduces

genetic content

Meiosis Forms Haploid Gametes

  • Meiosis must reduce the chromosome number by half

  • Fertilization then restores the 2n number

The right number!

Prophase i
Prophase I

  • Late prophase

  • Chromosomes condense.

  • Spindle forms.

  • Nuclear envelope fragments.

  • Early prophase

  • Homologs pair.

  • Crossing over occurs.

Crossing over

  • Homologous chromosomes in a tetrad cross over each other

  • Pieces of chromosomes or genes are exchanged

  • Produces Genetic recombination in the offspring

Metaphase i
Metaphase I

Homologous pairs of chromosomes align along the equator of the cell

Anaphase i
Anaphase I

Homologs separate and move to opposite poles.

Sister chromatids remain

attached at their centromeres.

Telophase i
Telophase I

Nuclear envelopes reassemble.

Spindle disappears.

Cytokinesis divides cell into two.

Meiosis ii

Sister chromatids carry

identical genetic


Meiosis II produces gametes with

one copy of each chromosome and thus one copy of each gene.

Meiosis II

Only one homolog of each chromosome is present in the cell.

Gene X

Meiosis ii reducing chromosome number
Meiosis II: Reducing Chromosome Number

Prophase II

Metaphase II

Telophase II

Anaphase II

4 Identical haploid cells

Prophase ii
Prophase II

Nuclear envelope fragments.

Spindle forms.

Metaphase ii
Metaphase II

Chromosomes align

along equator of cell.

Anaphase ii
Anaphase II



Sister chromatids separate and move to opposite poles.

Telophase ii
Telophase II

Nuclear envelope assembles.

Chromosomes decondense.

Spindle disappears.

Cytokinesis divides cell into two.

Results of meiosis
Results of Meiosis

Gametes (egg & sperm) form

Four haploid cells with one copy of each chromosome

One allele of each gene

Different combinations of alleles for different genes along the chromosome


  • Occurs in the testes

  • Two divisions produce 4 spermatids

  • Spermatids mature into sperm

  • Men produce about 250,000,000 sperm per day


  • Occurs in the ovaries

  • Two divisions produce 3 polar bodies that die and 1 functional egg

  • Polar bodies die because of unequal division of cytoplasm

  • Immature egg is called an oocyte

  • Starting at puberty, one oocyte matures into an ovum (egg) every 28 days


  • A picture of the chromosomes from a human cell arranged in pairs by size

  • First 22 pairs are called autosomes

  • Last pair are the sex chromosomes

  • XX female or XY male


  • When chromosomes fail to separate during Anaphase I or Anaphase II of meiosis

  • Chromosome number can be more (47) or less (45), than the normal chromosome number (46)

  • Example: Down’s Syndrome-has extra chromosome on #21 also called Trisomy 21


1. If a cat has 38 chromosomes in each of

its body cells, how many chromosomes will

be in each daughter cell after mitosis?

F 11

G 19

H 38

J 76


2. If a cat has 38 chromosomes in each of

its cells, how many chromosomes will

be in each gamete after meiosis?

F 11

G 19

H 38

J 76


3. Which of these symbols represents the

normal karyotype of a human female?






4. This human karyotype is

unusual because

chromosome set —

A 5 has chromosomes of

different shapes

B 10 is missing genetic


C 14 has enlarged


D 21 has extra genetic


Unusual Human Karyotype


5. The diagram represents the chromosomes of a person with a genetic disorder caused by nondisjunction, in which the chromosomes fail to separate properly. Which chromosome set displays nondisjunction?

F 2

G 8

H 21

J 23