CHAPTER 4: CELL DIVISION

4.1: MITOSIS

4.2: MEIOSIS

Lesson Learning Outcome

1.Describe basic principles and related

process in mitosis.

2.Describe basic principles and related

process in meiosis.

3.Compare models and processes involve in

mitosis and meiosis.

Diagram of a replicated and

condensed metaphase

eukaryotic chromosome.

(1) Chromatid – one of the

two identical parts of the

chromosome after S

phase.

(2) Centromere – the point

where the two chromatids

touch.

(3) Short arm.

(4) Long arm.

CELL DIVISION

Cell divide so that organisms can grow.

In order for organisms to grow, body cells

either have to increase their size or divide.

Most cells divide, because smaller is better

when it comes to cells.

Cells division is needed to replace damaged

or dying cells.

Mitotic spindle:

where the event of mitosis begins to form in the

cytoplasm during prophase

Centrosome:

microtubule organizing center, has two centrioles

Aster:

a radial array of short microtubules, extends from each

centrosome

Centromere:

the specialized region of the chromosome where two

sister chromatids are most closely attached

Kinetochore:

a structure of proteins attached to the centromere that

links each sister chromatid to the mitotic spindle

INTERPHASE

The period between cell divisions.

During this phase (the longest phase of the cell

cycle) the cell grows, DNA is replicated and

the centrioles divide.

Interphase: 3 subphases

G1 phase

“first gap” is a growth phase for the cell.

S phase

“synthesis” is when the cell copies its chromosomes.

G2 phase

“second gap” is a second growth phase where further

growth and preparations for division occur.

mitosis

Process, in the cell cycle, by which the

chromosomes in the cell nucleus are

separated into two identical sets of

chromosomes, each in its own nucleus.

Occurs only in eukaryotic cells.

During prophase the

nucleoli disappear and the

chromatin (DNA and

associated proteins)

condenses into discrete

chromosomes.

Each replicated

chromosome is composed of

two sister chromatids, both

containing the same genetic

information.

The sister chromatids are joined together at their

centromeres.

The mitotic spindle forms from the centrioles and

begins to elongate.

As the centrioles reach opposite ends of the cell

the spindle fibers from each of the centrioles attach

to each chromosome at a specialized protein

structure called the kinetochore.

Other spindle fibers elongate, but

instead of attaching to chromosomes, they

interact with spindle from the opposite

pole.

It is during this stage that the

tension applied by the mitotic

spindle fibers aligns all of the

chromosomes along the

metaphase plate, an imaginary

line the divides the cell in two.

This organization is necessary

to ensure that in the next phase,

when the chromosomes are

separated, each new nucleus

will receive one copy of each

chromosome.

Anaphase is the shortest

stage of mitosis. The spindle

fibers shorten during

anaphase, pulling the sister

chromatids apart towards

opposite ends of the cell.

Telophase marks the stage

where the daughter

chromosomes arrive at the

poles and the spindle fibers

begin to disperse. Two

daughter nuclei form, nuclear

envelopes are constructed

and the chromosomes

become less

condensed. Mitosis, which

describes the division of the

nucleus, is now complete.

Cytokinesis describes the

division of the cytoplasm and

while it is not a stage of mitosis

(nuclear division), but does

result in the completion of cell

division and the end of the cell

cycle.

In animal cells, cytokinesis

involves the formation of a

cleavage furrow, which pinches

the cell into two distinct

daughter cells.

MEIOSIS

Used to produce gametes (sperm and eggs).

Assures that genetic diversity is achieved during sexual

reproduction.

Consists of 2 cell divisions: Meiosis I and Meiosis II.

Meiosis starts with a diploid (2n) parent

cell that divides to make 4 haploid (n) cells.

In sexual reproduction, haploid gametes

from two different individuals combine to

produce a diploid zygote.

The resulting offspring is genetically

different from both parents.

Chromosome characteristics:

Haploid (n):

one set of chromosomes

Diploid (2n):

two sets of chromosomes

Eggs and sperm (gametes): haploid

Diploid set for humans: 2n = 46

Interphase before Meiosis: During the interphase

preceding meiosis, DNA replication takes place.

MEIOSIS

I

Prophase I:

Homologous chromosomes pair up and form

tetrads. This pairing is known as synapsis. While paired,

the homologous chromosomes exchange genetic

material in a process called crossing over. Crossing over

contributes to the genetic variation of sexual

reproduction. While all this is occurring, the nuclear

envelope and nucleoli begin to disappear. Spindle fibers

attach to the chromosomes and begin moving them to

the equatorial plate.

Metaphase I:

Homologous chromosomes, in a pair-wise fashion, have

lined up on the equatorial plate. One homologue is

positioned on each side of the equatorial plate. The

orientation is random, which means that there is a 50-

50 chance for the daughter cells to get either the

maternal or paternal homologue for each

chromosome. This is known as independent

assortment.

Anaphase I:

Chromosomes from each pair move to opposite poles of

the cell. Each chromosome still consists of two sister

chromatids.

Telophase I:

Nuclear envelopes may reform, or the cell may

immediately start meiosis II. DNA replication does NOT

take place. There are now only a haploid number of

chromosomes in each cell.

Summary of Meiosis I:

Crossing over occurs between

homologous chromosomes.

Homologous chromosomes separate

from each other and 2 haploid cells

are formed.

MEIOSIS

II

Prophase I:

Chromatin once again condenses into discrete

chromosomes. The spindle apparatus forms.

Metaphase II:

Chromosomes are lined up along the equatorial plate,

similar to metaphase in mitosis. Due to crossing over

in meiosis I, the two sister chromatids of each

chromosome are no longer genetically

identical. Microtubules from opposite poles attach

to each sister chromatid of a chromosome.

Anaphase II:

Sister chromatids separate and move toward opposite

poles as individual chromosomes.

Telophase II:

Chromosomes decondense and nuclear envelopes

reform. Meiotic division has produced 4 daughter cells,

each with a haploid set of chromosomes and each

chromosome has only one chromatid. Each of the 4

daughter cells is genetically distinct from each other and

the parent cell.

Summary of Meiosis II:

Sister chromatids separate from

each other (similar to mitosis)

and 4 haploid gamete cells are

formed.

Q & A

QUESTION 1:

1. Put the phases of mitosis in the correct order.

A. prophase, metaphase, telophase, anaphase

B. metaphase, telophase, prophase, anaphase

C. prophase, metaphase, anaphase, telophase

D. anaphase, metaphase, prophase, telophase

QUESTION 2:

2. Mitosis is completed at which stage?

A. Prophase

B. Metaphase

C. Anaphase

D. Telophase

E. Cytokinesis

QUESTION 3:

3. Chromatin is

A. the fibrous tissue that binds DNA together

B. the complex of DNA and protein that make up

the chromosomes

C. a fluorescent dye use to stain DNA

D. a chemical that triggers the replication of DNA

QUESTION 4:

4. During anaphase:

A. Chromosomes are aligned at the metaphase

plate

B. The mitotic spindle checkpoint is created

C. Microtubules being to invade the nuclear space

D. Chromosomes separate

QUESTION 5:

5. Which of the following events do NOT occur in

prophase?

A. Chromosomes are replicated

B. Nuclear membrane breaks down

C. DNA condenses to form distinct chromosomes

D. Mitotic spindle begins to form

QUESTION 6:

6. The division of cytoplasm is called

A. Telophase

B. Cytokinesis

C. Cleavage

D. Mitosis

QUESTION 7:

7. The chromosomes are replicated during

A. Mitosis

B. G1 phase

C. S phase

D. Prophase

QUESTION 8:

8. Which of the following represents the correct

order of the phases of the cell cycle?

A. G1 to S to G2 to M

B. M to S to G1 to G2

C. G1 to G2 to S to M

D. S to M to G1 to G2

QUESTION 9:

9. Each chromosome is replicated (copied) prior

to cell division. The products of this replication

are attached by a centromere and are called:

A. Homologous chromosomes

B. Sister chromosomes

C. Kinetochores

D. Sister chromatids

QUESTION 10:

10. In mitosis, chromosomes line up at the

equatorial plate during

A. Anaphase

B. Metaphase

C. Prophase

D. Telophase

QUESTION 11:

11. During which stage of meiosis do chromatids

separate completely?

A. Metaphase I

B. Anaphase I

C. Telophase II

D. Anaphase II

QUESTION 12:

12. Which event occurs in meiosis but not mitosis?

A. Chromosome condensation

B. Chromatid separation

C. Crossing over

D. Chromosome movement to poles

QUESTION 13:

13. Meiosis results in

A. 2 haploid daughter cells

B. 4 diploid daughter cells

C. 2 diploid daughter cells

D. 4 haploid daughter cells

QUESTION 14:

14. Crossing over occurs during

A. Prophase I

B. Prophase II

C. Metaphase I

D. Anaphase II

QUESTION 15:

15. A cell with a diploid number of 24 goes

through meiosis. How many chromosomes are

in each daughter cell?

A. 24

B. 12

C. 48

D. 6

QUESTION 16:

16. The 4 homologous chromatids which come

together in meiosis I is referred to as a(n)

A. Centromere

B. Kinetochore

C. Metaphase plate

D. Tetrad

QUESTION 17:

17. During which stage of the meiotic cell cycle

does DNA replication occur?

A. Interphase

B. Anaphase II

C. Metaphase

D. Prophase I

QUESTION 18:

18. During which phase of meiosis does the

nuclear envelope reform around the

chromosomes?

A. Interphase

B. Anaphase II

C. Telophase II

D. Prophase I

QUESTION 19:

19. Homologous chromosomes move towards

opposite poles of a cell during

A. Anaphase II

B. Anaphase I

C. Telophase II

D. Metaphase I

QUESTION 20:

20. Metaphase of meiosis I and meiosis II differ

in that

A. Homologues line up in meiosis I and

duplicated chromosomes line up in meiosis II

B. Chromosomes line up at the equator

C. Sister chromatids line up in meiosis I and duplicated

chromosomes line up in meiosis II

D. There are the same number of chromosomes