chapter 16: cell division

Question 1

how does growth take place in plants and animals?

Answer 1

in plants:
- growth is restricted to the tips of shoots and roots
> known as growing points
> the shoot and root tips can continue to grow throughout the life of the plant

in animals:
- growth occurs throughout the body
- for multicellular organisms to grow normally
> cells must divide
- cell division that takes place during growth is called mitosis

Question 2

what is mitosis?

Answer 2

• it is a form of nuclear division that produces daughter nuclei containing the same number of chromosomes as the parent nucleus
  > the daughter nuclei are genetically identical

Question 3

what are chromosomes made of?

Answer 3

• chromosomes are made of deoxyribonucleic acid (DNA)
  > DNA stores all the information that a cell needs in order to grow and to carry out vital activities
  > this information is stored as sections of DNA or genes
• the daughter cells produced by mitosis are genetically identical to the parent cell

Question 4

why is it important to produce genetically identical daughter cells?

Answer 4

• DNA replication copies all the information stored within the chromosomes
  > this ensures that daughter cells produced by mitosis contain all the sections of DNA (genes) needed for subsequent cell division and differentiation
  > resulting daughter cells are genetically identical
• a zygote (cell) divides to form an embryo
  > the cells formed as the zygote divides must be genetically identical for the embryo to develop normally
• if an error occurs during DNA replication or mitosis
  > it will be transmitted tot he daughter cells which may lead to harmful changes to the genes and affect how the cells function
  > embryo may not develop normally
• changes in the genes during DNA replication can also cause abnormal proteins to be produced
• cells (containing abnormal proteins) may be rejected or destroyed by the body’s immune system
• in adults, mistakes made in DNA replication/ mitosis can result in an uncontrolled division of cells (cancer)
  > cancer can be fatal
  > as cells lose the ability to control the way they divide and cannot perform their normal functions

Question 4

what are the three stages of the cell cycle?

Answer 4

1. interphase or ‘resting stage’
2. mitosis, nuclear division
3. cell division, which is division of the cytoplasm, cytokinesis

Question 5

what happens during interphase?

Answer 5

• called ‘resting stage’ but cells do not actually rest
• they carry out activities such as absorbing nutrients and building up protoplasm
• at interphase, chromosomes appear as long thin threads called chromatin
  > chromatin threads cannot be distinguished individually
• during interphase, centrioles divide in an animal cell
• just before the cells divide, each chromatin thread (contains 1 DNA molecule) REPLICATES
  > producing two IDENTICAL chromatin threads ( two identical DNA molecule)
  > joined at a point called the CENTROMERE
• the two chromatids attached to the same centromere are called sister chromatids

Question 6

what are the four stages of mitosis?

Answer 6

prophase, metaphase, anaphase and telophase

Question 7

what happens during prophase?
(stages of mitosis)

Answer 7

-chromatin threads condense, coil and shorten to become chromosomes
> each chromosomes consists of two sister chromatids attached to the centromere
> chromosomes now visible under the microscope as x-shaped structures

• in an animal cell, ASTERS form around the centrioles
  > ASTERS are made of MICROTUBULES
• the two pairs of centrioles move to opposite poles of the cell
• the nucleolus and nuclear envelope disappear
• a spindle forms with the spindle fibres extending from one pole of the cell to the other

Question 8

what happens during metaphase?
(stages of mitosis)

Answer 8

• chromosomes line up along the equatorial plane of the spindle
• the centromere of each chromosome is attached on both sides to a spindle fibre

Question 9

what happens during anaphase?
(stages of mitosis)

Answer 9

• each centromere divides
• the spindle fibres shorten and pull the chromatids apart to opposite poles of the cell
  > once the chromatids are separated, they are called daughter chromosomes

Question 10

what happens during telophase?
(stages of mitosis)

Answer 10

• spindle fibres break down
• a nuclear envelope forms around the chromosomes at each pole of the cell
• a nucleolus reforms in each nucleus and the chromosomes uncoil and lengthen to become thin chromatin threads

Question 11

what happens during cytokinesis?

Answer 11

• the division of the cytoplasm
• in animal cells, FURROWS appear in the cytoplasm between the two nuclei
• furrows deepen and two identical daughter cells are finally produced

Question 12

what happens during mitosis in plant cells?

Answer 12

it is similar to animal cell except that:
- centrioles are absent in a plant cell
- cleavage of the cytoplasm does not occur during cytokinesis
> instead, a cell plate is formed between the two daughter nuclei, dividing the cell into two
> the cell plate is formed by the fusion of small fluid-filled vesicles by the Golgi apparatus

Question 13

why is mitosis important?

Answer 13

• it enables the growth of an organism
  > for multicellular organisms to grow, new cells must be produced by mitosis
• it is needed for the repair of worn-out parts of the body
• it allows asexual reproduction to occur

Question 14

what are some examples of mitosis being needed for the repair of worn out parts of the body?

Answer 14

• each time you bruh your teeth, hundreds of dead and worn-out cheek cells are being shed into your saliva
  > mitosis constantly replaces dead cells
• mitosis helps heal wounds
  > if you cut, skin produces new cells to help seal the cut
• cells in malpighian layer of the skin divide constantly by mitosis
  > replace worn out cells from upper layers of the skin

Question 15

what are some examples of mitosis being needed for the repair of worn out parts of the body?

Answer 15

• each time you bruh your teeth, hundreds of dead and worn-out cheek cells are being shed into your saliva
  > mitosis constantly replaces dead cells
• mitosis helps heal wounds
  > if you cut, skin produces new cells to help seal the cut
• cells in malpighian layer of the skin divide constantly by mitosis
  > produce new cells to replace worn-out cells from the upper layers of the skin

Question 16

what are gametes?

Answer 16

• they are reproductive cells that contain half the number of chromosomes as the normal body cells
• in sex organs, cells undergo MEIOSIS (reduction division)
  > each of the daughter cells produced contains half the number of chromosomes as the parent cell

eg. sperm is the male gamete and the ovum (egg) is the female gamete

Question 17

how do gametes differ from normal cells?

Answer 17

• the number of chromosomes in the normal body cell is called the DIPLOID NUMBER (2n)
• half the diploid number is called HAPLOID NUMBER (n)
  > in humans, 46 is the diploid number and 23 is the haploid number

Question 18

how are gametes involved in sexual reproduction?

Answer 18

• sexual reproduction involves the fusion of two gametes during fertilisation
• fertilisation occurs when the nucleus of the male gamete fuses with the nucleus of the female gamete to form a zygote
  > zygote is the fertilised egg
  > the normal diploid number of chromosomes is restored in the zygote
  > zygote then divides by mitosis to form an embryo

Question 19

what is meiosis?

Answer 19

it is a form of nuclear division that produces daughter nuclei containing half the number of chromosomes as the parent nucleus

Question 20

what happens during prophase I?
(stages of meiosis I)

Answer 20

• chromatin threads condense, coil and shorten to form chromosomes
  > each chromosome has two sister chromatids attached at the centromere
  >chromosomes now visible under the microscope
• homologous chromosomes pair along their whole length
  > synapsis
• homologous chromosomes have the same shape, genes and length
  > one homologous chromosome comes from the male parent and the other comes from the female parent
• chromatids of homologous chromosomes my cross and twist around one another
  > point where they cross over one another : chiasma
  > they strength of their coiling may cause them to break and exchange parts > CROSSING OVER
• crossing over produces new combinations of alleles along the chromosomes
• as the chromosomes shorten further, homologous chromosomes appear to repel each other
  > asters form around the centrioles which moves apart to opposite poles of the cell
  > nuclear envelope and nucleolus disappear
  > spindle fibres form

Question 21

what happens during metaphase I?
(stages of meiosis I)

Answer 21

• pairs of homologous chromosomes arrange themselves along the equatorial plane of the spindle
• the two chromosomes of each pair faces opposite poles of the cells
• each chromosome is attached to a spindle fibre

Question 22

what happens during anaphase I?
(stages of meiosis I)

Answer 22

• homologous chromosomes separate and are pulled to opposite poles of the cell as the spindle fibres shorten

Question 23

what happens during telophase I?
(stages of meiosis I)

Answer 23

• a nuclear envelope forms around the chromosomes at each pole
  > division of cytoplasm

Question 24

what happens during cytokinesis I? (stages of meiosis I)

Answer 24

the cytoplasm cleaves into 2, producing two daughter cells, each with the haploid number of chromosomes > the centrioles divide

Question 25

what happens during prophase II? (stages of meiosis II)

Answer 25

- the two pairs of centrioles move to opposite poles of the cell - the nuclear envelope disappears and the spindle fibres appears

Question 26

what happens during metaphase II? (stages of meiosis II)

Answer 26

- chromosomes arrange themselves along the equatorial plane of the spindle

Question 27

what happens during anaphase II? (stages of meiosis II)

Answer 27

- the centromeres divide - sister chromatids separate and becomes daughter chromosomes > pulled towards opposite poles of the cell

Question 28

what happens during telophase II? (stages of meiosis II)

Answer 28

- spindle fibres disappear - nuclear envelopes forms around the two daughter chromosomes at each pole > nucleolus reforms

Question 29

what happens during cytokinesis II? (stages of meiosis II)

Answer 29

- the cleavage of cytoplasm results in four daughter cells being produced > each with half the number of chromosomes as the parent cell

Question 30

why is meiosis important?

Answer 30

1. produces haploid gametes - when the nucleus of a male gamete fuses with the nucleus of a female gamete > the diploid number of chromosomes is restored in the zygote > maintains a normal diploid number of chromosomes in the species 2. meiosis results in variations in the gametes produced - variations occur due to crossing over and independent assortment of chromosomes independent assortment of chromosomes: one chromosome from each pair can combine with either chromosome of the other pair > results in four different gametes being produced from two chromosomes - since fertilisation is random- sperm fuses with any egg > variation in the gametes produces variations in the offspring - variation increases the chances of survival of species during changes in environment - when environmental conditions change, nature will 'select' those organisms with favourable conditions to survive, while others die > those that survive can pass on their favourable genes to their offspring > species live on

Question 31

what are the differences between mitosis and meiosis?

Answer 31

mitosis: daughter cells contain same number of chromosomes as the parent cell meiosis: daughter cells contain half the number of chromosomes as the parent cell mitosis: no pairing of homologous chromosomes meiosis: homologous chromosomes pair at prophase I mitosis: no crossing over meiosis: crossing over may occur mitosis: daughter cells are genetically identical to the parent cell meiosis: theyre not identical mitosis: 2 daughter cells produced from one parent cell meiosis: 4 daughter cells produced from one parent cell mitosis: involves 1 nuclear division meiosis: involves 2 nuclear division mitosis: occurs in normal body cells during growth or repair of body parts meiosis: occurs in the gonads during gamete formation