Chapter 2.6-Cell division, cell diversity and cellular organisation

Question 1

What is mitosis?

Answer 1

type of nuclear division that produces daughter cells genetically identical to each other and to the parent cell

Question 2

What is cytokinesis?

Answer 2

cytoplasmic division following nuclear division, resulting in two new daughter cells

Question 3

What is interphase?

Answer 3

• phase of cell cycle where the cell is not dividing; it is subdivided into growth and synthesis phases
• G1, S and G2 phase

Question 4

What happens at G1(gap 1) phase of the cell cycle of interphase?

Answer 4

• cell grows and increases in size
• transcriptions of genes to make RNA occurs
• organelles duplicate
• biosynthesis

Question 5

What happens in the S (synthesis) phase of interphase?

Answer 5

• once the cell enters this phase, it is committed to finishing the cell cycle
• DNA replicates
• phase is rapid because the espoused DNA base pairs are more susceptible to mutagenic agents, this reduces the chances of spontaneous mutations happening

Question 6

What happens in the G2 (gap 2) phase of interphase?

Answer 6

cell grows

Question 7

What happens in the G0 (gap 0) phase of interphase?

Answer 7

• apoptosis (programmed cell death), differentiation or senescence
• some type of cells, such as neurones, remain in this phase for a very long time or indefinitely

Question 8

What are the purpose of checkpoints in the cell-cycle?

Answer 8

• prevent uncontrolled division that would lead to tumours (cancer)
• detect and repair damage to DNA (e.g. caused by UV light)
• the cycle cannot be reversed
• the DNA is only duplicated once during the cell cycle

Question 9

Where are the two main checkpoints in the cell cycle?

Answer 9

• G2/M checkpoint

- G1/S checkpoint (restriction point)

Question 10

What happens at the G2/M checkpoint?

Answer 10

chemicals stimulate spindle-forming proteins

• checkpoint chemical triggers condensation of chromatin
• metaphase checkpoint ensures proper separation of sister chromatids

Question 11

What happens at the G1/S checkpoint?

Answer 11

• ensures that the cell is ready to enter S phase

- some cells (triggered by a checkpoint chemical enter G0)

Question 12

What does the p53 gene do in the cell cycle?

Answer 12

• triggers the two main checkpoints in the regulation of the cell cycle
• it is known as the tumour suppressor gene

Question 13

Why do living organisms need to produce genetically identical daughter cells by mitosis?

Answer 13

• Asexual reproduction: single-celled protoctists such as Amoeba and Paramecium divide by mitosis to produce new individuals. Other organisms which do this are strawberry’s, fungi and some female sharks kept in captivity without any males have produced females identical to themselves
• Growth: all multicellular organisms grow by producing more cells that are genetically identical to each other and to the parent cell from which they arose by mitosis.
• Tissue repair: wounds heal when growth factors, secreted by platelets and macrophages (white blood cells) and damaged cells of the blood-vessels walls, stimulate proliferation of endothelial and smooth muscle cells to repair damaged blood vessels

Question 14

What is the anagram to remember the stages of mitosis?

Answer 14

• Peas Make Awful Tarts

- prophase, metaphase, anaphase and telophase

Question 15

What happens at prophase in mitosis?

Answer 15

• chromosomes that have replicated during the S phase of interphase and consist of two identical sister chromatids, now shorten and thicken as the DNA supercoils.
• nuclear envelope breaks down
• centrioles divide and the two new daughter centrioles move to opposite poles of the cell
• cytoskeleton protein (tubulin) threads form a spindle between these centrioles.

Question 16

What happens at the metaphase in mitosis?

Answer 16

• the pair of chromatids attach to the spindle threads at the equator region
• they attack by their centromeres

Question 17

What happens at the anaphase in mitosis?

Answer 17

• the centromere of each pair of chromatids splits
• Motor proteins, walking along the tubular threads, pull each sister chromatid of a pair, in opposite directions, towards opposite poles
• Because their centromere goes first, the chromatids, now called chromosomes, assume a V shape.

Question 18

What happens at the telophase in mitosis?

Answer 18

• the separated chromosomes reach the poles
• a new nuclear envelope forms around each set of chromosomes
• The cell now contains two nuclei each genetically identical to each other and to the parent cell from which they arose.

Question 19

What happens during cytokinesis in animal cells?

Answer 19

-plasma membrane folds inwards and ‘nips in’ the cytoplasm

-

Question 20

What happens during cytokinesis in plant cells?

Answer 20

• end plate forms where the equator of the spindle was

- new plasma membrane and cellulose cell-wall material are laid down on either side along this end plate

Question 21

What is meiosis?

Answer 21

type of nuclear division that results in the formation of cells containing half the number of chromosomes of the parent cell

Question 22

What are diploid cells?

Answer 22

• body cells (somatic/autosomal)

- contain 2 sets of chromosomes

Question 23

What are haploid cells?

Answer 23

• sex cells (gametes)

- having only one set of chromosomes; represented by the symbol ‘n’

Question 24

What happens during fertilisation?

Answer 24

two haploid cells fuse therefore the cell then has the correct number of chromosomes (2n)

Question 25

What are homologous chromosomes?

Answer 25

matching chromosomes, containing the same genes at the same place (loci). They may contain different alleles for some of the genes

Question 26

What happens at prophase 1 of meiosis?

Answer 26

• chromatin condenses and each chromosome supercoils.
• nucelar envelope breaks down, and spindle threads of tubular protein form from the centriole in animal cells
• the chromosomes come together in their homologous pairs
• each member of the pair consists of two chromatids
• crossing over occurs when non-sister chromatids wrap around each other and may swap section so that alleles are shuffled

Question 27

What happens at metaphase 1 of meiosis?

Answer 27

• the pairs of homologous chromosomes, still in their crossed state, attach along the equator of the spindle
• Each attaches to a spindle thread by its centromere
• the homologous pairs are arranged randomnly , with the members of each pair facing opposite poles of the cell. This is called independent assortment
• the way that they line up in metaphase determines how they will segregate independently when pulled apart during anaphase.

Question 28

What happens at anaphase 1 of meiosis?

Answer 28

• member of each pair of homologous chromosomes are pulled apart by motor proteins that drag them along the tubular threads of the spindle
• the centromeres do not divide, and each chromosome consists of two chromatids
• the crossed-over areas separate from each other, resulting in swapped areas of chromosomes and allele shuffling

Question 29

What happens at telophase 1 of meiosis?

Answer 29

• two nuclear envelopes form around each set of chromosomes, and the cell divides by cytokinesis. There is a short interphase when the chromosomes uncoil.
• each new nucleus contains half the original number of chromosomes, but each chromosome consists of two chromatids
• plant cells go straight from anaphase 1 into prophase 2.

Question 30

What happens at prophase 2 of meiosis?

Answer 30

• if nuclear envelopes have reformed, they now break down
• the chromosomes coil and condense, each one consisting of two chromatids
• the chromatids of each chromosome are no longer identical, due to crossing over in prophase 1
• spindle forms

Question 31

What happens at metaphase 2 of meiosis?

Answer 31

• chromosomes attach, by their centromere, to the equator of the spindle
• the chromatids of each chromosome are randomly arranged
• the way that they are arranged will determine how the chromatids separate during anaphase

Question 32

What happens at anaphase 2 of meiosis?

Answer 32

• centromeres divide
• chromatids of each chromosome are pulled apart by motor proteins that drag them along the tubular threads of the spindle, towards opposite poles
• the chromatids are therefore randomly segregated

Question 33

What happens at telophase 2 of meiosis?

Answer 33

• nuclear envelopes form around each of the 4 haploid nuclei
• in animals, the two cells now divide to give 4 haploid cells
• in plants, a tetrad of four haploid cells is formed

Question 34

How does meiosis produce genetic variation?

Answer 34

• crossing over during prophase 1 shuffle alleles
• independent assortment of chromosomes in anaphase 1 leads to random distribution of maternal and paternal chromosomes of each pair
• independent assortment of chromatids in anaphase 2 leads to further random distribution of genetic material
• haploid gametes are produced, which can undergo random fusion with gametes derived from another organism of the same species

Question 35

What is differentiation?

Answer 35

process by which stem cells become specialised into different types of cell

Question 36

What are erythrocytes?

Answer 36

red blood cells that carry oxygen from the lungs to respiring cells

Question 37

How are erythrocytes adapted to carry out their function?

Answer 37

• very small so have a large SA:V ratio. This means that oxygen can diffuse across their membranes and easily reach all regions inside the cell. Biconcave shape
• flexible, well developed cytoskeleton allows the erythrocytes to change shape so that they can twist and turn, as they travel through very narrow capillaries.
• most of the organelles are lost at differentiation, so they have no nucleus, mitochondria or endoplasmic reticulum. This means there is more room for the many haemoglobin molecules housed inside them

Question 38

What are neutrophils?

Answer 38

type of white blood cell that is phagocytic (can ingest microbes and small particles)

Question 39

Where are both neutrophils and erythrocytes derived from in the body?

Answer 39

stem cells in the bone marrow

Question 40

How do neutrophils work?

Answer 40

• attracted to and travel towards infection sites by chemotaxis
• ingest bacteria and some fungi by phagocytosis
• they are about twice the size of erythrocytes, and each neutrophil contains a multi lobed nucleus.

Question 41

What are spermatozoa?

Answer 41

sperm cell

Question 42

How are spermatozoa specialised to carry out their function?

Answer 42

• many mitochondria carry out respiration. The ATP provides energy for the undulipodium (tail) to move and propel the cell towards the ovum
• long and thin allows them to be able to move easily
• once it reaches the ovum, enzymes are released from the acrosome (specialised lysosome). The enzyme digests the outer protective covering of the ovum, allowing the sperm head to enter the ovum
• head contains the haploid male gamete nucleus and very little cytoplasm.

Question 43

What are epithelial cells?

Answer 43

cells that constitute lining tissue

Question 44

What are the features of epithelial cells?

Answer 44

• squamous epithelial cells are flattened in shape

- many of the cells in epithelium have cilia.

Question 45

What are palisade cells?

Answer 45

closely-packed photosynthetic cells within leaves

Question 46

How are palisade cells well adapted to their function?

Answer 46

• long and cylindrical, so they can pack together quite closely but with a little space between them for air to circulate; carbon dioxide in these air spaces diffuses into the cells
• large vacuole so that the chloroplasts are positioned nearer to the periphery of the cell, reducing the diffusion distance for carbon dioxide
• contain many chloroplasts- the organelles that carry out photosynthesis
• contain cytoskeleton threads and motor proteins to move the chloroplasts- nearer to the upper surface of the leaf when sunlight intensity is low, but further down when it is high

Question 47

What is a guard cell?

Answer 47

in leaf epidermis, cells that surround stomata