2.1.6 Cell Division, Cell Diversity and Cell Differentiation

Question 1

Understand the eukaryotic cell cycle diagram

Answer 1

• M is the division phase
• interphase is divided int G₁, S and G₂
• cells may also enter G₀

Question 2

What are the two main cell cycle checkpoints?

Answer 2

• G₁ /S checkpoint: also called restriction point
• G₂ / M checkpoint

Question 3

What is the purpose of cell-cycle checkpoints?

Answer 3

• to prevent uncontrolled division that would lead to tumours
• to detect and repair damage to DNA

Question 4

What is the point of the specific order of events in the cell cycle?

Answer 4

it ensure that:

• the cycle cannot be reversed
• DNA is only duplicated once during each cycle

Question 5

Describe the M phase of the cell-cycle

Answer 5

• a checkpoint chemical triggers condensation of chromatin
• halfway through the cycle, the metaphase checkpoint ensures that the cell is ready to complete mitosis

Events within the cell:

• cell growth stops
• mitosis consisting of stages: prophase, metaphase, anaphase, telophase
• cytokinesis

Question 6

Describe the G₀ (gap 0) phase in the cell-cycle

Answer 6

• a resting phase triggered during early G₁ at the restriction point by a checkpoint chemical
• some cells e.g. epithelial cells lining the gut, do not have this phase

Events within the cell:

• the cell may undergo apoptosis, differentiation or senescence (deterioration with age)
• some cells (e.g. neurones) remain in this phase for a very long time or indefinitely

Question 7

Describe G₁ (gap 1) - the growth phase- in the cell cycle

Answer 7

• a G₁ checkpoint control mechanism ensures that the cell is ready to enter S phase and begin DNA synthesis

Events within the cell:

• cells grow and increase in size
• transcription of genes to make RNA occurs
• organelles duplicate
• biosynthesis e.g. protein synthesis, including making enzymes needed for DNA replication
• p53 gene helps control this phase

Question 8

Describe S (synthesis) phase of interphase in the cell cycle

Answer 8

• every molecule of DNA is replicated
• housekeeping genes, which are active in all types of cells are duplicated first and genes that are normally inactive in specific cell types are replicated last

Events within the cell:

• when all the chromosomes have been duplicated, each one consists of a pair of identical sister chromatids
• a rapid phase as exposed DNA base pairs are more susceptible to mutagenic agents

Question 9

Describe G₂ (gap 2) phase of interphase in the cell-cycle

Answer 9

• special chemicals ensure the cell is ready for mitosis by stimulating proteins involved in making chromosomes condense and the formation of the spindle

Events within the cell:

• cells grow

Question 10

What is the significance of mitosis in the life cycle?

Answer 10

Asexual reproduction:

• single-celled protoctists such as Amoeba and Paramecium divide by mitosis
• some plants reproduce asexually by forming new plantlets at the end of stolons
• fungi can reproduce asexually by mitosis

Growth:

• all multicellular organisms grow by producing more cells that are genetically identical to each other and to the parent cell

Tissue repair:

• wound heal when growth factors, secreted by platelets and macrophages and damaged cells of the blood vessel walls, stimulate the proliferation of endothelial and smooth muscle cells to repair damaged blood vessels

Question 11

Describe prophase in mitosis

Answer 11

• 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
• centriole divides and the two new daughter centrioles move to opposite poles of the cell
• tubulin threads form a spindle between these centrioles

Question 12

Describe metaphase in mitosis

Answer 12

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

Question 13

Describe anaphase in mitosis

Answer 13

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

Question 14

Describe telophase in mitosis

Answer 14

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

Question 15

Describe cytokinesis in plants and animal cells

Answer 15

• the cell splits into two, so that each new cell contains a nucleus

animal cells:

• the plasma membrane folds inwards

plant cells:

• an end plate forms where the equator of the spindle was and new plasma membrane and cellulose cell wall material are laid down on either side along this end plate

Question 16

What does haploid mean?

Answer 16

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

Question 17

What is the significance of meiosis in life cycles?

Answer 17

• sexual reproduction increases genetic variation as it combines genetic material from two(usually) unrelated individuals of thr same species by fertilisatiion
• genetic variation within a population increases chances of survival when environment changes
• in many organisms, body cells are diploid, but for sexual reproduction to occur, they must produce haploid gametes

Question 18

What are homologous chromosomes?

Answer 18

• matching chromosomes, containing the same genes at the same loci
• may contain different alleles for some of the genes

Question 19

Briefly describe the main stages of meiosis

Answer 19

first meiotic division:

• prophase 1
• metaphase 1
• anaphase 1
• telophase 1

second meiotic division:

• prophase 2
• metaphse 2
• anaphase 2
• telophase 2

at the end, cytokinesis may occur

Question 20

What happens in prophase I stage of meiosis?

Answer 20

• the chromatin condenses and each chromosome supercoils
• they can take up stains and be seen with a light microscope
• nuclear envelope breaks down and spindle threads of tubulin protein form from the centriole in animal cells
• the chromosomes come toghether in their homologous pairs
• each member of the pair consists of two chromatids
• crossing over occurs where non-sister chromatids wrap around each other and may swap section to shuffle alleles

Question 21

Describe metaphase 1

Answer 21

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

Question 22

Describe anaphase 1

Answer 22

• the members of each pair of homologous chromosomes are pulled apart by motor proteins that drag them along the tubulin 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 chromosome and allele shuffling

Question 23

Describe telophase 1

Answer 23

• in most animal cells, two new nuclear envelopes form around each set of chromosomes, and cell divides by cytokinesis
• there is then a short interphase when the chroomsomes uncoil
• each new nucleus contains half the original number of chromosomes, but each chromosome consists of two chromatids
• in most plant cells, the cell goes straight from anaphase 1 iinto prophase 2

Question 24

Describe prophase 2

Answer 24

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

Question 25

Describe metaphase 2

Answer 25

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

Question 26

Describe anaphase 2

Answer 26

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

Question 27

Describe telophase 2

Answer 27

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

Question 28

How does meiosis produce genetic variation?

Answer 28

• crossing over during prophase 1 shuffles 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 29

Why do single-celled organisms not require different cells?

Answer 29

• each organelle has a specific function
• single-celled organisms are small and have a larger surface area to volume ratio
• therefore, oxygen can diffuse across their plasma membrane and waste products can diffuse out via the same membrane

Question 30

Why do multicellular organisms need specialised cells?

Answer 30

• they are larger
• therefore have a smaller SA/vol ratio
• most of their cells are not in direct contact with external environment
• thus, they need specialised cells to carry out particular functions

Question 31

Describe how a stem cell is formed in multicellular eukaryotic organisms

Answer 31

• a zygote is made when an ovum is fertilised by a spermatozoon
• the two haploid nuclei fuse to give a cell with diploid nucleus
• the zygote is not specialised and all the genes in its genome are able to be expressed
• it is also able to divide by mitosis

Question 32

How do embryonic stem cells differentiate?

Answer 32

• certain genes are switched off
• and other genes may be expressed more, so that
• the proportions of different organelles differs from those of other cells
• the shape of the cell changes
• some of the contents of the cell change

Question 33

What are erythrocytes?

Answer 33

• specialised cells in mammals that carry oxygen from the lungs to respiring cells
• derived from stem cells in the bone marrow

Question 34

What are neutrophils?

Answer 34

• specialised cells in mammals that ingest invading pathogens
• derived from stem cells in the bone marrow

Question 35

How are erythrocytes adapted to carry oxygen from lungs to respiriing cells?

Answer 35

• they are very small, about 7.5 µm in diameter
• so it has a larger SA/Vol ratio
• oxygen can diffuse across their membranes and easily reach all regions inside the cell
• their biconcave shape also increases their SA/Vol ratio
• they are flexible:
• have a well-developed skeleton that allows the erythrocytes to change shape, so that they can twist and turn as they travel through narrow capillaries
• most of their organelles lost at differentiation:
• no nucleus
• no mitochondria
• no endoplasmic reticulum
• very little cytoplasm
• this provides more space for many haemoglobin molecules inside
• haemoglobin is synthesised with immature erythrocytes when they still have the organelles above

Question 36

How are neutrophils specialised to ingest pathogens?

Answer 36

• they make up about 50% of white blood cells in our body
• about twice the size of erythrocytes
• each neutrophil contains a multilobed nucleus
• attracted to and travel towards infection sites by chemotaxis
• function is to ingest bacteria and some fungi by phagocytosis

Question 37

How are spermatozoa specialised?

Answer 37

• many mitochondria to carry out aerobic respiration
• ATP provides energy for the undulipodium (tail) to move and propel the cell towards the ovum
• small, long and thin: so can move easily
• once the sperm reaches an ovum, enzymes are released from the acrosome (a specialised lysosome)
• the enzymes digest the outer protective covering of the ovum, allowing sperm to enter
• head of sperm contains haploid male gamete nucleus and very little cytoplasm

Question 38

Describe how epithelial tissues are adapted for their function

Answer 38

• epithelium is lining tissue
• found outside and inside the body
• squamous epithelial cells are flattened in shape
• many of the cells in epithelium have cilia

Question 39

Label the transverse section through a leaf

Answer 39

Question 40

Label the stomata diagram

Answer 40

Question 41

How are palisade cells well adapted for photosynthesis?

Answer 41

• long and cylindrical
• can pack together quite closely, but with little space between for air to circulate
• CO₂ in these air spaces diffuses into cells
• large vacuole
• so that chloroplasts are positioned nearer the periphery of the cell
• this reduces the diffusion distance for CO₂
• contain many chloroplasts
• contain cytoskeleton threads and motor proteins to move chloroplasts
• nearer to the upper surface of the lead when low sunlight intensity, but further down when high

Question 42

What are guard cells?

Answer 42

• a pair of specialised cells found within the lower epidermis of leaves
• do contain chloroplasts
• but cannot carry out photosynthesis, as they do not have the enzymes needed for the second stage

Question 43

How are guard cells adapted to their function?

Answer 43

• light energy is used to produce ATP
• ATP actively transports potassium ions from surrounding epidermal cells into guard cells, lowering their water potential
• water now enters the guard cells from neighbouring epidermal cells by osmosis
• guard cells swell, but at the tips, the cellulose cell wall is more flexible and is more rigid where it is thicker
• so the tips bulge and the gap between them, the stoma, enlarges
• as these stomata open, air can enter the spaces within the layer of cells beneath the palisade cells
• gaseous exchange can occur and CO₂ will diffuse into the palisade cells
• as they use it for photosynthesis, this will maintain a steep concentration gradient
• oxygen produced during photosynthesis can diffuse out of the palisade cells into air spaces and out through open stomata

Question 44

How are root hair cells adapted for their function?

Answer 44

• hair like projection
• greatly increases surface area for absorption of water and mineral ions, e.g. nitrates, from the soil
• mineral ions are actively transported into root hair cells, lowering the water potential within them
• this causes water to follow by osmosis, down the water-potential gradient
• root hair cells have special carrier proteins in the plasma membranes in order to actively transport the mineral ions in
• cells will produce ATP, needed for active transport

Question 45

What is a tissue?

Answer 45

• a group of cells that work together to perform a specific function/set of functions

Question 46

What are the four main tissue tyoes in the body?

Answer 46

• epithelial
• connective tissues: hold structures together and provide support
• muscle tissue: cells specialised to contract and cause movement
• nervous tissue: cells specialised to conduct electrical impulses

Question 47

What are the characteristics of epithelials tissue?

Answer 47

• epithelial tissue made up of almost entirely of cells
• cells very close to each other and form continuous sheets
• adjacent cells are bound together by lateral contracts
• no blood vessels within epithelial tissue
• cells receive nutrients by diffusion from tissue fluid in the underlying connective tissue
• some have smooth surfaces, some have cilia or microvilli
• they have short cell cycles and divide up to two to three times a day to replace worn or damaged tissue
• specialised for protection, absorption, filtration, excretion and secretion

Question 48

What does connective tiissue consist of?

Answer 48

• non-living extracellular matrix containing proteins (collagen and elastin) and polysaccharides (such as hyaluronic acid)
• this matrix separates living cells within the tissue and enables it to withstand forces

Question 49

What are some examples of connective tissue?

Answer 49

• blood
• bone
• cartilage
• tendons
• ligaments

Question 50

How is cartilage made?

Answer 50

• immature cells in cartilage are called chondroblasts
• they divide by mitosis and secrete the extracellular matrix
• once the matrix has been synthesised, chondroblasts become mature, less active chondrocytes, which maintain the matrix

Question 51

What are the three types of cartilage?

Answer 51

• hyaline cartilage:
• forms the embryonic skeleton
• covers the ends of long bones in adults, joins ribs in the sternum
• found in nose, trachea and larynx
• fibrous cartilage:
• occurs in discs between vertebrae in the spine and in the knee joint
• elastic cartilage:
• makes up the outer ear (pinna) and the epiglottis

Question 52

Describe muscle tissue

Answer 52

• well vascularised (has many blood vessels)
• muscles cells are called fibres, which are elongated
• they contain organelles called myofilaments made up of proteins actin and myosin

Question 53

What are the functions of the three types of muscle?

Answer 53

• skeletal muscles:
• packaged by connective tissue sheets, joined to bones by tendons
• when they contract, they cause bones to move
• cardiac muscle:
• make up the walls of the heart and allows hear to beat and pump blood
• smooth muscle:
• occurs in walls of intestine, blood vessels, uterus and uterine tracts
• propels substances along these tracts

Question 54

Describe epidermal tissue in plants?

Answer 54

• consists of flattened cells
• apart from the guard cells, they lack chloroplasts and form a protective covering over leaves, stems and roots
• some epidermal cells have walls impregnated with a waxy substance, forming a cuticle
• this helps to reduce water loss

Question 55

What is vascular tissue in plants and give the two types

Answer 55

• tissue that is concerned with transport
• xylem:
• xylem vessels carry water and minerals from roots to all parts of the plant
• phloem:
• sieve tubes transfer the products of photosynthesis in solution, from leaves to part of the plants that do not photosynthesise

Question 56

What is meristematic tissue?

Answer 56

• contains stem cells
• other plant tissues are derived from this tissue by cell differentiation
• found at root and shoot tips and in cambium of vascular bundles
• these areas are called meristems

Question 57

Describe the features of the cells in meristems

Answer 57

• have thin walls containing very little cellulose
• do not have chloroplasts
• do not have large vacuole
• can divide by mitosis and differentiate into other types of cells

Question 58

How do cambium cells differentiate into xylem vessels?

Answer 58

• lignin is deposited in their cell walls to reinforce and waterproof them
• but this also kills the cells
• ends of cells breaks down so that xylem forms continuous columns with wide lumens to carry water and dissolved minerals

Question 59

How do other cambium cells differentiate into phloem sieve tubes or companion cells?

Answer 59

• sieve tubes lose most of their organelles and sieve plates develop between them
• companion cells retain their organelles and continue metabolic functions to provide ATP for active loading of sugars into the sieve tubes

Question 60

What are some examples of plant organs and their functions?

Answer 60

Question 61

What are the organ systems in animals and their functions?

Answer 61

Question 62

What are some characteristics of stem cells?

Answer 62

• undifferentiated cells, capable of becoming any cell types in an organism
• pluripotent
• able to express all their genes
• can divide by mitosis and provide more cell that can then differentiate into specialised cells for growth and tissue repair

Question 63

What are some sources of stem cells?

Answer 63

• embryonic stem cells:
• present in an early embryo
• stem cells in umbilical-cord blood
• adult stem cells:
• found in developed tissues, such as blood, brain, muscle etc
• acts as a repair system
• induced pluripotent stem cells:
• developed in labs by reprogramming differentiated cells to switch on certain genes and become undifferentiated

Question 64

What are some potential uses of stem cells in research and medicine?

Answer 64

• bone-marrow transplants
• drug research
• developmental biology
• repair of damaged tissue or replacement of lost tissues

Question 65

How can stem cells be used in bone-marrow transplants?

Answer 65

• treats diseases of the blood and immune system
• also used to restore the patient’s blood system after cancer treatment

Question 66

How can stem cells be used in drug research?

Answer 66

• stem cells can be made to develop into particular types of human tissue
• new drugs can then be tested first on these tissues, rather than on animal tissue

Question 67

How can stem cells be used in developmental biology?

Answer 67

• study how these cells develop to make particular cell types and learn how each cell type functions and see what goes wrong when they are diseased
• try to find out if they can extend the capacity that embryos have for growth and tissue repair, into later life

Question 68

How are stem cells used to repair damaged tissue or replacement of lost tissues?

Answer 68

• stem cells have been used to treat mice with type 1 diabetes by programming iPS cells to become pancreatic beta cells
• bone-marrow stem cells can be made into hepatocytes and could be used to treat liver disease
• stem cells directed to become nerve tissue could be used to treat Alzheimers and Parkinsons
• stem cells can be used to populate a bioscaffold of an organ, then grown into specific organs - regenerative medicine - if iPS cells are obtained from the patients, immunosuppressants are not needed