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OCR Biology Paper 1 > 2.6 - Cell Division > Flashcards

2.6 - Cell Division Flashcards

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1
Q

What is the cell cycle?

A

Sequence of events between one cell division and the next

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2
Q

Outline the stages of the cell cycle

A
  • Interphase is the longest phase
  • Interphase includes G1, S and G2
  • In G1 and G2 cell performs normal functions, organelles are replicated and checks are
    made to ensure replication has occurred correctly
  • DNA replicates in S phase
  • Mitosis is when nucleus divides
  • Stages of mitosis - prophase, metaphase, anaphase, telophase
  • Cytokinesis occurs - division of cytoplasm
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3
Q

Which phase usually takes the most time in the cell cycle?

A

Interphase

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4
Q

During which stage does the cell surface area to volume ratio decrease?

A

Interphase

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5
Q

Describe what happens in G1

A

G1 phase
- Cell synthesises mRNA and proteins required for DNA replication
- Some organelles copied
- Cell increases in size

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6
Q

Describes what happens in S phase

A

S phase
- Genetic material replicated (DNA replication)

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7
Q

Describes what happens in G2

A

G2 phase
- Additional growth and organelle replication
- Cytoskeleton dismantled
- DNA replication checked for errors

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8
Q

Describe what happens during G0

A

Cell leaves cell cycle due to:
- Differentiation - cell becomes specialised
- DNA damage - cell ‘dies’ (apoptosis)

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9
Q

How is the cell cycle controlled?

A

Checkpoints at end of each stage of interphase

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10
Q

Describe the checkpoints that occur during the cell cycle

A

G1 checkpoint
- End of G1 phase
- Check for cell size, nutrients, growth factors, DNA damage
- If anything wrong, cell enters G0

G2 checkpoint
- End of G2 phase
- Check for cell size, correct DNA replication, cell damage

Spindle assembly checkpoint
- During metaphase of mitosis
- Check for chromosome attachment to spindle

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11
Q

Describe the structure of a chromosome

A
  • Made up of two sister chromatids
  • Joined at the middle by a centromere
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12
Q

Summarise the stages involved in mitosis

A
  • Step 1: Prophase
  • Step 2: Metaphase
  • Step 3: Anaphase
  • Step 4: Telophase
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13
Q

Describe the stages in prophase

A
  • Chromosomes condense (become visible under light microscope)
  • Nuclear envelope breaks down
  • Spindle microtubules extend from centrioles at two ends of the cell to its equator
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14
Q

Describe the stages in metaphase

A
  • Sister chromatids of each chromosome attach to the spindle microtubules
  • Via the centromere
  • Line up at equator of cell
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15
Q

Describe the stages in anaphase

A
  • Centromeres divide
  • Chromosomes start to migrate to opposite poles of cell
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16
Q

Describe the stages in telophase

A
  • Chromosomes reach opposite poles of the cell
  • Spindle microtubules break down
  • Nuclear envelope reforms
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17
Q

Define cytokinesis

A

Division of cytoplasm to form two daughter cells

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18
Q

Explain how cytokinesis occurs in animal cells

A
  • Microtubules form ring around centre of cell
  • Microtubules constrict, forming a cleavage furrow
  • Cell eventually pinched in two, forming two genetically identical daughter cells
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19
Q

Explain how cytokinesis occurs in plant cells

A
  • New cell wall produced across the equator of cell from vesicles containing carbohydrates
  • Vesicles fuse together to form cell plate, which fuses with existing cell wall
  • There is plasma membrane either side
  • Cell divides into two to form two genetically identical daughter cells
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20
Q

What is the product of one round of mitosis?

A

Two genetically identical daughter cells

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21
Q

Why is mitosis necessary in living organisms?

A
  • Growth, replacement and repair of tissues
  • Asexual reproduction
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22
Q

Why do prokaryotes not divide by mitosis?

A
  • No nucleus
  • Reproduce by binary fission
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23
Q

Define fertilisation

A

Fusion of a male (sperm) and female (egg) gamete

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24
Q

Define homologous chromosomes

A
  • Pair of chromosomes
  • Position of the genes on each chromosome is the same
  • The alleles may be different
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25
Q

What is the same in all parts of homologous chromosomes?

A

Sequence of genes

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26
Q

What is the difference between sister chromatids and chromosomes?

A
  • The DNA molecules formed by replication prior to cell division are called sister chromatids
  • After the centromere splits at the start of anaphase, they are individual chromosomes
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27
Q

Describe the function of meiosis

A
  • Type of reductive cell division
  • Produces genetically varied gametes with a haploid number of chromosomes (n)
  • Maintains chromosome number after sexual reproduction
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28
Q

Summarise the stages involved in meiosis

A
  • Prophase I & II
  • Metaphase I & II
  • Anaphase I & II
  • Telophase I & II
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29
Q

Describe the stages in prophase I

A
  • Chromosomes condense and pair up - the chromosomes in each pair are homologous
  • Crossing over between homologous chromosomes occurs before condensation
  • Spindle microtubules extend from the two ends of the cell to its equator
  • Nuclear envelope breaks down
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30
Q

Describe the stages in metaphase I

A
  • Sister chromatids of each chromosome attach to spindle microtubules via the centromere
  • Homologous chromosomes randomly assemble at equator of cell (independent assortment)
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31
Q

Describe the stages in anaphase I

A
  • Homologous chromosomes pulled to opposite poles (this halves the chromosome number)
  • Each chromosome still consists of two chromatids
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32
Q

Describe the stages in telophase I

A
  • Sister chromatids arrive at the poles of the cell and decondense
  • Nuclear envelope reforms
  • After telophase, the cell divides by cytokinesis
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33
Q

Describe the stages in prophase II

A
  • Sister chromatids condense and pair up
  • No crossing over
  • Spindle microtubules extend from the two ends of the cell to its equator
  • Nuclear envelope breaks down
34
Q

Describe the stages in metaphase II

A
  • Sister chromatids of each chromosome attach to the spindle microtubules via the
    centromere
  • Sister chromatids randomly assemble at the equator of the cell (independent assortment)
35
Q

Describe the stages in anaphase II

A
  • Centromeres divide
  • Chromosomes move to opposite poles
36
Q

Describe the stages in telophase II

A
  • Chromosomes arrive at the poles of the cell and decondense
  • Nuclear envelope reforms
  • After telophase, the cell divides by cytokinesis
  • Results in four haploid cells
  • Each with 23 chromosomes made from a single chromatid
37
Q

What is the end point of the first division of meiosis?

A
  • Separation of homologous chromosomes
  • Creating 2 genetically varied haploid cells
38
Q

What is the end point of the second division of meiosis?

A
  • Separation of sister chromatids
  • Creating 4 genetically varied haploid cells
39
Q

During which stage of meiosis does crossing over usually occur?

A

Prophase I

40
Q

How does meiosis promote variation?

A
  • Crossing over of non-sister chromatids in prophase I
  • Independent assortment in metaphase I and II
  • Second division of meiosis separates alleles further
  • Possible combinations of alleles in gametes is 2n
41
Q

Describe two ways in which genetic variation is brought about during meiosis

A

Crossing over during prophase I
- Homologous chromosomes pair up and sections of non-sister chromatids are
swapped
- Produces chromatids with new combinations of alleles
Random orientation of pairs of homologous chromosomes in metaphase I and II
- For each pair of chromosomes there are two possible orientations when aligned at
equator of cell
Different combinations of alleles can be made in each daughter cell

42
Q

Define bivalent

A

Pair of homologous chromosomes

43
Q

Define chiasmata

A
  • Physical connection between non-sister chromatids
  • Occurs during prophase I
  • Hold homologous chromosomes together as a bivalent until anaphase I
  • Can result in the exchange of alleles (crossing over)
44
Q

Define recombinant chromosomes

A

Homologous chromosomes that have exchanged alleles by crossing over

45
Q

Explain how the recombinants are formed during meiosis

A
  • Recombination occurs in prophase I of meiosis
  • Homologous chromosomes come together in pairs
  • Chiasmata form between non-sister chromatids
  • Crossing over takes place
46
Q

Explain how independent assortment of homologous chromosomes in metaphase I leads to variation

A
  • Bivalents are orientated randomly on the equator during metaphase I of meiosis
  • Orientation of one bivalent does not affect orientation of other bivalents
  • Pole to which allele on one bivalent moves during anaphase I does not affect the pole to
    which alleles on another bivalent move
  • Independent assortment will not occur if two genes are linked
47
Q

How are gametes responsible for introducing variation?

A
  • Gametes underpin variation within all species which reproduce sexually
  • Fertilisation is a random process
  • One gamete is from a male, and one from a female
48
Q

Define tissue

A

A group of cells working together to perform a function

49
Q

Define organ

A

A group of tissues working together to perform a function

50
Q

Define organ system

A

A group of organs working together to perform a function

51
Q

Define organism

A

A group of organ systems working together to perform a function

52
Q

How are specialised tissues produced in multicellular organisms?

A
  • By differentiation
  • Some genes in a genome are expressed whilst others are not
53
Q

How are sperm cells specialised for their function?

A
  • Acrosome - contains digestive enzymes
  • Helical mitochondrion - produces ATP needed to help sperm swim
  • Microtubules - ensure tail beats side to side to propel sperm forwards
  • Protein fibres - strengthen tail
  • Haploid nucleus - contains 23 chromosomes in humans
  • Whiplash tail (flagellum) - propels sperm up vagina
54
Q

How are erythrocytes (red blood cells) specialised for their function?

A
  • Flattened, biconcave shape - increases SA:V ratio
  • No nuclei and few organelles - increases space for haemoglobin (carries oxygen)
  • Flexible - can squeeze through narrow capillaries
55
Q

How are neutrophils specialised for their function?

A
  • Type of white blood cell
  • Multi-lobed nucleus - easier to squeeze through small gaps
  • Granular cytoplasm - contains many lysosomes containing hydrolytic enzymes to digest
    pathogens
56
Q

How are squamous epithelial cells specialised for their function?

A
  • Line blood vessels
  • Flat, smooth and thin - short diffusion pathway
57
Q

How are ciliated epithelial cells specialised for their function?

A
  • Line trachea to ‘waft’ mucus up to the throat
  • Hair-like structures - collect and move mucus up the trachea
58
Q

How are palisade cells adapted for their function of absorbing light?

A
  • Lots of chloroplasts - absorb light and carry out
    photosynthesis - Rectangular shape - can be packed close
    together - Thin walls - increases rate of diffusion of CO2
59
Q

How are root hair cells adapted for their function of absorbing water and minerals?

A

Long root hairs - increase surface area,
maximises uptake of water and minerals

60
Q

How are guard cells adapted for their function of opening and closing the stomata?

A
  • Found in pairs either side of stomata - Become less swollen when lose water - closes
    stomata to prevent further water loss
61
Q

How is cartilage adapted for its function of reducing friction between bones?

A

Contains fibres of elastin and collagen

62
Q

How is muscle adapted for its function of moving parts of the body?

A

Contains myofibrils of actin and myosin - allow muscle to contract and relax

63
Q

How is epidermis adapted for its function of covering the surface of leaves?

A
  • Covered by waxy cuticle - reduces water loss - Stomata present - controls gas exchange and
    water loss
64
Q

How is xylem tissue adapted for its function of transporting water and ions?

A
  • Cells are dead and hollow - forms tube for water
  • Cells strengthened by lignin - provides support for plants
65
Q

How is phloem tissue adapted for its function of transporting sucrose?

A

Sieve tube elements have few organelles - more
room for transport of sucrose
Companion cells present to provide key
metabolic products for sieve tube elements Sieve plates are perforated - let sucrose through to the next cell

66
Q

What are stem cells?

A

Undifferentiated cells

67
Q

Define potency

A

Ability to differentiate into different cell types

68
Q

Define totipotent

A
  • Stem cells that can differentiate into any type of cell
  • Including umbilical cord and amnion
  • e.g. a fertilised egg or a zygote
    Define pluripotent
69
Q

Define pluripotent

A

Stem cells that can differentiate into all tissue types
- e.g. embryo cells

70
Q

Define multipotent

A

Stem cells that can differentiate into a range of cells within a specific tissue
- e.g. stem cells in bone marrow differentiate into different blood cells

71
Q

What are the two sources of animal stem cell?

A
  • Embryonic
  • Adult e.g. those found in bone marrow
72
Q

Why are stem cells useful in repairing diseased or damaged organs?

A
  • Continuously divide and replicate
  • Can differentiate into different types of specialised cell
73
Q

Where are plant stem cells found?

A
  • Meristem tissue
  • Present wherever plant is growing e.g. root and shoot tips
  • Present in cambium between xylem and phloem
  • Produce xylem vessels and sieve tube elements
74
Q

List some potential treatments involving stem cells

A
  • Heart disease - replace damaged muscle tissue
  • Type 1 diabetes - replace damaged insulin-producing cells in pancreas
  • Parkinson’s disease - replace damaged cells in the brain
75
Q

What is developmental biology?

A

Study of the changes that occur as multicellular organisms grow and develop

76
Q

What role can stem cells play in developmental biology?

A

Enable scientists to study differentiation of any type of cell

77
Q

When compared to other body cells, which characteristic of stem cells is the most important for therapeutic uses?

A

Enable scientists to study differentiation of any type of cell

78
Q

When compared to other body cells, which characteristic of stem cells is the most important for therapeutic uses?

A

Ability to differentiate into different types of specialised cell (potency)

79
Q

Describe some ethical concerns with using embryonic stem cells

A
  • Embryos count as human life
  • Embryos are unable to decide if they want to be used in this way
80
Q

Provide arguments for the use of embryonic stem cells

A
  • Early embryos are ‘only’ small balls of cells
  • Large embryos used by IVF are never implanted - surely better use is to help save lives
  • No nervous system - embryos unable to feel pain
81
Q

Advantages of adult stem cells over embryonic stem cells

A
  • Adults can give consent for use of their stem cells
  • No rejection problems
  • No death of embryos used to provide stem
    cells

OCR Biology Paper 1 (20 decks)

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