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LGS A-Level OCR Biology > Unit 2 - The cell cycle, mitosis, meiosis > Flashcards

Unit 2 - The cell cycle, mitosis, meiosis Flashcards

1
Q

What is the cell cycle?

A

The sequence of events that takes place in a cell to enable growth and repair

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2
Q

What are the two main phases of the cell cycle?

A
  1. Interphase (longest phase ~95% of time)
  2. Mitotic phase
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3
Q

What does a cell do during interphase?

A
  • Cells carry out their major functions
  • Protein synthesis occurs in the cytoplasm
  • Organelles grow and divide in the cytoplasm
  • Normal metabolic processes (some continue throughout cell division e.g. respiration)
  • DNA is replicated and checked for errors in the nucleus
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4
Q

What are the 3 stages of interphase?

A
  • G1 - growth phase 1
  • S - synthesis phase
  • G2 - growth phase 2
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5
Q

What occurs in G1 of interphase?

A
  • Proteins from which organelles are synthesised are produced
  • Organelles are replicated
  • The cell increases in size
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6
Q

What occurs in G2 of interphase?

A
  • Cell continues growth
  • Energy stores are increased
  • Cell double checks duplicated chromosomes for errors and makes any necessary repairs
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7
Q

What occurs in S of interphase?

A

Chromosomes are duplicated in the nucleus

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8
Q

What is G0 of the cell cycle?

A

Phase when cells leave the cell cycle either temporarily or permanently.

A few types of cells that enter G0 can be stimulated to go back into the cell cycle and start dividing again e.g., lymphocyctes in an immune response.

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9
Q

Why might a cell leave the cell cycle?

A

Phase when cells leave the cell cycle either temporarily or permanently because:

  • The cell has differentiated and is no longer able to divide
  • The DNA of the cell is damaged so the cell is no longer viable and therefore becomes senescent.
  • Age. The number of senescent cells increases with age and has been linked with age related diseases.
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10
Q

What occurs in the G1 checkpoint?

A
  • Checks that chemicals needed for replication (growth factors) are present
  • Checks for any damage to DNA before S phase
  • Checks for cell size
  • Checks that the nutrients required for replication are present
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11
Q

What occurs in the G2 checkpoint?

A
  • Checks for cell size
  • Checks for DNA replication
  • Checks for any errors in the DNA and repairs mistakes
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12
Q

What occurs in the spindle assembly checkpoint?

A
  • Checks if spindle fibres are connected to chromosomes
  • Checks that the chromosomes have aligned ready for metaphase

*Also known as metaphase checkpoint - mitosis cannot proceed unti this checkpoint is passed.

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13
Q

What are the two parts to the mitotic stage of the cell cycle?

A
  1. Mitosis
  2. Cytokinesis
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14
Q

What are the two types of cell division?

A

Mitosis and Meiosis

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15
Q

When does meiosis occur?

A

The production of gametes

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16
Q

What occurs in mitosis?

A

The division of the nucleus into two genetically identical nuclei via a 5 step process:

  • Early prophase
  • Late prophase
  • Metaphase
  • Anaphase
  • Telophase
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17
Q

What occurs in cytokinesis?

A
  • The cytoplasm divides and the two identical daughter cells are produced
  • Starts in late telophase
  • Characterised by the formation of a cleavage furrow, which pinches the cell in two, in aminal cells.
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18
Q

Why is mitosis important?

A
  • It ensures the two daughter cells produced are genetically identical
  • Each cell produced has an exact copy of the DNA present in the parent cell, and the same number of chromosomes
  • Growth, repair and replacement of cells in multicellular organisms
  • Assexual reproduction by Eukaryotic organisms
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19
Q

Define chromatid

A
  • Two identical copies of DNA (a chromosome)
  • Two chromatids are held together at a centromere
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20
Q

Define chromatin

A

Uncondensed DNA which is in complex with histone proteins

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21
Q

Define chromosomes

A

Structures of condensed and coiled DNA molecoles in the form of chromatin

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22
Q

Define sister chromatid.

A

Two identical DNA molecules joined by a common centromere

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23
Q

Define centromere.

A
  • Region at which two sister chromatids are held together
  • Area of constriction of cells chromosomes
  • Point at which the chromosomes short arm (P) and long arm (Q) are separated.
  • Point where mitotic spindle fibers attach to pull sister chromatids apart during cell division.
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24
Q

Define centrioles.

A

Component of the cytoskeleton made of microtubles and associated protiens, involved in the development of spindle fibres.

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25
Q

Define spindle fibres

A

A structure made of microtubles and associated proteins that assemble from the centrosome to provide the structure that moves chromosomes.

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26
Q

Define homologous pairs

A
  • A pair of chromosomes, one maternal and one paternal
  • Contain the same genes on the same loci
  • Do not necessarily have the same alleles
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27
Q

How is DNA packaged?

A
  • Wrapped around histone proteins to form chromatin
  • Coiled around more proteins to form chromosomes
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28
Q

When is DNA found as visible chromosomes?

A

Only during cell division.

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29
Q

Why does DNA form chromosomes for cell division?

A

Helps protect DNA from damage when it is being moved and replicated

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30
Q

Why does DNA wrap around histones?

A
  • Makes it more compact
  • Makes it easy to access
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31
Q

What are the 4 stages of mitosis in order?

A
  1. Prophase
  2. Metaphase
  3. Anaphase
  4. Telophase
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32
Q

What happens in prophase of mitosis?

A
  • Chromatin fibres become more tightly coiled, condensing into discrete chromosomes visible with a light microscope
  • Nuclear membrane begins to break down and nucleolus disappears
  • Each duplicated chromosome appears as two identical sister chromatids joined at their centromeres and, in some species, all along their arms by cohesins
  • The mitotic spindle begins to form from centrosomes and the microtubles from them. N.b. shorter, radial arrays of microtubles are called asters ‘stars’.
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33
Q

What happens in early metaphase (prometaphase) of mitosis?

A
  • The nuclear envelope fragments
  • Microtubles extending from each chromosome can now invade the nuclear area
  • The chromosomes have become even more condensed
  • Each of the two chromatids of each chromosome now has a kinetochore, a specialised protein structure at the centromere.
  • Some of the microtuble attach to the kinetochores becoming ‘kinetochore microtubles’ which jerk the chromosomes back an forth
  • Non-kinetochore fibres interact with those from the opposite pole of the spindle.
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34
Q

What happens in (late) metaphase of mitosis?

A
  • Centrosomes are now at opposite poles of the cell
  • Chromosomes are moved by spindle fibres to line up along the equatorial plate of the cell known as the ‘metaphase plate’
  • Each chromosome has the kinetochore of the sister chromatid attached to the kinetochore microtuble from the opposite pole.
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35
Q

What happens in anaphase of mitosis?

A
  • Shortest phase lasting only a few minutes
  • Cohesion protiens are cleaved allowing the sister chromatids to be pulled apart by spindle fibres
  • Daughter chromosomes move towards the poles of the cell as their kinetochore microtubles shorten
  • Chromosomes move centromere first at ~1μm/min
  • The cell elongates as the non-kinetochore microtubles lengthen
  • Results in two poles of cell having equivalent, and complete, collections of chromosomes.
36
Q

What happens in telophase of mitosis?

A
  • Two daughter nuclei form in the cell - nuclear membrane forms from fragments of parents and portions of endomembrane system
  • Nucleoli reappears
  • Chromatids now called chromosomes which become less condensed and indistinct
  • Remaining spindle fibres are depolymerized
  • Mitosis, the division of one nucleus into two genetically identical nuclei, is now complete.
37
Q

Why are centrioles important in cell division?

A
  • Pairs of centrioles called centrosomes are the subcellular region which organises the cell’s microtubles
  • During mitosis the spindle microtubles (fibres) start at the centrosomes
  • Spindle fibres are responsible for the movement of chromosomes within the cell
38
Q

What happens in cytokinesis of animal cells?

A
  • A cleavage furrow forms
  • Microfilaments form a ring around the edge of the plasma membrane pulling the membrane inwards pinching the cell in two
  • The plasma membrane fuses in the middle to form two cells
39
Q

What happens in the cytokinesis of plants?

A
  • Vesicles carry cell membrane and cell wall components assemble along the plane of division
  • The cell membrane first forms as vesicles fuse
  • Cell wall forms alongside
40
Q

What do single celled organisms use mitosis for?

A

To reproduce assexually

41
Q

What are 4 functions of mitosis?

A
  1. Growth of an organism
  2. Repair/replacement of tissues
  3. Asexual reproduction
  4. Development of embryos
42
Q

How do yeast undergo mitosis?

A
  • Yeast reproduces asexually through mitosis
  • A bud forms before DNA replicates
  • One large and one small cell are produced
43
Q

What is mitosis in bacteria called?

A

Binary fission

44
Q

Define diploid

A

Normal chromosome number, two chromosomes of each type inherited from each parent

45
Q

Define haploid

A

Half the normal number of chromosomes, one chromosome of each type

46
Q

Define gametes

A

Haploid sex cell produced by meiosis in organisms that reproduce sexually

47
Q

Define meiosis

A

Form of cell division where the nucleus divides twice resulting in a halving of chromosome number and producing 4 haploid cells

48
Q

Define reduction division

A
  • The first cell division in meiosis, the process by which germ cells are formed.
  • Cell division resulting in the production of haploid cells from a diploid cell
49
Q

Define zygote.

A

The initial diploid cell formed when the nuclei of gametes fuse during the fertilisation stage of sexual reproduction.

50
Q

How many rounds of cell division occur in meiosis?

A

2

51
Q

What occurs in prophase I of meiosis?

A
  • Chromosomes begin to condense, and homologs loosly pair along their lengths, aligned gene by gene
  • Synapsis - Pair homologs become physically connected to each other along their lengths by a ‘zipper-like’ protein structure
  • Crossing over, genetic rearrangement between non-sister chromatids, leaving at chiasmata‘x-shaped’ regoins at the point of crossover
  • Centrosome movement, spindle formation and nuclear membrane disintegration
  • Bivalents are formed
52
Q

What occurs in metaphase I of meiosis?

A
  • Pairs of homologous pairs of chromosomes line up along the metaphase plate, with one chromosome in each pair facing each pole
  • Both chromatids of one homolog are attached to kinetochore microtubles from one pole; those of the other homolog are attached to microtubles from the opposite pole
53
Q

What occurs in anaphase I of meiosis?

A
  • Breakdown of protiens responsible for sister chromatid cohesion along chromatid arms allows homologs to separate
  • Spindle fibres pull homologous chromosomes to opposite poles
  • Random assortment of alleles as pairs move apart
  • Sister chromatid cohesion persists at the centrome, causing chromatids to move as a unit toward the same pole
54
Q

What occurs in telophase I of meiosis?

A
  • Starts with each half of the cell having a complete haploid set of duplicated chromosomes
  • Each is composed of two sister chromatids; one or both chromatids include regiods of non-sister chromatid DNA
  • Cytokinesis occurs simultaneously forming two haploid daughter cells (which are unidentical)
  • Nuclear envelope reforms
55
Q

What occurs in prophase II of meiosis?

A
  • Chromosomes, which still consist of two chromatids, recondense
  • Nuclear membrane disintegrates
  • A spindle apparatus forms from centrosomes
  • Chromatids start to move towards the metaphase plate II
56
Q

What occurs in metaphase II of meiosis?

A
  • Pairs of chromatids line up along the (equatorial) metaphase plate as in mitosis
  • Because of crossing over in meiosis I, the two sister chromatids are not genetically identical
  • Random assortment of homologous chromosomes at the metaphase plate is the second mechanism that introduces variation into the gametes = Independent assortment
57
Q

What occurs in anaphase II of meiosis?

A
  • Breakdown of proteins that hold the sister chromatids together at the centromere
  • Allows chromatids to be pulled apart by spindle fibres
  • The chromatids move towards opposite poles as individual chromosomes
58
Q

What occurs in telophase II of meiosis?

A
  • Nuclei form, the chromosomes begin decondensing, and cytokinesis continues
  • Mitotic division of one parent cell to produce four haploid daughter cells which are genetically distinct from each other
59
Q

How does independent assortment occur?

A

The random arrangement of bivalents/homologs at the metaphase plate.

60
Q

How does independent assortment lead to variation?

A

Different alleles can face the poles e.g. the maternal or paternal chromosomes can therefore end up at either pole during anophase.

61
Q

Define bivalent.

A

Homologous chromosomes that are associated in pairs physically held together with at least one crossover

62
Q

Define chiasmata

A
  • An x-shaped region in a homologous pair which denotes where sections of DNA have become entangled and ‘crossed over’, break and rejoin
  • Singular chiasma
  • Appears as a cross becasue sister chrmatid cohesion still holds the original two sister chromatids together, even in regions beyond the crossover point.
63
Q

At what stage in meiosis are chiasmata formed?

A

Prophase I

64
Q

Define recombiant chromatids

A

Chromatids with a combination of DNA from both homologous chromosomes formed by crossing over at chiasmata.

65
Q

What two processes occur in meiosis to introduce genetic variation?

A
  1. Crossing over
  2. Independent assortment
66
Q

What is crossing over?

A

A genetic rearrangement between two non-sister chromatids involving the corresponding segments of DNA molecules.

67
Q

How does crossing over lead to genetic variation?

A

The genes exchanged can be different alleles so the recombiant chromosomes have different sets of alleles to the original chromatids

68
Q

Besides meiosis, what are the two ways in which variation can be introduced?

A
  1. Random fertilisation
  2. Gene mutation
69
Q

How does random fertilisation introduce variation?

A
  • Each gamete has a unique combination of genes
  • Any of the numerous male gametes can fertilise the egg
70
Q

When does random fertilisation occur?

A

When two the nuclei of two gametes fuse to form a zygote.

71
Q

How does gene mutation introduce variation?

A

Bases within a gene can be altered during DNA replication.

72
Q

Can mutation in the germline be inherited?

A

Yes e.g. inherited diesase, variation - sexual canibalism (behavourial), neck length (anatomical), production of enzyme (physiological).

73
Q

Can mutation in somatic cells be inherited?

A

no

74
Q

Why are different alleles within a population important?

A
  • They introduce genetic variation
  • Some combinations can provide an organism with an advantageous characteristic e.g resistance to a disease, longer neck to acces food.
  • If the environment alters organisms with favourable mutations/alleles survive
  • Natural selection/selection pressure/survival of fittest
75
Q

Why is meiosis important in organisms which reproduce sexually?

A
  • Results in genetically distinct haploid cells
  • Ensures diploid number of chromosomes is restored in fertilisation
  • Variation generated through crossing over and independent assortment
  • Allows for a genetically different zygote to be formed through the fusion of the nuclei of random gametes during random fertilisation.
76
Q

Give five differences between mitosis and meiosis.

A
  1. Mitosis has one cell division but meiosis has two
  2. Mitosis maintains the original diploid chromosome number whilst meiosis halves chromosome number
  3. Mitosis produces two daughter cells whilst meiosis produces four
  4. Daughter cells are genetically identical to the parent cell in mitosis but genetically different in meisosis
  5. Mitosis produces somatic cells whilst meiosis produces gametes
77
Q

Compare the genetic make up of daughter cells produced by mitosis with the original parent cell.

A
  • Genetically identical
  • Have same number of chromosomes
  • Have the same genes as the parent cell
78
Q

Why is mitosis important in the life cycle of a human?

A
  • It allows for the growth of a human
  • Repairs tissues
  • Genetically identical cells produced
  • Throughout life maintain chromosome number
79
Q

Why is mitosis important in the repair of tissues?

A
  • Cells produced in mitosis are genetically identical
  • Allows cells to carry out the same function as the parent cell
  • Daughter cells have the correct information to develop the same type of cell as parent cells
80
Q

What is the difference between sister and non-sister chromatids?

A

They have different alleles.

81
Q

When does independent assortment occur?

A

Metaphase I and Metaphase II

82
Q

Why is it important in sexual reproduction that gametes are haploid?

A
  • Without gametes less variation
  • Gametes fuse to form a diploid cell
  • Prevents doubling number of chromosome number in each successive generation
83
Q

Why does meiosis need to have twice as many stages as mitosis?

A
  • Gametes must be haploid as when they fuse a diploid cell must form
  • To halve the number chromosomes from 2n to n
  • To separate homologous pairs of chromosomes
84
Q

What is the difference between prophase I and prophase II?

A
  • In prophase I chromosomes condense to become visible and homologous chromosomes pair up to form bivalents
  • In prophase II the chromasomes consist of two chromatids that condense to become visible again.
85
Q

Why is it important that the genetic information is checked at the checkpoints?

A
  • Otherwise mutation of a gene could occur
  • Which would mean a protein necessary for a particular function might not be produced
  • A cell could end up with too many (or little) chromosomes
86
Q

Why is replication of DNA before division of the nucleus in mitosis essential?

A
  • Cells produced are genetically identical, both daughter cells receive a full copy of the parent cell’s DNA
  • Diploid daughter cells are produced
  • To maitain the chromosomal number

LGS A-Level OCR Biology (42 decks)

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