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AS Biology > Chapter 6 - Cell Division > Flashcards

Chapter 6 - Cell Division Flashcards

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1
Q

What is the cell cycle?

A

The highly ordered sequence of events that takes place in a cell, resulting in division of the nucleus and the formation of two genetically identical daughter cells

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2
Q

What are the 2 main phases of the cell cycle in eukaryotic cells?

A
  • Interphase
  • Mitotic phase
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3
Q

What is interphase?

A

Growth period of the cell cycle, between cell divisions (mitotic phase). Consists of stages G1, S and G2.

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4
Q

When does the cell carry out all of its major functions e.g. producing enzymes/hormones? Interphase or mitotic phase?

A

Interphase

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5
Q

What happens during interphase? (5)

A
  • DNA is replicated and checked for errors in the nucleus
  • Protein synthesis occurs in the cytoplasm
  • Mitochondria grow and divide, increasing in number in the cytoplasm
  • Chloroplasts grow and divide in plant and algal cell cytoplasm, increasing in number
  • Normal metabolic processes of the cell occur (some, including respiration, also occur throughout cell division)
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6
Q

What are the 3 stages of interphase?

A
  • G1 - 1st growth phase
  • S - synthesis phase
  • G2 - 2nd growth phase
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7
Q

What happens during G1? (3)

A
  • Proteins from which organelles are synthesised are produced
  • Organelles replicate
  • The cell increase in size
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8
Q

What happens during S?

A

DNA is replicated in the nucleus

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9
Q

What happens during G2? (3)

A
  • Cell continues to increase in size
  • Energy stores are increased
  • The duplicated DNA is checked for errors
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10
Q

What is the mitotic phase?

A

Period of cell divine of the cell cycle. Consists of the stages mitosis (when the nucleus divides) and cytokinesis (the cytoplasm divides and two cells are produced)

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11
Q

What is the G0 phase?

A

The phase when the cell leaves the cell cycle either temporarily or permanently

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12
Q

List reasons why a cel may inter G0 (3)

A
  • Differentiation - A cell that becomes differentiated can no longer divide; it carries out its function indefinitely and doesn’t enter the cell cycle again
  • Damaged DNA - A damaged cell can no longer divide and enters a period of permanent cell arrest (G0). Most normal cells only divide a limited number of times, eventually becoming senescent
  • Ageing - As you age the number of senescent cells in the body increases, linking with diseases e.g. cancer and arthritis
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13
Q

Give an example of a cell that can leave G0 after entering

A

Lymphocytes (white blood cells) can go back into the cell cycle and start diving again in an immune response

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14
Q

What are checkpoints and why are they important?

A

Control mechanisms of the cell cycle

They monitor and verify whether the processes at each phase of the cell cycle have been accurately completed before the cell is allowed to progress into the next phase

They ensure the fidelity of cell division - that two identical daughter cells are created from the parent cell

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15
Q

Describe what happens at each checkpoint in the cell cycle:
1. G1 checkpoint
2. G2 checkpoint
3. Spindle assembly/ metaphase checkpoint

A
  1. End of G1 before entry into S. Check for: cell size, nutrients, growth factors, DNA damage
  2. End of G2 before start of mitotic phase. Check for: cell size, DNA replication, DNA damage
  3. At the metaphase point in mitosis. Check for: chromosome attachment to spindle
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16
Q

What is the importance of mitosis? (2)

A
  • Ensures that the 2 daughter cells produced are genetically identical to each other and to the parent cell
  • Needed for growth; replacement and repair of tissues in multicellular organisms; and asexual reproduction in eukaryotes
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17
Q

What is a chromosome?

A

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

They become visible under the light microscope when cells are preparing to divide

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18
Q

What is chromatin?

A

Uncondensed DNA in a complex with histones

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19
Q

What are chromatids?

A

2 identical copies of DNA (a chromosome) held together at a centromere

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20
Q

What is a centromere?

A

Region at which 2 chromatids are held together

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21
Q

What happens before mitosis? (4)

A
  • All of the DNA in the nucleus is replicated during interphase
  • Each DNA molecule is converted into 2 identical DNA molecules called chromatids
  • The 2 chromatids are joined at a centromere
  • Chromatids must be kept together during mitosis so that the can be segregated equally, 1 in each of the 2 new daughter cells
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22
Q

What are the stages of mitosis (in order)? (4)

A
  1. Early prophase
  2. Metaphase
  3. Anaphase
  4. Telophase
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23
Q

What happens in early prophase

A
  • Chromatin fibres coil & condense to form chromosomes that take up stain to become visible under light microscope
  • Nucleolus disappears & nuclear membrane begins to break down
  • The centriole divides and the 2 new daughter centrioles move to opposite poles of the cell
  • Protein microtubules form a spindle fibres that will form a spindle between these centrioles
  • Spindle fibres attach to specific areas on the centromeres and start to move the chromosomes to the centre of the cell
  • By the end of prophase the nuclear envelope has disappeared
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24
Q

What happens in metaphase?

A

Chromosomes are moved by the spindle fibres to form a plane in the centre of the cell, called the metaphase plate equator, and then held in position

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25
Q

What happens in anaphase?

A
  • Centromere of each pair of chromatids divide
  • Chromatids are separated and pulled to opposite poles of the cell by the shortening spindle fibres
  • Chromosomes assume a V shape
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26
Q

What happens in telophase?

A
  • Chromosomes reach the poles
  • New nuclear envelope forms around each set
  • Chromosomes start to uncoil and nucleolus is formed
  • Cytokinesis begins
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27
Q

Describe cytokinesis in animal cells

A
  • Cleavage furrow forms around the middle of the cell
  • Cell-surface membrane pulled inwards by cytoskeleton until it is close enough to fuse around the middle forming 2 cells
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28
Q

Describe cytokinesis in plant cells

A
  • Have cell walls so cleavage furrow can’t be formed
  • Vesicles from Golgi apparatus assemble in the same place as metaphase plate equator
  • Vesicles fuse with each other and cell surface membrane dividing the cell into 2
  • New sections of cell wall then form along the new sections of membrane
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29
Q

What is meiosis?

A

Form of cell division when the nucleus divides twice (meiosis I and meiosis II) resulting in a halving of the chromosome number and producing 4 haploid cells from one diploid cell.

Reduction division: cell division resulting in the production of haploid cells from a diploid cell.

30
Q

Why is meiosis needed?

A

To produce gametes that are haploid cells, so that when they fuse in fertilisation during sexual reproduction to produce a zygote, the zygote is a diploid cell.

31
Q

What are homologous chromosomes?

A

Matching pair of chromosomes, one inherited from each parent

Alleles are different versions of the same gene

As they have the same genes in the same positions, they will be the same length and size when they are visible in prophase. The centromeres will also be in the same positions

32
Q

What are the stages of meiosis?

A

Meiosis I
* The 1st division is the reduction division when the pairs of homologous chromes are separated into 2 cells
* Each intermediate cell will only contain 1 full set of genes instead of 2, so the cells are haploid

Meiosis II
* The 2nd division is similar to mitosis
* The pairs of chromatids present in each daughter cell are separated forming 2 more cells
* 4 haploid daughter cells are produced in total

33
Q

Describe Prophase I

A
  • Chromosomes condense; nuclear envelope disintegrates, nucleolus disappears, spindle formation begins
  • The homologous chromosomes pair up, forming bivalents
  • Chromosomes are large DNA molecules and moving them through liquid cytoplasm as they are brought together results in chromatids entangling
  • This is called crossing over
34
Q

Describe Metaphase I

A
  • Pairs of homologous chromosomes still in their crossed over state assemble along the metaphase plate equator
  • Orientation of each homologous pair on the metaphase plate is random and independent of any other homologous pair
  • Independent assortment results in genetic variation
35
Q

Describe Anaphase I

A
  • Homologous chromosomes are pulled to the opposite poles and the chromatids stay joined together
  • Sections of DNA on ‘sister’ chromatids, which became entangled during crossing over, now break off and rejoin -sometimes resulting in DNA exchange
  • When exchange occurs, recombinant chromatids are formed, with genes being exchanged between chromatids
  • Results in swapped areas of chromosome and allele shuffling
36
Q

Describe Telophase I

A
  • The chromosomes assemble at each pole and the nuclear membrane reforms
  • Chromosomes uncoil
  • Cell undergoes cytokinesis and divides into 2 cells
  • reduction of chromosome number from diploid to haploid is completeD
37
Q

Describe Prophase 2

A
  • The chromosomes, which still consist of 2 chromatids, condense and become visible again
  • The nuclear envelope breaks down and spindle formation begins
38
Q

Describe Metaphase 2

A
  • Individual chromosomes assemble on the metaphase plate equator
  • Due to crossing over, the chromatids are no longer identical soother is independent assortment again and more genetic variation is produced
39
Q

Describe Anaphase 2

A

Chromatids of the individual chromosomes are pulled to opposite poles after division of the centromeres

40
Q

Describe Telophase 2

A
  • The chromatids assemble at the poles
  • The chromosomes uncoil and form chromatin again
  • Nuclear envelope reforms and the nucleolus becomes visible
  • Cytokinesis results in division of the cells forming 4 daughter cells in total
  • Cells are haploid due to reduction division
  • Cells are genetically different to each other and the parent cell due to crossing over and independent assortment
41
Q

Describe erythrocytes (3)

A
  • Red blood cells
  • Flattened biconcave shape to increase SA:V ratio, for transporting O2 around the body
  • In mammals don’t have a nuclei or many other organelles to increase space for haemoglobin
  • Flexible to squeeze through narrow capillaries
42
Q

Describe neutrophils (4)

A
  • Type of white blood cell
  • Important role in immune system
  • Multi-lobed nucleus making it easier to squeeze through small gaps to get to the site of infections
  • Granular cytoplasm containing many lysosomes that have enzymes used to attack pathogens
43
Q

Describe sperm cells (4)

A
  • Male gametes
  • Deliver genetic info to female gamete (ovum or egg)
  • Have a tail/ flagellum so they can move
  • Contain many mitochondria to supply energy needed to swim
  • Acrosome on the head contains digestive enzymes that digest the protective layers around the egg, allowing the sperm to penetrate and leading to fertilisation
44
Q

Describe palisade cells (6)

A
  • Present in the mesophyll
  • Contain chloroplasts to absorb light for photosynthesis
  • Rectangular box shapes that can be closely packed to form a continuous layer
  • Thin walls to increase rate of diffusion of CO2
  • Large vacuole maintain turgor pressure
  • Chloroplasts can move within the cytoplasm in order to absorb more light
45
Q

Describe root hair cells (3)

A
  • Present at the surfaces of roots near growing tips
  • Have long extensions called root hairs which increase the surface area of the cell
  • Maximises the uptake of water and minerals from the cell
46
Q

Describe guard cells (4)

A
  • Pairs of guard cells on surfaces of leaves form small openings called stomata
  • Stomata are necessary for CO2 to enter for photosynthesis
  • When guard cells lose water, they become less swollen (because of osmosis), they change shape and the stoma closes to prevent further water loss
  • The cell wall is thicker on one side so the cell doesn’t change shape symmetrically as its volume changes
47
Q

What are the 4 main categories of tissues in animals?

A
  1. Nervous tissue: Adapted to support the transmission of electrical impulses
  2. Epithelial tissue: Adapted to cover body surfaces, internal and external
  3. Muscle tissue: Adapted to contract
  4. Connective tissue: Adapted either to hold other tissues together, or as a transport medium
48
Q

Describe the squamous epithelium (5)

A
  • Made up of specialised squamous epithelial cels
  • Flat appearance
  • Very thin due to the flat cells that make it up, and because it’s only 1 cell thick
  • Present when rapid diffusion across a surface is needed
  • Lining of the lungs; allows rapid diffusion of O2 into the blood
49
Q

Describe the ciliated epithelium (5)

A
  • Made up of ciliated epithelial cells
  • Cells have cilia on one surface that move in a rhythmic manner
  • Lines the trachea, causing mucus to be swept away from the lungs
  • Goblet cells are also present, releasing mucus to trap any unwanted particles present
  • Prevents particles e.g. bacteria from reaching the alveoli once inside the lungs
50
Q

Describe cartilage (5)

A
  • Connective tissue found in the outer ear, nose, at the end of (and between bones)
  • Contains fibres of the proteins elastin and collagen
  • Firm, flexible tissue composed of chondrocyte cells embedded in an extracellular matrix
  • Helps prevent the ends of bones rubbing together an causing damage
  • Many fish have skeletons made of cartilage not bone
51
Q

Describe muscle (2)

A
  • A tissue that needs to be able to contract in order to move bones, which move different parts of the body
  • Skeletal muscle fibres - muscles which are attached to bone; contain myofibrils which contain contractile proteins
52
Q

What are the difference types of tissue in plants?

A
  • Epidermis tissue - Adapted to cover plant surfaces
  • Vascular tissue - Adapted for transport of water and nutrients
53
Q

Describe the epidermis (3)

A
  • Single-layer of closely packed cells covering the surfaces of plants
  • Usually covered by waxy, waterproof cuticle to reduce water loss
  • Stomata are present here; allow CO2 in and out, and water vapour and O2 in and out
54
Q

Describe xylem tissue (3)

A
  • Vascular tissue responsible for the transport of water and minerals throughout plants
  • Composed of vessel elements, which are elongated dead cells
  • Walls of cells strengthened with lignin, providing structural support
55
Q

Describe phloem tissue (2)

A
  • Vascular tissue responsible for the transport of organic nutrients (esp. sucrose) from leaves and stems where its made by photosynthesis to the rest of the plant
  • Composed of column of sieve tube cells, separated by perforated walls called sieve plates
56
Q

Give 3 examples of organ systems in animals

A
  • Digestive system: takes in food; breaks down the large insoluble molecules into small soluble ones; absorbs nutrients into the blood; retains water needed by the body; removes undigested material from the body
  • Cardiovascular system: moves blood around the body to provide an effective transport system for the substances it carries
  • Gaseous Exchange system: brings air into the body so O2 can be extracted for respiration, and CO2 can be expelled
57
Q

What are stem cells?

A

Undifferentiated cells with the potential to differentiate into a variety of the specialised cell types of the organism

58
Q

Describe cell division in stem cells

A
  • Stem cells can undergo cell division many times, and are a source of new cells necessary for growth, development, and tissue repair
  • Once stem cells have become specialised, they lose the ability to divide and enter G0
  • If they don’t divide fast enough, tissues are not efficiently replaced, leading to ageing
  • If there’s uncontrolled division the form tumours, which can lead to cancer
59
Q

What is meant by the term ‘potency’?

A

A stem cell’s ability to differentiate into different cell types. The great the number of cell types it can differentiate into, the greater its potency

60
Q

Describe totipotent stem cells

A
  • Can differentiate into any type of cell
  • A zygote and the 8 or 16 cells from its first few mitotic divisions are totipotent cells
  • Eventually produce a whole organism
  • Can also differentiate into extra-embryonic tissues like the aminos and umbilicus
61
Q

Describe pluripotent stem cells

A
  • Can form all tissue types, but not whole organisms
  • Are present in early embryos and are the origin of the different types of tissue within an organism
62
Q

Describe multipotent stem cells

A
  • Can only form a range of cells within a certain type of tissue
  • e.g. Haematopoetic stem cells in bone marrow, because they can form the various types of blood cell
63
Q

What is differentiation?

A

The process of a cell becoming differentiated. Involves the selective expression of genes in a cell’s genome

64
Q

Why is differentiation necessary in multi-cellular organisms?

A

Cells have to specialise total on different roles in tissues and organs

65
Q

Describe the replacement of red blood cells

A
  • Erythrocytes (red blood cells) have a short lifespan of 120 days
  • The stem colonies in the bone marrow produce 3 billion erythrocytes per kilogram of body mass per day to keep up with the demand
66
Q

Describe the replacement of white blood cells

A
  • Neutrophils (white blood cells) have a lifespan of 6 hours
  • The stem colonies in the bone marrow produce 1.6 billion per kg per hour, which increases during infection
67
Q

What are the sources of plant stem cells

A

Meristematic tissue (meristems) in plants

  • Found at the tips of roots and shoots (apical meristems)
  • Found in between phloem and xylem tissues (in the vascular cambium).

Cell in this region differentiate into different cells present in xylem and phloem

Pluripotent nature of stem cells in meristems continues throughout the life of the plant

68
Q

What are the sources of animal stem cells?

A

Embryonic stem cells
* Present at early stage of embryo development and are totipotent
* After 7 days a blastocyst has formed and the cells remain in a pluripotent state in the foetus until birth

Tissue (adult) stem cells
* Present throughout life from birth
* Found in specific areas e.g. bone marrow
* Multipotent
* Stem cells can also be taken from the umbilical cords of newborn babies

69
Q

List some of the potential areas where stem cells could be used in human medical care? (6)

A
  • Heart disease
  • Type 1 diabetes
  • Parkinson’s disease
  • Alzheimer’s disease
  • Macular degeneration
  • Birth defects
    Spinal injuries
70
Q

List some of the areas of human medical care where stem cells are already being used? (3)

A
  • Burn treatment: Stem cells grown on biodegradable meshes can produce new skin for burn patients faster than the normal process of taking a graft from another part of the body
  • Drug trials: Potential new drugs can be tested on cultures of stem cells before animals and humans
  • Developmental biology: Allows more study of the changes that occur as multicellular organisms grown and develop, and why sometimes things go wrong
71
Q

What are the arguments agaisnt the use of stem cells? (3)

A
  • Religious and moral objections
  • Many people believe that life begins at conception, and so the destruction of embryos is murder
  • Lack of consensus as to when the embryo itself has rights, and who owns the genetic material that is being used for research

AS Biology (5 decks)

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