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Medicine 1 - 02 Cell Basics > 5 Chromosomes, the Cell Cycle, and Intro to its regulation > Flashcards

5 Chromosomes, the Cell Cycle, and Intro to its regulation Flashcards

1
Q

Define chromatin

A

In eukaryotic cells, chromosomal DNA forms a complex with RNA and proteins

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2
Q

Describe the structure of chromatin

A

The proteins in chromatin are of 2 main types:

  • basic (mainly histones)
  • and acidic

Chromatin is also associated with phospholipids, enzymes and other molecules

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3
Q

Describe histones

A

Histones are rich in amino acids

  • Core histones are H2A, H2B, H3, H4
  • 2 of each forms the core histone octamer
  • DNA winds twice around it
  • Linker histones are H1 and H5, which hold the DNA to the histone core

These are important in compacting DNA and in chromatin regulation

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4
Q

Describe how chromatin is packed into bead-like structures (and how chromosomes are formed)

A

Chromatin is packed into bead-like structures called nucleosomes
- each nucleosome has 2 loops of DNA double helix warped around a cluster of 8 histones

Chromosomes form by the supercoiling and condensation of chromatin

Genes are specialised functional sites arranged along the chromosomes

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5
Q

Describe the appearance of chromatin in Interphase

A

In interphase nuclei, chromatin appears in 2 main forms:

Heterochromatin
- more condensed and densely stained
- tends to be found near nuclear envelope
- represents genes which are switched off (not expressed)
- Two types:
> constitutive
> facultative (sometimes on)

Euchromatin

  • less condensed and lightly staining
  • more central location
  • represents genes which are switched on
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6
Q

How and why do cells grow and proliferate?

A

Cells can

  • increase in cell mass
  • increase in cell number

Cells grow because:

  • foetal development
  • growth
  • repair
  • cell replacement
  • reproduction
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7
Q

Describe tissues and cell proliferation

A

Tissues of the body can be divided into 3 main groups according to their rates of turnover:

Continuously renewing
- epidermis of skin, intestinal epithelium

Conditionally renewing
- liver, kidney, endocrine glands

Static or non-proliferative
- cardiac, nerve cells

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8
Q

Define The cell cycle

A

The cell cycle is the process where cell materials duplicates and divide, resulting in 2 daughter cells

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9
Q

List the different stages of the cell cycle

A

Interphase

  • G1, S, G2
  • M phase (mitosis + cytokinesis)
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10
Q

Describe the interphase stage of the cell cycle

A

Interphase:

G1 (gap/growth 1)
- cell growth, 6-12h

S phase (synthesis)
- DNA replication 6-8h

G2 (gap/growth 2)
- preparation for cell division 3-4h

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11
Q

Describe the M phase of the cells cycle

A

Mitosis and cytokinesis
- the formation of 2 identical daughter cells

Mitosis results in

  • nuclear division (karyokinesis)
  • cytoplasmic division (cytokinesis)

The nuclear division of mitosis has several stages:

  • prophase
  • pro-metaphase
  • metaphase
  • anaphase
  • telophase
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12
Q

Describe regulation of the cell cycle

A

Cyclins and Cyclin-dependent Kinases (CDKs)

  • they are regulatory molecules
  • these proteins coordinate the Cell’s entry into the next phase of the cell cycle
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13
Q

Describe Cyclins and Cyclin-dependent Kinases (CDK’s)

A

CDKs become activated upon binding with cyclin

  • activated CDKs then cause phosphorylation to activate/inactivate target proteins
  • these activated proteins coordinate cell’s entry into the next phase of the cell cycle
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14
Q

Describe the regulation of cell cycle progression by CDK-cyclin complex

A
  • Cyclins are made and degraded cyclically
  • They activate CDK’s
  • So that the cell cycle cannot go backwards
  • different CDK-cyclin complexes control different stages of the cell cycle
  • relative concentration levels of the different CDK-cyclin complexes fluctuate, to trigger the next stage of the cell cycle
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15
Q

Which CDK inactivation control mitotic exit

A

CDK 1 inactivation

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16
Q

Describe the different cell cycle checkpoints

A

G1 checkpoints

  • check if there are plenty of nutrients
  • check at end of G1
  • if ok - enter the S phase
  • or pause/enter G0

G2 checkpoints

  • check if DNA replicated correctly and intact
  • if ok, enter mitosis

Mitotic checkpoint

  • check if chromosomes are properly aligned
  • M-cyclin degradation, CDK inactivation, and anaphase-cytokinesis
17
Q

Describe what happens if a checkpoint fails

A

This can lead to complications like

- Cancer - a disease of uncontrolled cell proliferation

18
Q

Describe a proto-oncogene

A

Proto-oncogene

  • A normal gene which, when altered by mutation, becomes an oncogene that can contribute to cancer
  • Some proto-oncogenes provide signals that lead to cell division
19
Q

Describe inhibitors of Cyclin-CDK complexes

A

Two families of genes prevent the progression of the cell cycle
- e.g. CIP/KIP family halt the cell cycle in G1, by binding and inactivating a complex of Cyclin-CDK
- These proteins are of medical interest - as they arrest the cell cycle
- could be anticancer agents
aka tumour suppressors

20
Q

Describe which type of tissue cancer can only arise from

A

Cancer can only arise from cells with proliferative potential

  • e.g. epithelium of the skin, GI tract, bone marrow cells
  • Labile cells
21
Q

Define meiosis

A

Meiosis is a process of cell division for reducing the somatic quantity of DNA (2c) to that found in sex cells or gametes (1c)

22
Q

Describe where meiosis occurs

A

The process occurs in:

  • the ovaries (oogenesis)
  • the testes (spermatogenesis)

When male (1c) and female (1c) gametes fuse to form the zygotes, the 2c quantity of DNA is restored

23
Q

Describe meiosis

A

Since DNA is duplicated in the S phase prior to cell division, we begin with 4c quantity of DNA
- therefore, there must be 2 separate reduction division to achieve the 1c amount in gametes

These are called 1st and 2nd reduction division or meiosis 1 and meiosis 2:

  • Meiosis 1 reduces 4c to 2c
  • Meiosis 2 reduces 2c to 1c
24
Q

List the stages of meiosis 1

A

Prophase I (5 stages)
Metaphase I
Anaphase I
Telophase I

25
Q

Describe prophase I (and name the different substages)

A

Recombination and pairing occur during prophase I of the first meiotic division

Prophase I is divided into 5 stages:

  • Leptotene
  • Zygotene
  • Pachytene
  • Diplotene
  • Diakinesis
26
Q

Describe the first stage of Prophase I (L)

A

Leptotene

  • Chromatin condenses to form chromosomes that are not yet split into sister chromatids
  • Along their length, chromomeres are visible
27
Q

Describe the second stage of Prophase I (Z)

A

Zygotene

  • homologous chromosomes (maternal-paternal) pair together to form bivalents
  • Chromosomes are now split into sister chromatids and local sites of alignment (synapsis) develop between sister chromatids of homologous pairs
28
Q

Describe the third stage of Prophase I (P)

A

Pachytene
- synapsis is completed and ‘crossing over’ occurs
> chiasmata formation
- At chiasmata, genetic information is exchanged between homologous pairs
- This is more likely to happen between long stretches of chromatid arms than short ones

29
Q

Describe the fourth stage of Prophase I (Dip)

A

Diplotene

- Chromatids are held together by chiasmata as well as by the centromeres

30
Q

Describe the fifth/last stage of Prophase I (Dia)

A

Diakinesis

- chiasmata appear to move towards the ends of chromatids (terminalisation)

31
Q

Describe metaphase I

A

Each bivalent has 2 centromeres, and these arrange themselves by chance on opposite sides of the metaphase plate

32
Q

Describe the genetic variation that occurs at Anaphase I

A

The impact of independent assortment of chromosomes

chance of bivalents arranging themselves along the metaphase plate

33
Q

Describe anaphase I

A

Chromosomes move to opposite poles of the cell
- Each pole has a mixture of maternal and paternal chromosomes
(a source of genetic variation)

34
Q

Describe telophase I

A

Each daughter cell has half the diploid number of chromosomes
- nuclear envelope may reassemble before prophase II

35
Q

Describe the stages of meiosis II

A

These are morphologically similar to those seen in mitosis

  • centromeres are now split at the kinetochores and sister chromatids move to opposite poles
  • At the end of the 3 divisions (meiosis I and II), potentially 4 gametes (tetrad of haploid cells) are formed
36
Q

Describe the important times genetic variation occurs during meiosis

A

Pachytene of prophase I
- genetic information is exchanged between homologous pairs of chromosomes at chiasmata

Anaphase I
- each pole has a mixture of maternal and paternal assortment, resulting in independent assortment

Fertilisation
- by chance

37
Q

Describe numerical chromosomal abnormalities

A

A numerical abnormality means an individual is either:

  • Missing one of the chromosomes from a pair (trisomy)
  • Or has more than 2 chromosomes
  • One extra is trisomy
    2 extra is tetrasomy

e.g. most common is trisomy
(Down syndrome)

38
Q

Describe structural chromosomal abnormalities

A

A structural abnormality means the chromosome’s structure has been altered in one of several ways

Medicine 1 - 02 Cell Basics (4 decks)

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