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BMS 242 - Advanced Cell and Molecular > Eukaryotic Chromosome Structure and Function > Flashcards

Eukaryotic Chromosome Structure and Function Flashcards

1
Q

What did Boveri and Sutton do in 1902?

A
  • Spotted the chromosome under the light microscope
  • Saw them double in number and then divide into 2 cells
  • Speculated heritable factors determined cell phenotypes
  • CHROMOSOME THEORY OF INHERITANCE
  • Thought genes are on chromosomes
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2
Q

What did Morgan do in 1915?

A
  • Identified using the light microscope STRUCTURAL REARRANGEMENTS which correlate with phenotypic changes in fruit flies
  • FULLY established the link between genes and chromosomes
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3
Q

What do chromosomes contain?

A

Linear DNA

And many other proteins that confer specialist functions

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4
Q

What functions do the proteins carried by chromosomes have? (5)

A

1) Packing and unfolding of DNA within the nucleus (in a regulated way)
2) Genetic recombination (in germ cells)
3) Maintain chromosome integrity (prevent loss of end sequences)
4) Govern proper chromosome segregation
5) Regulate gene expression

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5
Q

Where are chromosomes located in eukaryotes?

A

In the nucleus

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6
Q

What does unfolding of DNA allow?

A

Genes to be accessible to machineries for replication/repair/transcription

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7
Q

What organelles contain chromosomes and how?

A

Mitochondria and chloroplasts

Small, circular chromosomes

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8
Q

At what stage in the cell cycle can chromosomes be easily distinguished?

A

Metaphase - they have been replicated

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9
Q

How do chromosomes differ from each other?

A

Size, DNA sequence content (genes), banding

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10
Q

How many chromosomes are present in eukaryotic DNA?

A

23 pairs

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11
Q

How is a ‘metaphase spread’ made?

A

By smashing dividing cells (to break open the nucleus) onto a microscopic slide

Chromosomes laid out in a random arrangement

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12
Q

What is ‘chromosome painting’?

A

Used to distinguish chromosomes from each other - done in metaphase

  • Hybridise DNA with probes that have sequences which match the DNA sequence of a specific chromosome
  • Each probe is labelled different colour of fluorescence
  • Each chromosome will have a specific colours to them
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13
Q

What is a karyotype?

A

The organised representation of all the chromosomes in a eukaryotic cell at METAPHASE

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14
Q

What can a karyotype be used for?

A

To spot abnormalities in the karyotype and therefore in the genome

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15
Q

In chromosome painting/ karyotyping, what does a chromosome of 2 different colours show?

What is this typical of?

A

Translocation

Typical of leukemias

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16
Q

What do individual chromosomes occupy, even in interphase nuclei?

How is this seen?

A

Distinct 3D regions of the nuclei, which is separate to regions of the other chromosomes

Each chromosome is specific relations to other chromosomes - may influence each other

Seen using chromosome paints - each area of the nucleus is a different colour

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17
Q

When in the cell cycle does transcription occur?

A

In interphase

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18
Q

What is contained in the nuclear periphery?

A

Highly condensed transcriptionally inactive DNA

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19
Q

What is contained in the centre of the nucleus?

A

Active genes

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20
Q

What happens to a gene when it becomes active?

How is this seen?

A

Moves from the periphery to the centre of the nucleus

Seen using chromosome paints and following the movement

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21
Q

What is the structure of a chromosome?

A
  • Highly ordered chromatin
  • 10nm thick fibre of DNA wrapped TWICE around a histone octamer to form a HISTONE
  • ‘Beads on a string’
  • Can be supercoiled to give 30nm fiber and then again to give fully condensed chromosomes
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22
Q

How many base pairs of DNA wrap around a histone octamer?

A

146

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23
Q

What is the structure of a core histone?

A

Octamer:

- Each subunit has a terminal end tail of 30 amino acids which project away from the nucleosomal core

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24
Q

What does the tail of a histone monomer allow?

A
  • Interaction with the regulatory proteins which covalently modify the properties of the end terminal tails (modifications are reversible)
  • To add or remove METHYL, ACETYL, PHOSPHORYL groups
  • Which act as signals to machinery (DNA repair, replication etc.) to carry out their functions
  • Facilitating the regulation of chromatin structure and function
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25
Q

Which groups can be added/removed from the N terminal tails of histones?

What do these acts as signals for?

A

Methyl
Acetyl
Phosphoryl

Act as signals to machinery (DNA repair, replication etc.) to carry out their functions

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26
Q

What is histone H1?

What does it do?

A
  • A linker histone
  • Fixes the entry and exit strand of DNA around a histone together - stablising the position of the histone with respect to DNA and limiting the movement of DNA
  • Governs the transition of chromatin from a condensed, to an accessible state
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27
Q

How does histone H1 interact with DNA?

A

H1 is net positive (rich in basic amino acids)

DNA is net negative

28
Q

What is bound/unbound to transcriptionally active DNA

A

Actively transcribed DNA is FREE of histones and H1

Bound by sequence-specific binding proteins

29
Q

What does chromatin remodelling allow?

A

Accommodation of protein complexes involved in gene transcription/DNA replication

30
Q

When does chromatin remodelling occur?

A

In response to changes in cell differentiation status and environmental signals which change transcription factors

31
Q

What is a replication origin?

Where are they located?

A

A specific DNA sequence where DNA replication is initiated

Located in multiple positions a long the DNA of most eukaryotic chromosomes

32
Q

How many replication origins are present in yeast?

A

One

33
Q

What are telomeres?

What do they do?

A

Specific DNA sequences at the ends of linear chromosomes

  • Maintain chromosome integrity, preventing the chromosome from shortening at each cell cycle
  • Preventing the loss of genes
34
Q

What is a centromere?

What does it mediate?

A

DNA sequences on which the kinetochore assembles and the mitotic spindle attaches

Mediates the segregation of chromosomes in mitosis and meosis

35
Q

What is the kinetochore?

A

A protein complex which binds microtubules in the mitotic spindle to the DNA sequence in the centromere

36
Q

What is the sequence of a telomere?

A

TTAGGG

37
Q

What is the structure of a telomere?

A

Initially double stranded but at the extreme 3’ end there are many single stranded repeats - forming an overhang

38
Q

How are repeats added to the 3’ overhang of the telomere?

A

By a special DNA polymerase TELOMERASE

39
Q

What is the structure of the centromere?

A

Has short alpha satellite-DNA repeats, within higher order repeats

Forms condensed heterochromatin with histone octomers which contain unusual subunits

40
Q

What allows the centromere to interact with the kinetochore proteins?

A

Specialised histones which interact with the DNA repeats in the centromere (which have formed heterochromatin)

41
Q

What is the structure of the kinetochore?

A

Bilayer structure:

  • Inner plate - binds to alpha satellite DNA in the centromere
  • Outer plate - binds to protein components of the mitotic spindle (microtubules)
42
Q

What histone allows the centromere to interact with the kinetochore?

What is the name of this histone?

A

Centromere-specific H3 variant

CENP-A
- Makes specific interactions with the components of the kinetochore

43
Q

Why do some parts of the centromere interact with the kinetochore and others don’t?

A

The histones which interact with the repeats of the centromere are different

44
Q

What is the structure of normal histone H3?

What do these histones do?

A

DImethylated at lysine 4

They act to hold sister chromatids together

45
Q

What is the name of the histone which holds the sister chromatids together?

A

H3-K4me

46
Q

What is the name of the histone which allows attachment to the kinetochore?

A

CENP-A

47
Q

What is the structure of the kinetochore in yeast?

A

Basket-like structure which links a SINGLE subunit of a SINGLE nucleosome of centromeric chromatin (containing CENP-A (centromere specific H3 varient) to a SINGLE microtubule

Inner kinetochore plate is a basket shaped structure and is linked through a set of helically interacting proteins to the OUTER kinetochore plate

Outer kinetochore plate exists as a RING of subunits, into which a SINGLE microtubule bundle is inserted

48
Q

How much of the DNA encodes information to make proteins?

A

1.5%

49
Q

How much of the DNA is made of repeated sequences which are not centromeric?

What DNA element does this include?

A

50%

Includes transposons

50
Q

How much of the DNA is made of unique sequences?

A

50%

51
Q

What in the DNA contributes to UNIQUE sequences?

A
  • Exons
  • Introns
  • Non-repetitive unique sequences that is neither exons or introns
52
Q

What percentage of the DNA is introns?

A

20%

53
Q

What percentage of the DNA is Non-repetitive unique sequences that is neither exons or introns?

What are these?

A

30%

These act as transcriptional regulatory sequences (eg. enhancer, promotors)

54
Q

What happens to the protein coding genes and non-protein coding genes in more complex organisms?

A

They increase

Can regulate and organise access to protein coding genes more precisely

55
Q

What are transposons?

What can they do?

A

Mobile genetic elements which can jump around the genome by transposition

Can reorganise the genome

56
Q

What are 3 classes of transposons?

A

1) DNA transposons
2) Retroviral transposons
3) Non-retroviral - polyA transposons

57
Q

How do DNA transposons move in the genome?

A

By ‘cut and paste’ WITHOUT self-duplication:

1) Chromosome is cut
2) Chromosome position is repaired
3) Transposon inserts elsewhere

  • Require enzyme transposase - which removes it from the genome
58
Q

What are examples of DNA transposons?

A

P-element (fly)
Ac-Ds (maize)
Tn3/Tn10 (E.coli)

59
Q

What can DNA transposons do?

A

Shuffle sequences - are powerful mutagens

60
Q

Who discovered DNA transposons and how?

A

Barbra McClintock

When she saw spontaneous mutations occurring in her maize which changes the pigmentation

61
Q

What are retroviral-like transposons?

How do they move in the genome?

A

Transposons which act like retroviruses
They exist as DNA within the genome

1) They are TRANSCRIBED - producing an RNA copy
2) One of the genes they encode is a REVERSE TRANSCRIPTASE

3) Reverse transcriptase uses the RNA to make a DNA copy - RNA/DNA hybrid intermediate
4) DNA copy is inserted elsewhere in the genome using INTERGRASE

62
Q

How are retroviral-like transposons different to retroviruses?

A

They cannot LEAVE the cell BUT they are passed on to descendants of the cell

(Lack a protein coat)

63
Q

What are examples of retroviral transposons?

A

Ty1-copia
Ty3-gypsy
ERV elements

64
Q

What are examples of non-retroviral retrotransposons?

A

LINE-1 elements (human)
Mouse B1 elements
LINEs (long interspersed elements)
SINEs (short interspersed elements)

65
Q

How are retroviral like transposons different to non-retroviral poly-A retrotransposons?

A

Differ in genetic structure:

Retroviral like transposons have a LTR sequence at the end - which are characteristic of retroviruses

Non-retroviral like transposons DONT

66
Q

How are retroviral like transposons the same as non-retroviral poly-A retrotransposons?

A

They both contain reverse transcriptase and move by ‘copy and past’ using intergrase

BMS 242 - Advanced Cell and Molecular (24 decks)

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