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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
Which groups can be added/removed from the N terminal tails of histones? What do these acts as signals for?
Methyl Acetyl Phosphoryl Act as signals to machinery (DNA repair, replication etc.) to carry out their functions
26
What is histone H1? What does it do?
- 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
27
How does histone H1 interact with DNA?
H1 is net positive (rich in basic amino acids) DNA is net negative
28
What is bound/unbound to transcriptionally active DNA
Actively transcribed DNA is FREE of histones and H1 Bound by sequence-specific binding proteins
29
What does chromatin remodelling allow?
Accommodation of protein complexes involved in gene transcription/DNA replication
30
When does chromatin remodelling occur?
In response to changes in cell differentiation status and environmental signals which change transcription factors
31
What is a replication origin? Where are they located?
A specific DNA sequence where DNA replication is initiated Located in multiple positions a long the DNA of most eukaryotic chromosomes
32
How many replication origins are present in yeast?
One
33
What are telomeres? What do they do?
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
What is a centromere? What does it mediate?
DNA sequences on which the kinetochore assembles and the mitotic spindle attaches Mediates the segregation of chromosomes in mitosis and meosis
35
What is the kinetochore?
A protein complex which binds microtubules in the mitotic spindle to the DNA sequence in the centromere
36
What is the sequence of a telomere?
TTAGGG
37
What is the structure of a telomere?
Initially double stranded but at the extreme 3' end there are many single stranded repeats - forming an overhang
38
How are repeats added to the 3' overhang of the telomere?
By a special DNA polymerase TELOMERASE
39
What is the structure of the centromere?
Has short alpha satellite-DNA repeats, within higher order repeats Forms condensed heterochromatin with histone octomers which contain unusual subunits
40
What allows the centromere to interact with the kinetochore proteins?
Specialised histones which interact with the DNA repeats in the centromere (which have formed heterochromatin)
41
What is the structure of the kinetochore?
Bilayer structure: - Inner plate - binds to alpha satellite DNA in the centromere - Outer plate - binds to protein components of the mitotic spindle (microtubules)
42
What histone allows the centromere to interact with the kinetochore? What is the name of this histone?
Centromere-specific H3 variant CENP-A - Makes specific interactions with the components of the kinetochore
43
Why do some parts of the centromere interact with the kinetochore and others don't?
The histones which interact with the repeats of the centromere are different
44
What is the structure of normal histone H3? What do these histones do?
DImethylated at lysine 4 They act to hold sister chromatids together
45
What is the name of the histone which holds the sister chromatids together?
H3-K4me
46
What is the name of the histone which allows attachment to the kinetochore?
CENP-A
47
What is the structure of the kinetochore in yeast?
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
How much of the DNA encodes information to make proteins?
1.5%
49
How much of the DNA is made of repeated sequences which are not centromeric? What DNA element does this include?
50% Includes transposons
50
How much of the DNA is made of unique sequences?
50%
51
What in the DNA contributes to UNIQUE sequences?
- Exons - Introns - Non-repetitive unique sequences that is neither exons or introns
52
What percentage of the DNA is introns?
20%
53
What percentage of the DNA is Non-repetitive unique sequences that is neither exons or introns? What are these?
30% These act as transcriptional regulatory sequences (eg. enhancer, promotors)
54
What happens to the protein coding genes and non-protein coding genes in more complex organisms?
They increase Can regulate and organise access to protein coding genes more precisely
55
What are transposons? What can they do?
Mobile genetic elements which can jump around the genome by transposition Can reorganise the genome
56
What are 3 classes of transposons?
1) DNA transposons 2) Retroviral transposons 3) Non-retroviral - polyA transposons
57
How do DNA transposons move in the genome?
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
What are examples of DNA transposons?
P-element (fly) Ac-Ds (maize) Tn3/Tn10 (E.coli)
59
What can DNA transposons do?
Shuffle sequences - are powerful mutagens
60
Who discovered DNA transposons and how?
Barbra McClintock When she saw spontaneous mutations occurring in her maize which changes the pigmentation
61
What are retroviral-like transposons? How do they move in the genome?
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
How are retroviral-like transposons different to retroviruses?
They cannot LEAVE the cell BUT they are passed on to descendants of the cell (Lack a protein coat)
63
What are examples of retroviral transposons?
Ty1-copia Ty3-gypsy ERV elements
64
What are examples of non-retroviral retrotransposons?
LINE-1 elements (human) Mouse B1 elements LINEs (long interspersed elements) SINEs (short interspersed elements)
65
How are retroviral like transposons different to non-retroviral poly-A retrotransposons?
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
How are retroviral like transposons the same as non-retroviral poly-A retrotransposons?
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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