L3 - Eukaryotic Chromosome Structure

Question 1

Nuclear chromosomes

Answer 1

Linear DNA molecule and proteins with many functions

• Packaging and unfolding DNA
• Controlling DNA replication and repair
• Maintain chromosome integrity
• Govern proper chromosome segregation
• Regulate gene expression

Question 2

When can individual chromosomes be identified?

Answer 2

Metaphase
It is replicated and fully condensed at this stage
Karyotype - organised representation of all chromosomes at metaphase

Question 3

How to identify chromosome abnormalities?

Answer 3

Chromosome paints

DNA probes specific for individual chromosomes labelled with fluorescent dye

Question 4

How to identify interphase nucleus?

Answer 4

Chromosome paints used to identify chromosome occupying distinct subnuclear territories
Under microscope it resembles beads on string - 10nm
- This undergoes supercoiling to produce 30nm fibre

Question 5

Transcriptional activation

Answer 5

The nuclear periphery is composed of transcriptionally inactive DNA
Transcriptional activation of a gene Is accompanied by movement from periphery towards centre

Question 6

What is chromatin?

Answer 6

A chromosome is a highly coiled fibre of chromatin

Allows flexible responses to altered transcription factor activity

Question 7

Identifying core histones

Answer 7

Biochemical analysis of nucleosomes reveals a protein core around which DNA is wound.
N terminals of 8 core histone subunits project out and can interact with other proteins
- Facilitates regulation of chromatin structure

Question 8

Linker histone

Answer 8

H1 strap DNA onto histone octamers and limit movement of DNA relative to histone octamer
Stabilises formation of 30nm fibre

Question 9

30nm chromatin fibre

Answer 9

DNA is packaged by histone octamers into a compact 30nm fibre
Remodelled to accommodate protein complexes involved in replication and transcription

Question 10

Telomere

Answer 10

At ends of chromosome and maintain chromosome integrity
Replicated by telomerase
Single stranded 3’ overhanging TTAGGG repeat arrays are synthesised by telomerase

Question 11

Replication origin

Answer 11

Where DNA reaction is initiated

Question 12

Centromere

Answer 12

Kinetochore assembles on the centromere
Mediate chromosome segregation
Contains specialised proteins and DNA sequences
Alpha satellite DNA repeats - form condensed heterochromatin with histone octamers

Question 13

Kinetochore

Answer 13

Protein complex that binds microtubules in the mitotic spindle
Inner plate proteins - bind to alpha satellite DNA
Outer plate proteins - bind to microtubules

Question 14

Chromatin contains which two chromatin histones?

Answer 14

Chromatin either contains

• Centromere specific histone H3 variant
• Normal histone H3

Question 15

Yeast kinetochore

Answer 15

Is a basket that links a nucleosome of centromeric chromatin to a microtubule

Question 16

DNA sequence contents

Answer 16

50% - repeated DNA elements
- most are copied of retrotransposons
Some non-protein coding DNA encodes transcriptional regulatory information
- Determines where and when adjacent protein coding genes are expressed

Question 17

More complex organisms have

Answer 17

More protein coding genes

More non-protein coding DNA

Question 18

Transposons

Answer 18

50% of human genome is made up of 3 repeated DNA sequences

• DNA transposons
• Retroviral retrotransposons
• Non-retroviral polyA retrotransposons

Question 19

DNA transposons mechanism

Answer 19

Move by cut and paste mechanism without self-duplication
Require transposase
Powerful mutagens

Question 20

Retroviral retrotransposons mechanism

Answer 20

Replicate via RNA intermediate producing new DNA copies

These integrate at new genomic locations using self-encoded reverse transcriptase

Question 21

Non-retroviral PolyA retrotransposons mechanism

Answer 21

Replicate via an RNA intermediate using its self-encoded reverse transcriptase - copy and paste
These integrate directly into the genome at new locations
- Do not need to be packaged into virus like particles
Expanded in number during evolution
- Originated from single copy of 7SL RNA gene

Question 22

Non-retroviral PolyA retrotransposons and disease

Answer 22

Some insertions are known to disrupt genes causing human disease - Haemophilia

Question 23

Centromeric chromatin contains?

Answer 23

Specialised histone (CENP-A) that mediates attachment of chromosome to kinetochore 
Methylated core histones (H3-K4me) that help to hold sister chromatids together

Question 24

Retroviral retrotransposons example

Answer 24

Ty1-copia
Ty3-gypsy
ERV elements

Question 25

DNA transposons example

Answer 25

P-element (fly)
Activator-Dissociator (maize)
Tn3 / Tn10 (E. coli)

Question 26

Non-retroviral transposons example

Answer 26

Human L1 elements - LINE-1 
Human Alu elements
Mouse B1 elements
Long and Short Interspersed Elements
 “LINEs”  “SINEs”