Lecture 4- Eukaryotic chromosome structure

Question 1

What is the karyotype of the parent organism?

Answer 1

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

Question 2

What is a highly coiled fibre of chromatin?

Answer 2

A chromosome

Question 3

Explain what the term ‘beads on a string’ means when describing interphase chromatin

Answer 3

The beads are nucleosomes and the string is double stranded DNA

Question 4

What are the protein subunits of the nucleosomes called?

Answer 4

Core histones

Question 5

Explain the structure of the nucleosomes

Answer 5

The N-terminal tails of the 8 core histone subunits project out from the nucleosomes core and are free to interact with other proteins, facilitating regulation of chromatin structure and function.

Question 6

What is the role of linker histones?

Answer 6

• Strap DNA onto histone octamers
• Limit movement of DNA relative to the histone octamer
• Facilitates the establishment of transcriptionally silent heterochromatin

Question 7

How and why is DNA packaged

Answer 7

• DNA is packaged by histone octamers into a compact, flexible chromatin scaffold
• Chromatin scaffold can be remodelled to accommodate protein complexes involved in gene transcription and DNA replication

Question 8

What is the role of chromatin?

Answer 8

Permits flexible responses to altered transcription factor activity caused by changes in cell differentiation status and changes in signalling pathways

Question 9

What is interphase chromatin?

Answer 9

A set of dynamic, fractal globules that can reversibly condense and decondense without becoming knotted

Question 10

What does the nuclear periphery in interphase cells include and exclude?

Answer 10

Includes transcriptionally inactive DNA and excludes RNA transcripts

Question 11

What follows the transcriptional activation of a gene?

Answer 11

The movement of the gene from the periphery towards the centre of the nucleus

Question 12

Chromosomes contain specialised DNA sequences that facilitate…

Answer 12

1. Reliable and complete DNA replication

2. Segregation of duplicated chromosomes during cell division

Question 13

Define telomere

Answer 13

DNA sequences at the end of linear chromosomes which maintain chromosomal integrity

Question 14

Define replication origin

Answer 14

DNA sequence where DNA replication is initiated

Question 15

Define centromere

Answer 15

A DNA sequence which is (indirectly) attached to the mitotic spindle and mediates chromosomes segregation at mitosis and meiosis

Question 16

Define kinetochore

Answer 16

Protein complex that binds to centromeric DNA sequences and the microtubules of the mitotic/meiotic spindle

Question 17

How are telomeres (TTAGGG) replicated?

Answer 17

Telomeres are replicated by a specialised DNA polymerase called telomerase

Question 18

What do kinetochore inner and outer plate proteins bind to?

Answer 18

Kinetochore inner plate proteins bind to chromatin containing alpha-satellite DNA

Kinetochore outer plate proteins bind to protein components of mitotic spindle

Question 19

Describe the structure of centromeres

Answer 19

Centromeres contain alpha-satellite DNA repeats that readily form condensed chromatin with histone octamers containing unusual subunits

Question 20

Describe the kinetochore in yeast

Answer 20

The kinetochore is a basket that links a single nucleosome of centromeric chromatin to a single microtubule

Question 21

What 2 things accompany increasing biological complexity?

Answer 21

1. Increase numbers of protein coding genes
2. Increasing amounts of non-protein coding DNA for regulating transcription and organising access to protein coding genes

Question 22

What does non protein encoding DNA that encodes cis-regulatory information determine?

Answer 22

Determines when and where in the body adjacent protein coding genes are transcribed

Question 23

What are the 3 types of transposons?

Answer 23

1. DNA transposons
2. Retroviral retrotransposons
3. Non-retroviral polyA retrotransposons

Question 24

How do DNA transposons move around the genome?

Answer 24

Move by a cut and paste mechanism without self duplication, requiring the transposon encoded enzyme Transposase

Question 25

How do other transposable elements behave like retroviruses?

Answer 25

Replicating via RNA intermediates, producing new DNA copies that integrate at new genomic locations during self encoded reverse transcriptase

Question 26

What are polyA retrotransposons?

Answer 26

• non-viral transposons
• abundant in vertebrate genomes
• replicates via an RNA intermediate using its own retrotransposons encoded reverse transcriptase

Question 27

What happens to products of L1 reverse transcription (an example of a polyA retrotransposons)?

Answer 27

They are integrated directly into the genome at new locations without the need to be packaged into a virus like particle.

Question 28

What has occurred to the numbers of non retroviral retrotransposons during higher mammal evolution?

Answer 28

Numbers have expanded hugely