8. Cell nucleus

Question 1

Describe the nucleus (1pt)

Answer 1

The nucleus stores and maintains the cells DNA

Question 2

Describe the nuclear Pore (1pt)

Answer 2

The nuclear pore allows communication between the nucleoplasm and the cytoplasm

Question 3

Describe a chromosome (6pts)

Answer 3

1. A single molecule of DNA
2. Linear in eukaryotes
3. Contains genes
4. Contains a telemore which protects chromosome ends
5. Contains a centromere which is needed during cell division.
6. Contains origins of replication along the chromosome. This is required to initiate DNA replication during the S phase.

Question 4

Describe centromeres (3pts)

Answer 4

1. Centromeres lock sister chromatids together after the s phase of the cell cycle.
2. Centromere is the attachment site for chromosomes to the mitotic spindle via the kinetochore.
3. In humans centromeres are megabases of repetitive DNA.

Question 5

Describe a telomere (1pts)

Answer 5

Telomere is the end of the chromosomes which are made up of a tandem repeat

Question 6

Describe the end replication problem (1pts)

Answer 6

With each round of replication we loose a little bit of the end of the chromosome. These repeats are lost as the cell divides.

Question 7

Describe the Hayflick limit (1pts)

Answer 7

Hayflick limit is the number of times a cell can divide before it dies

Question 8

Describe Telomerase (1pt)

Answer 8

A RNA-dependant DNA polymerase that adds telomeric DNA to telomeres

Question 9

Describe the activity of telomerase (3pts)

Answer 9

1. Removal of the RNA primer leads to the shortening of the chromosome after each round of replication. Chromosome shortening eventually leads to cell death.
2. An RNA sequence in telomerase acts as a template for DNA. This enzyme adds the telomeric sequence to the 3 prime end of the chromosome.
3. This continues until the original length of the chromosomal DNA has been restored.

Question 10

Describe origins of replication (4pts)

Answer 10

1. DNA synthesis begins at replication origins
2. Bacteria have a single origin of replication
3. Eukaryotic chromosomes are large and DNA replication is also slower so multiple origins must fire simultaneously for replication to be completed within a reasonable timescale.
4. Origins are clustered in replication units

Question 11

Describe G banding (4pts)

Answer 11

1. G banding produces a metaphase spread where you can visualize the chromosomes under a light microscope.
2. This then produces karytopying.
3. G dark= Gene poor, tend to be heterochromatin rich
4. G light= Gene rich, tend to be euchromatin rich.

Question 12

What key features are used to identify chromosomes ? (3pts)

Answer 12

1. Size
2. Banding pattern
3. Centromere position

Question 13

Describe Chromosome organisation in the interphase nucleus using an EM image? (1pt)

Answer 13

Cant identify information about chromosomes or see individual chromosomes so cannot say where they are located within the nucleus.

Question 14

Describe Chromosome organisation in the interphase nucleus using the FISH technique (3pts)

Answer 14

1. This technique allows decondensed chromosomes in the interphase nucleus to be visualised.
2. Metaphase chromosomes DNA= tightly packed and condensed.
3. Interphase chromosomes DNA= DNA is decondensed and relaxed

Question 15

Describe Chromosome organisation in the interphase nucleus using Spectral Karyotyping? (4pts)

Answer 15

1. Found that chromosomes form non-overlapping domains in the interphase nucleus
2. Allows you to clearly see coloured patterns of chromosomes
3. Does not require a computer program
4. Allowed researchers to understand that chromosome arms and bands are distinct and are mutually exclusive.

Question 16

What happens in response to transcriptional activation (1pts)

Answer 16

Genes have preferential locations at the surface of the chromosome territory and can dynamically loop out in response to transcription activation e.g if you add interferon to a cell.

Question 16

What happens in response to transcriptional activation (1pts)

Answer 16

Genes have preferential locations at the surface of the chromosome territory and can dynamically loop out in response to transcription activation e.g if you add interferon to a cell.

Question 17

Describe the Nucleolus (6pts)

Answer 17

1. The nucleolus is the largest substructure in the nucleus
2. The nucleolus is the site of ribosome subunit production
3. The nucleolus is the site of transcription
4. The nucleolus is a self organising nuclear compartment.
5. The nucleolus consists of 3 distinct zones which can be seen via electron microscopy.
6. The nucleolus forms around the NOR- (nucleolar organising regions). NOR’s are the locations of the RNA genes.

Question 18

Describe Acrocentric chromosomes (3pts)

Answer 18

1. There are 20 rNA gene copies per haploid genome. These genes are located in tandem copies on the acrocentric chromosomes.
2. Acrocentric chromosomes= 13, 14, 15, 21, 22. The p arm of all of them are very short and composed of heterochromative repetitive DNA and ribosomal RNA genes.

Question 19

Describe splicing speckles (7pts)

Answer 19

1. There are 20-50 per cell nucleus
2. They are composed of splicing factors and other mRNA processing factors
3. They vary in size and shape
4. They are used as a model system to study nuclear organisation.
5. Do not contain DNA
6. Not a site of transcription but are associated with highly active transcription sites.
7. Splicing speckle experiments change the internal conditions of the cell and observe the effect on the compartments.

Question 20

Experiment 1= Inhibit transcription so there is no splicing in the cell

Answer 20

The splicing speckles round up and became larger. This supports speckles as a storage compartment and it shows that it is not a direct site of splicing.

Question 21

Experiment 2= Add more intron containing genes to a cell

Answer 21

This increases splicing. Splicing factors redistribute to transcription sites and the speckles get smaller. This supports speckles as a reservoir of splicing factors.

Question 22

Describe super resolution microscopy (4pts)

Answer 22

1. There was no high quality 3D analysis due to limited resolution of optical microscopy.
2. Super resolution microscopy was therefore invented. It can revolve single replication foci in the s phase
3. Predicted number of replicons= 5149 firing at the same time.
4. It challenges the conventional interpretation of nucleus replication foci as replication factories.

Question 23

Describe transcription factories (4pts)

Answer 23

1. Transcription factories are sites where multiple active RNA polymerase are concentrated.
2. Termed transcription factories and proposed that genes pass through the factory as they are transcribed.
3. Proposed that genes from the same or different chromosomes may associate at the same factory.
4. Super resolution microscopy disproved transcription factories.

Question 24

Describe the nuclear envelope

Answer 24

The nuclear envelope is formed of two bilayers- an inner nuclear membrane and an outer nuclear membrane. The inner surface of the inner membrane is the nuclear lamina and the space between the two membranes is the peripheral space. The nuclear envelope has 3000-4000 nuclear pores in it that are the site of communication between the nucleoplasm and the cytoplasm.

Question 25

Describe the nuclear pore (3pts)

Answer 25

1. The nuclear pore allows communication to take place between the nucleoplasm and the cytoplasm.
2. Movement through the nuclear pore is strictly controlled and only small water soluble molecules can diffuse freely through the pore.
3. Large molecules must be actively transported through the nuclear pore.

Question 26

Describe nuclear export

Answer 26

Proteins require a nuclear export signal e.g ribosomal subunits and mrna

Question 27

Describe a nuclear import

Answer 27

Proteins require a nuclear localization sequence e.g histones, DNA/poymserases and other nuclear proteins.