Cell nucleus

Question 1

What are the five functions of the nucleus?

Answer 1

• Store and maintain the cells DNA
• DNA replication
• Transcription (making RNA)
• Ribosomal biogenesis
• Controls communication between nucleoplasm and cytoplasm

Question 2

What is a chromosome?

Answer 2

• A single molecule of DNA – double stranded
• Linear in eukaryotes
• Contains Genes
• Structural elements:
o Telomere – protects chromosome ends.
o Centromere – needed for mitotic cell division
o Origins of replication
• No correlation between complexity of organisms and number of chromosomes
• Inherited from one cell to the next

Question 3

How are chromosomes identified in the lab?

Answer 3

• Images are shown as idiograms, the banding pattern present can be used to identify the chromosome using three key features: size, banding pattern and centromere position.
• G-banding: chromosomes are partially digested and stained with Giemsa
• G dark: gene poor, heterochromatin rich.
• G light: euchromatin rich
• Forms a unique banding like a barcode
• Chromosomes have two arms, p and q, p arm is slightly shorter, and q is slightly longer.
• This can help us understand genetic diseases, by g banding them and checking how many chromosomes an individual has and if they are correct.
• In down syndrome it is caused by a third copy of chromosome 21.
• Metacentric – Similar arm length Submetacentric – Centromere towards one end Acrocentric – Very sharp short p arm that only contains repetitive DNA and rRNA genes.

Question 4

Compare and contrast heterochromatin and euchromatin

Answer 4

(as shown in table)

Question 5

Describe spectral karyotyping

Answer 5

• A lab technique that allows scientists to visualise all the human chromosomes at once by painting each pair of chromosomes in a different fluorescent colour.
• Uses fluorescent markers, in situ as chromosomes are placed on to a slide and then a hybridisation step is used and then a probe. Wherever the DNA is on the chromosomes it will bind and give a fluorescent signal.
• Fluorescence in situ hybridisation – A molecular cytogenetic technique that uses fluorescent probes that bind to only those parts of the chromosome with a high degree of sequence complementarity.

Question 6

Describe how chromosomes are organised in the nucleus during metaphase and interphase

Answer 6

on slides

Question 7

Describe chromosome territories

Answer 7

• Chromosomes arms and bands are distinct and mutually exclusive.
• A specific type of microscope is used that can take a picture through the inside of the cell at different (0.2 micron- μm//200 nm) intervals, it can focus at different focal planes within the cell nucleus at these intervals, we can then reconstruct and create a 3D image.
• P arm is red, Q arm is green. Part 3 the p and q arm are mutually exclusive within the nuclear volume.
• Chromosomes form non-overlapping domains in the interphase nucleus

Question 8

Describe the nuclear compartments

Answer 8

• Subnuclear compartments exist despite the absence of internal membranes.
• Chromosome territories- Store DNA and control access to DNA.
• Nascent RNA- New RNA, RNA being transcribed (transcription factories). Forms little dots called replication factories.
• Spliceosome- Irregular domains containing splicing factors. Forms speckled like distribution within the nucleoplasm.
• Nucleoli- Ribosome biogenesis. Large and prominent.
• PML nuclear bodies- Possible nuclear depot. Forms punctate dots (10-20 in a typical mammalian fibre blast).

Question 9

Describe the DNA replication factories and RNA transcriptase factories

Answer 9

DNA replication takes place here. Factories contain all the enzymes and other factors required to produce two new DNA strands.

This contains RNA polymerase 2, template DNA strand and newly synthesised messenger RNA

Question 10

Describe the functions of the nucleolus

Answer 10

Transcription of rRNA genes to produce large 45s rRNA precursor.
Cleavage/modification of rRNA into 18S, 5.8S and 28S rRNA.
Assembly of ribosomal subunit, 18s - small ribosomal subunit, 5.8s and 28s and 5 s large ribosomal subunit.

Question 11

Describe how the nucleolus is further compartmentalised

Answer 11

Dense fibrillar component - rRNA transcripts (the black spots).
Granular component- Processing and assembly (lighter areas with some white spots).
Fibrillar centre – ribosomal RNA genes (white area within the dense component).

Question 12

Describe nuclear pores

Answer 12

• The nuclear envelope of a typical mammalian cells contains around 3000-4000 nuclear pore complexes.
• Only small water-soluble molecules can diffuse through the nuclear pore.

Question 13

Describe nuclear export

Answer 13

• e.g. ribosomal subunits and mRNA.

* Proteins require a nuclear export signal.

Question 14

Describe nuclear import

Answer 14

• e.g. histones, DNA/RNA polymerases and other nuclear proteins.
• Proteins require localisation sequence.