Lecture 8 - The cell nucleus

Question 1

Functions of the nucleus (5)

Answer 1

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

Question 2

DNA is stored in chromosomes (3)

Answer 2

23 pairs of chromosomes.
6 x 10^9 base pairs of DNA.
Highest level of compaction: metaphase chromosome.

Question 3

Chromosome (6)

Answer 3

A single molecule of DNA.
Linear in eukaryotes.
Contains genes.
3 structural elements
Telomeres- Protects chromosomes ends.
Centromere- needed during cell division, has have a lot of repetitive DNA, this is where the mitotic spindle attaches.
Regions of replication – used in DNA replication.

Question 4

Homologous chromosomes (3)

Answer 4

During S phase these replication origins will fire, duplicating the chromosomes.
When cell enters G2 of cell cycle it is tetraploid.
When the cell enters mitosis, they become highly condensed (tightly packaged), these are recognisable chromosomes which can then attach to mitotic spindle.

Question 5

Identifying chromosomes in the lab (9)

Answer 5

Images known as idiograms.
1) Size 2) Banding pattern 3) Centromere position.
G banding: Chromosomes partially digested and stained with Giemsa.
G dark: gene poor, heterochromosome rich.
G light: gene rich, euchromatin rich.
Forms a unique banding pattern (like a barcode).
Chromosomes have two arms, p and q. P arm is slightly shorter and Q is slightly longer.

Question 6

Practical uses (2)

Answer 6

Understand genetic diseases, by g banding them and checking how many chromosomes an individual has and if they are the correct length.
e.g. In down syndrome which is caused by the presence of all or part of a third copy of chromosome 21.

Question 7

Arm lengths (3)

Answer 7

Metacentric – Similar arm length.
Submetacentric – Centromere towards one end.
Acrocentric – Very sharp short p arm that only contains repetitive DNA and rRNA genes.

Question 8

Heterochromatin (8)

Answer 8

• Gene poor
• Appears dark
• Found near centromeres and telomeres
• Highly condensed – usually resistant to gene expression
• Often associated with the nuclear envelope
• In a typical cell 10% of the genome is heterochromatin
• Periphery of the nucleus
• Tightly wound

Question 9

Euchromatin (7)

Answer 9

• Gene rich
• Location for active genes
• Less condensed
• Most of the genome is made up of euchromatin
• Appears white in images and microscope
• Interior of the nucleus
• Loosely wound

Question 10

Spectral karyotyping (3)

Answer 10

Allows scientists to visualise all the human chromosomes at one time 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 11

How are chromosomes organised in the interphase nucleus? (3)

Answer 11

Chromosomes DNA more relaxed (decondensed).
Whereas in metaphase the DNA is more condensed.
Chromosomes form non-overlapping domains in the interphase nucleus.

Question 12

Chromosomes territories (3)

Answer 12

Chromosomes arms and bands are distinct and mutually exclusive.
A specific type of microscope is used to 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.
Chromosomes form non-overlapping domains in the interphase nucleus.

Question 13

Nuclear compartments (6)

Answer 13

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 14

DNA replication factories (2)

Answer 14

DNA replication takes place here.

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

Question 15

RNA transcription factories (3)

Answer 15

Contains:

• > RNA polymerase II.
• > Template DNA strand.
• > Newly synthesised messenger RNA.

Question 16

Nucleolus (1) Function (4)

Answer 16

The largest substructure in the nucleus.
Function:
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.
rRNA folds into a specific shape depending on the nucleotide sequence.

Question 17

Compartmentalisation of the nucleolus (3)

Answer 17

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 18

rRNA in the nucleus (3)

Answer 18

200 rRNA gene copies per haploid genome.
The genes are located in tandem copies (one after the other, no intervening sequences) on the acrocentric chromosomes (13,14,15,21,22).

Question 19

Acrocentric chromosome (2)

Answer 19

A chromosome in which the centromere is located quite near one end of the chromosome.
Down syndrome is caused by an extra acrocentric chromosome (chromosome 21).

Question 20

Why does the cell need so many copies of RNA genes? (2)

Answer 20

A single mRNA molecule can be translated many times to give amplification of the final protein product.
The ribosomal RNA molecule is not translated into protein, the ribosomal RNA molecule transcribed is the final product and the cell requires many ribosomes.

Question 21

Nuclear envelope (1)

Answer 21

Has two lipid bilayers.

Question 22

Nuclear pore (2)

Answer 22

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 23

Examples of nuclear export (2)

Answer 23

Ribosomal subunits and mRNA.

Proteins require a nuclear export signal.

Question 24

Examples of nuclear import (2)

Answer 24

Histones, DNA/RNA polymerases and other nuclear proteins.

Proteins require localisation sequence.