Nucleus

Question 1

What are the functions of the nucleus?

Answer 1

stores and maintains the cell’s DNA in the form of chromosomes
DNA replication, transcription,
Ribosomal biogenesis (occurs in the nucleolus)
Controls the communication between the cytoplasm and the nucleoplasm

Question 2

How many pairs of chromosomes does a human cell have, how much DNA is this?

Answer 2

23 pairs of chromosomes there is 6x10^9 base pairs of DNA inside the nucleus

Question 3

How is DNA packaged?

Answer 3

DNA wraps around 8 histones (H2A, H2B, H3,H4). This forms an octamer core. There is also a H1 histone located on the outside which binds to the linker DNA. This forms a nucleosome which forms 30nm beads n a string this is then folded looped and coiled into a chromosome

Question 4

What is the structure of the chromosome in humans?

Answer 4

Composed of a single linear molecule of DNA.
Contains telomeres which protect the ends of the chromosomes when the cell divides.
(also unusual DNA structure contains hexamer repeats of guanine TTAGGG)
Contains a centromere -Involved in cell division and is the part which the mitotic spindle attaches

Question 5

Why does DNA need multiple replication origins?

Answer 5

So it can replicated DNA simultaneously in S phase

Question 6

What are maternal and paternal homologs?

Answer 6

They are homologous chromosomes one from each parent. Maternal and paternal

Question 7

How are chromosomes identified by G banding?

Answer 7

Chromosomes are stained with Gisema in metaphase when the chromosomes are condensed

Question 8

What are the key features used to identify chromosomes?

Answer 8

1. Size
2. Banding pattern
3. Centromere positioning
   (e. g acrocentric chromsomes is when the chromosome is located near the end of one chromosomes)

Question 9

What is spectral karyotyping?

Answer 9

Karyotyping is used to identify chromosome abnormalities.

Spectral karyotyping is when fluorescent in situ hybridisation = use of fluorescent probes to detect DNA sequences

Question 10

What are the features of heterochromatin?

Answer 10

Dark stained
Located around the periphery
Gene poor found near centromere and telomeres
Highly resistant to gene expression (condensed).
Compromises 10% of the genome

Question 11

What are the features of euchromatin? (ACTIVE)

Answer 11

Light stained
Located in the centre
Genes are actively transcribed
Majority of the genome

Question 12

What is functional nuclear compartmentalisation?

Answer 12

The nucleus is composed of many nuclear sub compartments despite the fact that there are no internal membranes

Question 13

What are the different nuclear sub compartments?

Answer 13

Chromosome territories = store DNA - control access to DNA
DNA replication factories = contain all the enzymes required for DNA replication
RNA transcription factories = contain all the machinery for transcription, RNA Polymerase II, template strand and newly synthesised mRNA
Spliceosome = contains irregular domains with splicing factors
Nucleoli = ribosomal biogenesis (largest sub structure)
PML (promyelotic leukaemia nuclear bodies) = possible nuclear depot and is associated with cellular disorders

Question 14

What are the features of the nucleolus?

Answer 14

Largest sub structure inside the nucleus involved in ribosomal biogenesis
. 1) Transcription of rRNA genes (FC) producing rRNA transcript (DFC)
2) Production of large 45S precursor
3) Cleaved into small ribosomal subunit 18S, 33 proteins and large ribosomal subunit 28S, 5.8S and 5S, 49 proteins.
4) Assembly of ribosomal subunits (GC)

Question 15

What are the distinct zones that were discovered by electron microscopy?

Answer 15

FR - fibrillar centre - contains RNA genes
DFC - dense fibrillar components - contains rRNA transcripts
GC - granular component - involved in processing and assembly

Question 16

What are the tandem copies of rRNA genes?

Answer 16

200 Repeats of the rRNA gene located on acrocentric chromosomes

Question 17

What is special about rRNA genes?

Answer 17

They are located in tandem copies (repeats over and over) on acrocentric chromosomes

Question 18

Why does the cell need tandem copies on the

Answer 18

To meet the needs of the ribosomes that need to be produced

Question 19

Can rRNA be amplified and translated?

Answer 19

No unlike mRNA, rRNA is not amplified or translated instead it is assembled to form the final product after transcription

Question 20

How many rRNA copies are there per haploid genome?

Answer 20

200 rRNA gene copies per haploid genome the tandem

copies are located on acrocentric chromosomes

Question 21

What is the nuclear envelope?

Answer 21

Lipid bilayer which enables communication between the nucleoplasm and cytoplasm, there are 3000-4000 nuclear pore complexes in the nuclear envelope

Question 22

How many holes are there in the nuclear envelope?

Answer 22

There are 3000-4000 nuclear pore complexes per nuclear envelope

Question 23

What molecules can be imported into the nucleoplasm?

Answer 23

DNA polymerase, RNA polymerase, histones.

Import requires a NUCLEAR LOCALISATION SEQUENCE

Question 24

What molecules are exported out of the nucleoplasm into the cytoplasm?

Answer 24

mRNA, ribosomal subunits

Requires a nuclear export signal

Question 25

What is the spatial location of gene poor vs gene rich chromosomes?

Answer 25

Gene poor chromosomes will be located on the periphery of the nucleus
Gene rich chromosomes will be located in the interior of the nucleus

Question 26

What does the nuclear lamina do?

Answer 26

Maintains the structure of the nucleus and nuclear lamina proteins are involved in regulating gene expression

Question 27

What do the chromosome territories look like in metaphase and interphase?

Answer 27

In interphase = non-overlapping domains, intermingling, DNA is relaxed and decondensed
In metaphase = chromosomes are tightly packed and less intermingling

Question 28

What are metacentric chromosomes?

Answer 28

The chromosome is in the centre and both arms are of equal length

Question 29

What are submetacentric chromosomes?

Answer 29

The chromosome is slightly offset the centre leading to a slight asymmetry in the length of the two parts

Question 30

What are acrocentric chromosomes?

Answer 30

The centromere is severly offset from the centre with one short arm and one very long arm. e.g the P arm is shorter than the q arm.