Cell nucleus

Question 1

What are the functional elements of a chromosome?

Answer 1

a chromosome is a single molecule of DNA

• linear in eukaryotes
• contains genes

functional elements

• origin of replication = site of replication, rewired to initiate DNA replication during the S phase
• centromere = needed during cell division
• telomeres = are at each end of the chromosome, used to protect the ends of chromosomes

Question 2

What is the function of centromeres?

Answer 2

are megabases of repetitive DNA, major component is the alpha satellite DNA in humans
- all chromosomes have different satellite DNA configurations = at the centre of centromeres

function 
- locks sister chromatids together

• is the attachment site for chromosomes to the mitotic spindle via a protein structure called the kinetochore

Question 3

What is the centromere composed of?

Answer 3

kinetochore outer plate
- formed from special proteins
- binds the mitotic spindle (spindle microtubules)
kinetochore inner plate
- formed by kinetochore proteins bound to the alpha satellite DNA

heterochromatin - type of chromatin
- formed on the alpha satellite DNA

Question 4

What is the function of telomeres?

Answer 4

made up of tandem repeat bases = TTAGGG

• join at the end of each chromosome and form a loop
• protects chromosomes by preventing lost of DNA during each replication/cell division by being lost instead = end replication problem

joins where the first RNA primer was removed from
- cannot be fused together to the rest of the strand because there was no initial strand = primer started transcription

Question 5

What are the origins of replication?

Answer 5

eukaryotes

• have multiple origins of replication
• have a large genome/chromosome = require multiple origins to allow replication simultaneously, happens at a reasonable rate

prokaryotes

• have a single origin of replication
• do not have an especially large genome = one origin allows them to replicate at a reasonable rate

Question 6

What is the specific characteristic of chromosomes which allow for identification?

Answer 6

G banding within the chromosomes

• G-banding is a technique used to produce thin, alternating bands along the length of the entire chromosome
• creates unique patterns on each homologous set and allows for their identification

G banding: chromosomes partially digested and stained with Giemsa
G dark – gene poor, tend to be heterochromatin rich
G light – gene rich, tend of be euchromatin rich

Question 7

What are the three identifying characteristics of chromosomes?

Answer 7

size
- chromosome 1 is the largest whilst chromosome 21 and 22 are the largest

banding pattern

• G dark - heterochromatin
• G light - euchromatin

centromere position

• each arm of the chromsome is called p or q
• check where p and q are in relation to the centromere portion

metacentric - centromere is roughly in the centre of the arms
submetacentric - centromere is offset towards the p or q arm (uneven arm length)
acrocentric - no genetic material in the p arm ecept for rRNA

Question 8

What is karyotyping?

Answer 8

karyotype

• is an individual’s collection of chromosomes
• refers to a laboratory technique that produces an image of an individual’s chromosomes
• is used to identify and evaluate the size, shape, and number of chromosomes in a sample of body cells
• is used to look for abnormal numbers or structures of chromosomes.

spectral karyotyping
- full colour map of chromosomes

Question 9

How are the types of chromatin and chromosomes positioned in the nucleus during interphase?
- electron microscopy

Answer 9

at interphase they are decondensed to enable visualisation

heterochromatin - stains dark
- positioned at the periphery and towards the edge
euchromatin - stains light
- positioned in the interior

chromsomes

• chromosome arms and bands are distinct and mutually exclusive
• chromosomes form non-overlapping domains in the interphase nucleus

Question 10

How do sub-compartments exist in the nucleus?

Answer 10

sub-compartments exists despite the absence of internal membranes

Question 11

What are the different sub-compartments within the nucleus?

Answer 11

chromosome territories
- store DNA and control access to DNA

replication factories
- nascent/new DNA production

transcription factories
- nascent/new RNA production

spliceosome
- irregular domains containing splicing factors

nucleoli
- ribosome biogenesis

PML nuclear bodies: possible nuclear depot

Question 12

What is the nucleolus? How does it function?

Answer 12

is the largest sub-structure of the nucleus

• is the self organising nuclear compartment
• site of ribosome subunit production

RNA polymerase transcribes the precursor rRNA
rRNA is processed to 18S, 5.8S and 28S RNA molecules by endonuclease and exonuclease cleavage
5S is transcribed in the nucleoplasm by RNA polymerase III and transported to the nucleolus
rRNA folded and associate with 79 ribosomal proteins to assemble the 40S and 60S ribosomal subunits (eukaryotic)
subunits are transported to the cytoplasm

Question 13

What are the distinct sections/features of the nucleolus?

Answer 13

shown by electron microscopy

• fibrillar centre = holds RNA polymerase and rRNA genes
• dense fibrillar component = RNA transcripts/transcription
• granular component = processing and assembling takes place

Question 14

What are NORs?

Answer 14

nucleolar organising regions = NORs

- site/location of rRNA genes

Question 15

What is the difference between mRNA and rRNA?

Answer 15

mRNA

• a single mRNA can be translated multiple times to give amplification of the final product
• can be reused until it is degraded

rRNA

• cell has multiple copies of the rRNA genes
• is not translated into proteins because it is the final product
• required to synthesise the many ribosomes required

Question 16

How can the nucleolus be examined?

Answer 16

nucleolus proteomic analysis

- examines what proteins are in the nucleolus

Question 17

What are splicing speckles? What are their features?

Answer 17

splicing speckles = spliceosome
- composed of splicing factors and other mRNA processing factors

features

• do not contain DNA
• are associated with active transcription factors
• not a site of transcription

Question 18

How can the function of splicing speckles be determined?

Answer 18

change the internal conditions for splicing speckles to determine if they are the site of transcription or a storage reservoir for transcription factors

inhibit transcription

• no splicing occurs
• splicing speckles round up and become larger
• supports the theory that it is a storage/assembly compartment not direct site of splicing

add more introns

• increases splicing
• splicing factors redistribute to transcription sites = splicing speckles become smaller
• supports the theory that splicing speckles are a reservoir of splicing factors with splicing factors moving between transcription sites and speckles

Question 19

What is the nuclear envelope?

Answer 19

2 lipid bilayers

• surrounds the nucleus
• has an inner and outer membrane
• is continuous with the endoplasmic reticulum
• has nuclear pores = allows communication between the nucleoplasm and the cytoplasm

Question 20

What is the nuclear pore?

Answer 20

movement through the nuclear pore is strictly controlled

• only small water soluble molecules can diffuse freely through the pore
• larger molecules must be actively transported through the nuclear pore = require specific tags for movement through the nuclear pore

Question 21

What is the difference between nuclear import and export?

Answer 21

nuclear import
- entrance into the nucleus = require nuclear localisation sequence

nuclear export
- exit out of the nucleus = proteins require a nuclear export signal to leave