1.8 - the nucleus – Bioch / OB

Question 1

size of the nucleus

Answer 1

5-10µm in diameter

Question 2

nuclear envelope

Answer 2

double membrane

outer membrane is continuous with the endoplasmic reticulum

inner and outer membranes are separated by a space = perinuclear space = around 20-40 nm

has nuclear pores

Question 3

nuclear pores

Answer 3

holes in the nuclear envelope that are made up of proteins forming a nuclear pore complex

Cylinder in hole spanning the two nuclear membranes

Question 4

nucleoplasm

Answer 4

interior of the nucleus is filled with a semi-fluid substance called the nucleoplasm = similar to cytoplasm

contains nucleotides, enzymes etc

Question 5

nucleolus

Answer 5

dense, spherical region within the nucleus that is not membrane-bound

site of ribosomal RNA synthesis and the assembly of ribosomal subunits.

Question 6

nuclear lamina

Answer 6

dense, fibrous network located just beneath the inner membrane of the nuclear envelope

composed of intermediate filament proteins called lamins

Question 7

functions of the nuclear lamina

Answer 7

Provides mechanical support to the nuclear envelope, helping to maintain the shape and stability of the nucleus.

Involved in the anchoring of chromatin to the nuclear periphery, which helps in organizing the DNA within the nucleus.

Plays a role in the disassembly and reassembly of the nuclear envelope during cell division (mitosis).

Question 8

nuclear functions

Answer 8

gene replication + repair

genetic transcription

ribosome production

Question 9

DNA in interphase

Answer 9

genetic material found as chromatin – largely invisible + thin strands of genetic material

more accessible for transcription factors and important enzymes such as RNA polymerase II.

Question 10

type of DNA in interphase

Answer 10

euchromatin or heterochromatin

Question 11

heterochromatin

Answer 11

more densely stored + wrapped tightly around deacetylated histones so that it is inaccessible for transcription

Question 12

deacetylated

Answer 12

positive charge on histones is retained = strengthens bonding

Question 13

histones

Answer 13

proteins around which DNA wraps to form nucleosomes – basic unit of chromatin

Question 14

euchromatin

Answer 14

loosely packed and less condensed than heterochromatin

structure allows for easier access to DNA by the transcription machinery

associated with active genes

Question 15

regulation of euchromatin

Answer 15

-regulated through acetylation, methylation, and other similar methods so that it can be appropriately expressed by the cell

Question 16

Acetylation

Answer 16

neutralises the positive charge on histones, loosening their interaction

Question 17

2 classes of heterochromatin

Answer 17

constitutive

facultative

Question 18

constitutive

Answer 18

poorly expressed
contains genes that are permanently silenced
genes transcriptionally inactive

often found in centromeres + telomeres

Question 19

function of constitutive

Answer 19

Aids in the physical separation of chromosomes during mitosis and meiosis, preventing chromosomal entanglement

Helps maintain chromosomal integrity and stability during cell division.

Question 20

Facultative

Answer 20

can switch between an active (euchromatic) state and an inactive (heterochromatic) state

reversible = can be decondensed into euchromatin when specific genes need to be expressed

Question 21

well known example of facultative heterochromatin

Answer 21

X chromosome inactivation in females

Question 22

descrive x chromosome inactivation

Answer 22

in early embryonic development – cells randomly choose one of the 2 X chromosomes to inactivate – some will inactive mother + some father = leads to mosaicism – different cells express different X-linked genes

inactivated X chromosome condenses into a dense structure called the Barr body

reversible process – that’s why its facultative

Question 23

why do we have x chromosome inactivation

Answer 23

ensures that females do not have a double dose of X-linked gene expression compared to males, who only have one X chromosome

Question 24

what is argued to define eukaryotes

Answer 24

nuclear envelope

Question 25

importance of nuclear pores

Answer 25

selectively permeable

two-way communication / bi-directional

Question 26

examples of two way communication in nuclear pores

Answer 26

from nucleus to cytoplasm – mRNA + rRNA that is assembled into ribosomal subunits = use exportins (transport proteins) to exit = energy-dependant

from cytoplasm to nucleus – transcription factors / DNA + RNA polymerase / histones – use importins

Question 27

how does a molecule know if it needs to go in or out of the nucleus

Answer 27

Special signals

Nuclear Localization Signals (NLS): These are signals on proteins that tell the cell, “I need to go into the nucleus.”

Nuclear Export Signals (NES): These are signals that tell molecules to leave the nucleus and go to the cytoplasm.