WEEK 8 (Nucleus)

Question 1

In which phase are chromosomes dyed to look out for deletion or translocation?

Answer 1

Metaphase

Question 2

What are the key properties of the Nuclear envelope?

Answer 2

• Envelope has TWO CONCENTRIC MEMBRANES separated by a narrow PERINUCLEAR SPACE
• The space and the outer nuclear membrane are continuous with the extensive cytoplasmic network of the ROUGH ENDOPLASMIC RETICULUM
• Closely associated with the INNER NUCLEAR MEMBRANE is the NUCLEAR LAMINA which stabilises the nuclear envelope
• Inner and outer nuclear membranes are bridges at NUCLEAR PORE COMPLEXES
• Various core proteins of a nuclear pore complex (NUCLEOPORINS) display eightfold symmetry around a lumen

Question 3

What are the properties of Nuclear Lamina?

Answer 3

• Highly organised meshwork of proteins
• Stabilises the nuclear envelope
• Contain LAMINS that bind to membrane proteins and associate with CHROMATIN in non-dividing cells

Question 4

Describe how proteins destined for the nucleus enter it

Answer 4

Proteins to be imported have NUCLEAR LOCALISATION SEQUENCES which bind specifically to transport proteins (IMPORTINS) that interact with proteins of the pore complexes for transfer across the nuclear envelope. Energy for the transport is derived from GTP with specific GTPases helping provide directionality to the transfer.

Question 5

What are the key properties of Chromatin?

Answer 5

• Consists of DNA and all of the associated proteins involved in the organisation and function of DNA
• Each cell’s chromatin (except gametes) contains 46 chromosomes (23 pairs)
• After DNA replication but before cell division, each chromosome consists of TWO IDENTICAL CHROMATIN UNITS called CHROMATIDS held together by complexes of COHESIN PROTEINS

Question 6

Describe the structure of Nucleosomes

Answer 6

Nucleosome is the structural unit of DNA and Histones which contains a core of EIGHT HISTONES (two copies each of histones H2A, H2B, H3 & H4) around which is wrapped 150 nucleotides. Each nucleosome also has H1 associated with both the wrapped DNA and the surface of the core.

Nucleosomes are structurally dynamic; modification and rearrangement of the histones allows temporary unwrapping of the DNA and arrival of enzymes and other proteins required for replication and gene transcription.

Question 7

Describe the organisation of Chromatin

Answer 7

• The 10nm fiber of nucleosomes and DNA undergoes HELICAL FOLDING to yield a fiber with a diameter of 30nm
• Many loops are tethered to central scaffold-like arrays containing large protein complexes (CONDENSINS) capable of compacting chromatin
• Further packaging during the first phase of cell division causes chromosomes to become visible as discrete structures by LIGHT MICROSCOPY

Question 8

What are the different stages from DNA to chromatin?

Answer 8

1) DNA
2) NUCLEOSOMES
3) Packed nucleosomes in 30nm chromatin fiber
4) Extended loops of transcriptionally active chromatin, tethered to protein scaffold
5) Condensed HETEROCHROMATIN and dispersed EUCHROMATIN
6) Entire chromosome at metaphase

Question 9

What are the two categories of chromatin that can be distinguished in nuclei of most non-dividing cells?

Answer 9

Euchromatin & Heterochromatin

Question 10

What is Euchromatin seen as microscopically?

Answer 10

Visible as finely dispersed granular material in the electron microscope and as lightly stained basophilic areas in the light microscope

Question 11

What is Heterochromatin seen as microscopically?

Answer 11

Appears as coarse, electron dense material in the electron microscope and as intensely basophilic clumps in the light microscope. It tends to be concentrated near the NUCLEAR LAMINA.

Question 12

What are the two types of Heterochromatin?

Answer 12

Constitutive & Facultative Heterochromatin

Question 13

What is the difference between Constitutive and Facultative Heterochromatin?

Answer 13

Constitutive heterochromatin contains mainly repetitive, gene-poor DNA sequences including centromeres and telomeres.

Facultative heterochromatin can undergo reversible transitions from compact, transcriptionally silent states to more open, transcriptionally active conformations. Also occurs in Barr bodies.

Question 14

Strutures occupy discrete _____________ territories within dispersed chromatin

Answer 14

chromosomal

Question 15

What is the correlation between chromosomal domains and location within the nucleus?

Answer 15

Chromosomal domains with few genes form a layer beneath the nuclear envelope, while domains with many active genes are located deeper in the nucleus

Question 16

Describe the stages of microscopic analysis of chromosomes

Answer 16

1) Begins with cultured cells in mitotic METAPHASE by COLCHICINE or other compounds that disrupt microtubules
2) After processing and staining the cells, the condensed chromosomes of one nucleus are photographed by LIGHT MICROSCOPY
3) They are rearranged digitally to produce a KARYOTYPE in which stained chromosomes can be analysed

Question 17

Describe the Nucleolus

Answer 17

• Spherical
• Highly basophilic
• Subdomain of nuclei in cells actively engaged in protein synthesis
• Intense basophilia of nuclei is due to the presence of densely concentrated ribosomal RNA (rRNA) which is transcribed, processed and assembled into ribosomal units
• Chromosomal regions with the genes for rRNA organize one or more nucleoli in cells requiring intense ribosome production for protein synthesis during growth or secretion.