Nuclear structure and function

Advantages of separate transcription and translation

• allows mRNA processing and quality control (ensures DNA was properly transcribed)

  • capping and splicing occur in the nucleus

    • prevent mRNA degradation

  • alternative splicing can produce many different protein isoforms (diff versions of a protein)

    • → allows tissue specific expression → alters how protein functions in different cell types

• Better regulation of gene expression

  • post-transcriptional, translational controls …

• Separate rates of transcription and translation

  • → you can build up lots of mRNA in a cell, and them quickly translate it when its needed

Different forms of chromatin

Heterochromatin = dense staining DNA

• formed by methylation

• gene is off

Euchromatin = less dense staining DNA

• formed by acylation

• gene is on

What is nucleolus and what does it contain

Nucleolus = highly dense staining of RNA

Contains:

• proteins - structural proteins, enzymes for transcription, incoming ribosomal proteins that combine with rRNA to make ribosome subunits

• rRNA

• DNA (rDNA = specific gene cluster that code for rRNA)

• ribosomal precursors (=partially assembled large and small ribosomal subunits)

How is chromosomal painting done and why is it important

Done by using different probes for each individual chromosome which has a different fluorescent label on it (diff. colour)

~shows where different chromosomes are located within the nucleus

Why is it important?

→ These chromosomal territories are inherited, but can change dues to mutation or disease. By being able to visualise chromosomes in the nucleus, we can get some inside to what happens when they are changed in response to diff. conditions (could be a response to DNA damage / chromosomal rearrangement / infectious disease)

How is transcription status of a gene is controlled?

Location of gene within a nucleus depends on its transcription status

Expressed: gene moves into euchromatin area (in the centre of the nucleus)

Not active: gene co-localises with heterochromatin (near the edge of the nucleus)

Lamina (what it is + function)

Lamina = protein connected to the inner nuclear membrane by peripheral lamination proteins

f: ~ maintain nuclear shape

~ allows nucleus to resist any force e.g. when cell has to change shape

(provides structural integrity)

~ interprets signals from outside of the cell to nucleus when cell is under mechanical stress

How does material enter/exit nucleus? And what is needed for this to happen?

Via nuclear pores in a size-dependent manner (pore can open up to 26 nm in diameter)

What is needed to enter the nucleus?

• energy (ATP)

• signal: specific peptide sequence (different from signalling sequence!!!)