NUCLEUS: STRUCTURE AN FUNCTION

→DNA is the structural basics of genetic information.

→Antiparallel double helix

SUPERCOILING

→In bacteria, DNA is condensed into supercoiled structures.

→Supercoiling effectively decreases the length of the chromosome.

→Positive supercoiled occur in the direction of the double helix, which tightens DNA (properly written of 8 ). Tight coil.

→Negative supercoils occur in the direction opposite the double helix twist. (inverted way of writing 8 ). More relaxed coil.

  • Negative supercoil→Relaxed DNA→ Positive supercoil, vise versa.

-Topoisomerases remove both positive and negative supercoils.

  • Topoisomerase I introduces the transient single strand breaks in DNA

  • Topoisomerase II introduce double-strand breaks

    • DNA gyrase both induce double strand breaks and supercoils.

    • In bacteria DNA gyrase can relax the DNA

Bacterial Chromosome

→Bacterial DNA is NEGATIVELY SUPERCOILED.

→Bacterial chromosomes are bound to few proteins that help stabilize the loops.

→Bacterial chromosome are located in the nucleoid.

Bacterial Plasmids

→Bacteria has bacterial DNA and circular plasmids.

→R factors are drug resistance genes

→Metabolic plasmids are non-essential metabolic enzymes.

→Colicinogenic factors synthesize bactericidal toxins.

→Virulence factors- synthesize disease-causing toxins.

Eukaryotic Chromosomes

→Eukaryotic DNA is organized into two types of structures: Chromatin and chromosomes

→Chromatin is long fibrous DNA or protein complexes dispersed in the nucleus.

→Chromosomes form during cell division and are larger condensed structures.

→Fluorescence In Situ Hybridization (FISH) is used for labeling DNA in cells.

  • DNA probes are synthesized with a fluorescent tag.

→Giemsa stain can be used to identify chromosomes in dividing cells.

→Nucleosomes are DNA complexed with histone proteins

→Chromatin fibres comes from condensed nucleosome structures.

→Heterochromatin is a further folded structure of chromatin during cell division.

→Nucleosome particle consists of DNA wrapped around a complex of histone proteins.

→Euchromatin is a loose structure of chromatin during interphase.

→Loops of chromatin fibres are stabilized by binding to cohesin proteins.

Chromatin remodelling

→Methylation of a histone tail by histone methyltransferase tightens chromatin packing.

→Acetylation by histone acetyltransferase (HAT) opens (looseen) chromatin to allow transcription.

  • Histone deacetylase (HDAC) reverses.

→Chromatin remodelling proteins read the histone code methylation and acetylation.

→Chromosome remodelling complexes can bind to acetylated histones and loosen the chromatin through an ATP-dependent process.

KEYPOINTS

-Centromeres

  • Centromeres maintain cohesions between sister chromatids during cell division

  • Centromeres are assembly sites for kinetochores, where microtubules attach during meiosis and mitosis.

  • Centromeres have a highly repetitive centromere sequence (CEN).

-Telomeres

  • The highly repetitive sequences can be found at the tips of chromosomes.

  • The telomeres protect the ends of chromosomes from falling apart during replication.

  • Telomeres have a highly conserved sequence of repeats in vertebrates (TTAGGG).

NUCLEUS

→The nucleus as both a storage sites for genetic information

→The nucleus is also a control centre for expression of information

NUCLEUS ENVELOP

→Nucleus is attached to a double-membraned nuclear envelope.

→Nuclear envelope has a perinuclear space 20-40nm across.

→The outer membrane of the nucleus envelop is continuous with the endoplasmic reticulum.

→Perinuclear space is continuous with the lumen of the endoplasmic reticulum.

  • Structure:

    • Nuclear envelop contains proteins that anchor the nucleus to the cytoskeleton.

    • Outer membrane is studded with ribosomes

    • Nuclear pores make the cell cytoplasm continuous with the nucleoplasm

→Nuclear Pore Complex is a large protein complex that spans the two bilayers of the nuclear envelop.

  • Transporter plays a role in the transport of macromolecules.

NUCLEAR PORE TRAFFIC

→Protein synthesis only occur in the cytoplasm

→Transcription occurs only in the nucleus.

→Nuclear pores drive the import and export of macromolecules across the nuclear envelop.

RAN/ IMPORTIN TRANSPORT

→Importin is a NLS binding protein that can be a transporter to shuttle proteins through the nuclear pore traffic.

→RAN is a GTPase and an exporter that shuttles the importin out of the nucleus once the the importin is unloaded.

→RAN-dependent transport also transport certain RNAs with nuclear export signals (NES) that bind exportins.

→ Nuclear Transport Factor 2 transport RAN-GDP from cytoplasm to the nucleus.

→Nuclear lamins form a nuclear scaffold to give shape to nucleus.

NUCLEOLUS

→Nucleolus is a site for rRNA synthesis

→Can be located within the nucleus that contains fibrils and granules.

  • Fibrils are sites of active rRNA synthesis

  • Granules are rRNA packaged with proteins to be exported out of the nucleus.