The Organization of

Cellular Genome

Asmarinah

Department of Medical Biology

Genome

The totality of genetic information belonging to a cell or an organism; in particular, the DNA that carries this information

Genomics

The science of studying the DNA sequences and properties of entire genomes

● DNA molecules are organized into chromosome

● Chromosome: A DNA molecule with its attendant proteins that moves as an independent units during mitosis and meiosis

Before DNA replication, each chromosome consists of a single DNA molecule plus protein, is called a chromatid

After replication, each chromosome consists of two identical DNA molecule plus proteins; this are called sister chromatids

● Chromatin: DNA plus the proteins that package it within the cell nucleus

A B C

A. The principal structural features of chromosome

B. An electron micrograph of human mitotic chromosomes

C.A diagram of various classes of human chromosomes

Centromere: The chromosomal locus that regulate the movement of the chromosome during mitosis and meiosis

Telomere: The specialized structure at either end of chromosomal DNA molecule that ensures the complete replication of the chromosomal end and protects the ends within the cell.

Chromosomes are classified based upon the position of the centomere relative to the arms, i.e:

-Metacentric

-Submetacentric

-Acrocentric

-Telocentric

Chromatin packing occurs on several levels

Nucleus contain two broad classes of chromatin, i.e:

● Heterochromatin

which is highly condensed throught the cell cycle and is generally inactive in transcription

● Euchromatin

which is less condensed and contains actively transcribed genes

Structural organization of nucleosome

Nucleosome contains

Octameric histon protein & DNA double helix

DNA and its building

DNA is made from subunits, called nucleotides

Each nucleotide consisting of

-Sugar (deoxyribose)

-Phosphate

-Base: Adenine

Thymine

Guanine

Cytosine

Building block of DNA DNA strand

Double-stranded DNA

DNA double helix

The most important of chromosomes is to carry genes

A gene is defined as a segment of DNA that contains the insctruction for making a particular protein

The Organization of human genome

Each human chromosome has a unique banding pattern

Human Genome

Conserved gene order in chromosome between the human (a) and mouse (b) genome

Closely related species can have very different chromosome numbers

Chromosomes from many eucaryotes (including human) contain a large excess of interspersed DNA that does not seem to carry critical information, called junk DNA

This DNA may not be important, but can play a role:

- as spacer material

- for long-term evolution of the species

- for the proper expression of genes

Bacteria and some single-celled eucaryotes have especially compact genomes. The complete nucleotide sequence of their genomes reveals that DNA molecules are little more than strings of closely packed genes

The genome of E. coli

A. E.coli strain K-12 has 4.639.221 nucleotides pairs

B. A diagram of the E. coli genome.

E. Coli genome is circular that forms a single, closed loop.

Yellow or orange bars shows protein-coding genes

Green arrows indicate genes encoding only RNA molecules

Genetic differences between pathogen and nonpathogen bacteria

Example: Genetic organization of Vibrio cholerae

A. Vibrio cholerae has two circular chromosomes, that each of its has distinct origin of replication (oriC1 &

oriC2)

CTXφ locus carriers the gene for cholera toxin

VPI island contains gene for factors required for intestinal colonization

B. Map of CTXφ locus consist of: - core region: gene for cholera toxin ctxA & ctxB

gene for involving of virulence (ace & zot)

- repeated sequence RS2 & RS1 that involved

chromosomal insertion of bacteriophage genome

Schematic drawings of several types of viral genomes

References:

-Albert et al., 2002. Molecular biology of the cell. 4th ed.

-Albert et al., 2004. Essential of cell biology.

-Pollard and Earnshaw. 2004. Cell biology.