5.1 DNA Organization

Organization of Genetic Material

Genome: the total genetic material of an organism

Gene: the functional unit of DNA (a specific DNA sequence that codes for RNA and then made into proteins)

Non-coding regions: no genes, may contain regulatory sequence

Prokaryotic Cells

Bacterial chromosome:

Circular, double-stranded DNA

No nucleus – not separated

in the nucleoid

Tightly packed supercoiling

Supercoilinglooped

proteins

Formation of

additional coils due to

twisting forces

Controlled by enzymes

topoisomerase I and II

Antibacterial drugs

block these enzymes

DNA of Prokaryotic Cells

Haploid:

• Have only one set of chromosomes

• Carry only one copy of each gene

Genome contains:

• Genes

• Regulatory sequences (control gene activity)

• has very little non-essential DNA

E. Coli

genome

Plasmid

in some prokaryotes

Small, circular or linear DNA

Often carry non-essential genes

Not part of nucleoid

Can be copied and transmitted between cells

Can be incorporated into chromosomal DNA and reproduced in cell division

DNA of Eukaryotic Cells

Double-stranded

Linear

Total amount much greater than in prokaryotic cells

inside nucleus

2m DNA fits into a 4μm nucleus Greater compacting

(protein)

Solenoid

DNA Organization in Eukaryotic Cells

“Beads-on-a-string

Chromatin

DNA Organization in Eukaryotic Cells

Chromosome

Variation in the Eukaryotic Genome

1. Diploid vs Haploid

• Most are diploid

• Some haploid (ferns, algae)

• Some triploid (seedless watermelon)

2. Genes are not evenly spaced, and not equally divided

• Chr19 : 72 million base pairs with 1450 genes

• Chr4 : 1.3 billion base pairs with 200 genes

Variation in the Eukaryotic Genome

3. Size of genome

4. Number of genes

• Vary greatly

• No correlation to organism’s

complexity

(A) Lungfish have 40 times more DNA

per cell than a human cell.

(B) Rice has 30 000 more protein-

coding genes than a human.

(C) C. elegans has the same number

of genes as humans but less DNA.

Variation in the Eukaryotic Genome

5. Coding:

• Can code for proteins

• Can code for RNA molecules (for cellular

processes, not protein-making)

increase biological complexity without

increasing the number of protein-coding genes