LEC 1: Genome Organisation

Question 1

Griffith Experiment

Answer 1

Findings: the introduction of virulent (IIIS) and non-virulent (IIR) bacteria into rats -> virulent bacteria extracted was now a different bacteria, discovery of gene transfer

Question 2

Hershey-Chase Experiment

Answer 2

Findings: Viruses are composed of DNA or RNA surrounded by a protective protein coat. Bacteriophages are viruses that infect bacteria

Question 3

Nucleotide Structure

Answer 3

Each nucleotide consists of:

• A pentose (5-carbon) sugar (Ribose in RNA and Deoxyribose in DNA)
• A nitrogenous base
• A phosphate group

Question 4

Nitrogenous Bases

Answer 4

There are two classes of nitrogenous bases:
1. Purines (double-ring, nine-membered structures)
− Adenine (A) and Guanine (G).

2. Pyrimidines (one-ring, six-membered structures)
   − Cytosine (C), Thymine (T) in DNA and Uracil (U) in RNA.

Question 5

Pairing Between Bases

Answer 5

Base-pairing rule:

• Cytosine pairs with Guanine
• Adenine with Thymine

Question 6

Assembly of DNA into Chromosomes

Answer 6

• Chromatin consists of DNA wrapped around one histone octamers (10 nm chromatin fiber)
• Further packing involves the action of histone H1
• Molecules interact with each other, causing the chromatin to form a spiral, with 6 to 8 nucleosomes per turn of the spiral (solenoid)

Question 7

Euchromatin

Answer 7

• It is actively transcribed, and lacks repetitive sequences.
• Euchromatin accounts for most of the genome in active cells

Question 8

Heterochromatic

Answer 8

Heterochromatin remains condensed throughout the cell cycle and is therefore, transcriptionally inactive. Exists in 2 forms:

1. Constitutive heterochromatin -mostly repetitive DNA (e.g., centromeres)
2. Facultative heterochromatin - varies between cell types or developmental stages (e.g., Barr bodies)

Question 9

Eukaryotic Genome Organisation

Answer 9

• One promoter for each gene (99% of genes)
• One open reading frame (ORF) only is translated each mRNA
• Exons spliced together in nucleus
• Alternative splicing gives additional complexity of possible mRNA products (10% of genes?)

Question 10

Transposon-Derived Repeats

Answer 10

Encode enzymes that can insert their sequence into new sites in genomic DNA, thus contributing to spontaneous mutation, genetic rearrangements, horizontal transfer of genetic material
- Involve RNA intermediates (retrotransposons) or DNA intermediates (DNA transposons)