Lecture 3: DNA

Question 1

DNA (Prokaryotes)

Answer 1

DNA molecules are short, circular, not associated with proteins

Question 2

DNA (Eukaryotes)

Answer 2

DNA molecules are long, thin, linear and associated with proteins called histones

Question 3

DNA (mitochondria and chloroplasts)

Answer 3

DNA like that of prokaryotes

Question 4

Homologous chromosomes

Answer 4

In eukaryotic cells the DNA is stored as chromosomes in the nucleus.
Humans have 23 pairs (46)
Matching chromosomes
Homologous pairs
Same size and same genes (could be different alleles)
Heterozygous
Homozygous

Question 5

Structure of DNA

Answer 5

DNA and RNA are both types of nucleic acids
Store of genetic information
Polymer of nucleotides
Pentose sugar, nitrogenous bases (purines and pyrimidines) and a phosphate group
Double helix (2 strands) held by hydrogen bases (antiparallel)
Pairs adenine-thymine, cytosine-guanine

Question 6

Purines and pyrimidines

Answer 6

Pyrimidines – one 6 membered ring of carbon and nitrogen atoms) C,T,U
Purines (larger 6 membered ring fused to a 5 membered ring) A,G

Question 7

Polynucleotide structure

Answer 7

Condensation reaction
Phosphate and sugar
Phosphodiester bond (between the nucleotides)
The chain of phosphates and sugar is known as a sugar-phosphate backbone
The ends of the DNA strand are called 5’ end (said as “5 prime end”) at the phosphate end and the 3’ end at the deoxyribose end.
The 2 strands of DNA are antiparallel (one strand runs in a 5’ to 3’ direction and the other runs in the 3’ to 5’ direction

Question 8

RNA

Answer 8

Single sugar-phosphate backbone
Sugar (ribose)
Uracil
Single polynucleotide
Shorter
mRNA, tRNA and rRNA

Question 9

DNA replication

Answer 9

DNA needs to replicate before cell division
Semi-conservative replication
Half of the strands of each new DNA molecule are from the original DNA molecule

Question 10

How does DNA replication occur step process

Answer 10

1. DNA untwists (enzyme DNA helicase breaks hydrogen bonds between bases)
2. Original strand is a template. Free-floating DNA nucleotides are attracted to complementary exposed bases
3. Condensation reactions join the adjacent nucleotides (catalysed by DNA polymerase). Hydrogen bonds form between the bases
4. Two Strands retwist

Question 11

Gene mutations during DNA replication

Answer 11

DNA replication is carefully controlled to preserve the genetic information
However, changes in the genome do sometimes occur.
These changes are known as mutations.