Year 12 HSC Biology Module 1: Heredity

What does DNA stand for?

Deoxyribonucleic acid

What does DNA carry?

Genetic code

What shape is DNA?

Double helix, a twisted ladder

What can be said about the two strand that make up DNA?

They are complementary

DNA strands are comprised of a sequence of what units?

Nucleotides

What is a single DNA nucleotide comprised of?

A deoxyribose unit (a five carbon sugar), a phosphate group, and a nitrogenous base

What are the four nitrogenous bases in DNA?

Adenine, Thymine, Guanine and Cytosine

Of the nitrogenous bases, which are the purines and what are their structure?

Adenine and Guanine, composed of two fused rings

Of the nitrogenous bases, which are the pyrimidines and what are their structure?

Cytosine and Thymine, composed of two fused rings

How do nucleotides of a DNA strand bond with one another?

The sugar in one nucleotide forms a phosphodiester bond with the phosphate group of the next nucleotide

How do the sugar-phosphate ‘sides’ bond with the nitrogenous base ‘rungs’ of DNA?

Via a glycosidic bond between the sugar and the base

How are base pairs bonded on the ‘rungs’ of the DNA ‘ladder’?

Hydrogen bonds

What is the DNA base pairing rule?

The structure of each nitrogenous base of DNA means that adenine will only bond with thymine, and guanine will only bond with cytosine

What is the 5’ end of the sugar-phosphate backbone in DNA?

The end where a phosphate group is connected to the 5th carbon in the sugar

What is the 3’ end of the sugar-phosphate backbone in DNA?

The end where the hydroxyl (OH) group is connected to the 3rd carbon in the sugar

Why are the backbones of the two DNA strands said to be ‘antiparallel’?

The 5’ end of one backbone is opposite the 3’ end of the other backbone

What gives DNA its characteristic ‘ladder-like’ structure?

The three types of bounds found within and between nucleotides

What causes the DNA double helix to be twisted?

The bases in the DNA being hydrophobic

What is the benefit of DNA being twisted?

It allows the nucleotides to pack closely together, preventing water in the cellular fluid from squeezing between the DNA

What is DNA helicase and what is its function in DNA replication?

An enzyme that unzips the DNA strands by breaking the hydrogen bonds between each base pair forming the replication fork

What are binding proteins’ function in DNA replication?

Attach to the single strands to prevent re-joining

What is Primase and what is its function in DNA replication?

An enzyme that adds a short, complementary RNA primer to the exposed 3’ end of each strand

Why are RNA primers necessary in DNA replication?

DNA polymerase can only extend a nucleotide chain, not start one

What is DNA Polymerase III and what is its function in DNA replication?

Attaches free nucleotides to the primers on each exposed strand according to the base pairing rule and synthesised in the 5’ to 3’ direction

What does the ‘leading strand’ refer to in DNA replication?

The strand that is synthesised continuously as it is synthesised in the same direction as the DNA is unzipped by Helicase

What does the ‘lagging strand’ refer to in DNA replication?

The strand that must be built in sections called Okazaki Fragments as it is synthesised in the opposite direction as the DNA is unzipped by Helicase

What is Exonuclease and what is its function in DNA replication?

An enzyme that removes RNA primers

What is DNA Polymerase I and what is its function in DNA replication?

An enzyme that fills the gaps of removed RNA primers with DNA nucleotides

What is DNA Ligase and what is its function in DNA replication?

An enzyme that links the Okazaki Fragments on the lagging strand of DNA

Why is DNA said to be ‘semi-conservative’?

When a new double-stranded DNA molecule is formed, one strand will be from the original template molecule and one strand will be newly synthesised

What must a cell do before it can divide?

Replicate its DNA