Session 4: DNA Structure & Replication

Question 1

Why do DNA strands have major strands?

Answer 1

Larger spacing allow proteins to bind to DNA and control their function

Question 2

What are autosomes? How many do humans have?

Answer 2

Chromosomes that code for proteins found in the body. Each person has 44 autosomes (one copy from each parent so 22x2)

Question 3

Why do humans contain 2 copies of each chromosome?

Answer 3

Humans are diploid so receive copy from each parent.

Question 4

How does heterochromatin stain under an electron microscope? Why?

Answer 4

Dark stain because chromosomes are heavily condensed

Question 5

How does euchromatin stain under an electron microscope? Why?

Answer 5

Light stain because chromosomes are more unwound and so less condensed

Question 6

How are histone proteins able to bind with DNA?

Answer 6

Histone core is an octamer so it is positively charged as so can bind to negatively charged DNA.

Question 7

Describe how eukaryotic DNA is packaged into chromosomes

Answer 7

DNA is wrapped around histone protein (positively charged histones bind to negatively charged DNA) and form complexes called nucleosomes which then coil and stack together to form a secondary solenoid structure which further loops and folds with the help additional proteins to form condensed chromosomes.

Question 7

Describe how eukaryotic DNA is packaged into chromosomes

Answer 7

DNA is wrapped around histone protein (positively charged histones bind to negatively charged DNA) and form complexes called nucleosomes which then coil and stack together to form a secondary solenoid structure which further loops and folds with the help additional proteins to form condensed chromosomes.

Question 8

What is chromatin?

Answer 8

a material consisting of DNA and associated proteins

Question 9

What are the 2 types of nitrogenous bases and which bases have each structure?

Answer 9

Purine = A & G
Pyrimidine = C, U & T

Question 10

What are the differences between purine and pyrimidine bases?

Answer 10

Purine = larger, double rings, 9 species on ring
Pyrimidine = smaller, single ring, 6 species on ring

Question 11

What type of bond are nucleotides held together by? And between what?

Answer 11

Phosphodiester bonds, between the phosphate group and sugar backbone of another nucleotide

Question 12

What are the differences between DNA and RNA?

Answer 12

DNA = made up of nucleotides (base, sugar & phosphate), A, G, T & C, double-stranded, stores genetic info, deoxyribose sugar, read by polymerases

RNA = made up of nucleosides (base & sugar), A, G, U & C, single-stranded, stores info about protein structure, ribose sugar, read by ribosomes

Question 13

Why are A-T and A-U basepair weaker than G-C base pair?

Answer 13

A-T and A-U contain 2 hydrogen bonds whilst G-C contain 3 hydrogen bonds

Question 14

What are the enzymes involved in DNA replication? What are their functions?

Answer 14

• DNA helicase = unwinds the double helix by breaking hydrogen bonds
• Primase (RNA polymerase) = Builds RNA primer on DNA strands to synthesise short RNA sequences that are complementary to a single-stranded piece of DNA which serves as a template.
• DNA polymerase = adds daughter nucleotides on to parent strands
• DNA ligase = joins Okazaki fragments to form a continuous strand

Question 15

Describe what is meant by semi-conservative replication

Answer 15

genetic replication in which a double-stranded molecule of nucleic acid separates into two single strands each of which serves as a template for the formation of a complementary strand that together with the template forms a complete molecule.

Question 16

Why is DNA packaging important?

Answer 16

• Helps DNA to fit well within the nucleus of a small cell
• Highly packed DNA makes it easy to turn genes on or off (gene expression)