Nucleic Acids

Question 1

Function of DNA

Answer 1

Holds genetic information which codes for polypeptides

Question 2

Function of RNA

Answer 2

Transfers genetic information from DNA to ribosomes

Question 3

Molecules from which ribosome are made

Answer 3

rRNA

Proteins

Question 4

Structure of DNA nucleotide

Answer 4

Phosphate group

Deoxyribose sugar

Nitrogen containing organic base

Question 5

Structure of RNA nucleotide

Answer 5

Phosphate group

Ribose sugar

Nitrogen containing organic base

Question 6

Difference between DNA and RNA nucleotide

Answer 6

DNA nucleotide - Deoxyribose sugar
RNA nucleotide - Ribose sugar

DNA nucleotide - Base can be thymine
RNA nucleotide - Base can be uracil

Question 7

How do nucleotides join together to form polynucleotides

Answer 7

Condensation reactions, removing water molecules

Between phosphate group of one nucleotide and deoxy/ribose of another

Forming phosphodiester bonds

Question 8

Why did many scientists initially doubt that DNA carried the genetic code

Answer 8

The relative simplicity of DNA - chemically simple molecule with few components.

Question 9

Structure of DNA

Answer 9

Polymer of nucleotides

Each nucleotide formed from deoxyribose, phosphate group and a nitrogen containing organic base

Phosphodiester bonds join adjacent nucleotides

2 polynucleotide chains helpd togeyher by hydrogen bonds

Between specific complementary base pairs - A&T C&G

Double helix

Question 10

Structure of mRNA

Answer 10

Polymer of nucleotides

Each nucleotides formed ribose, phosphate group and a nitrogen containing organic base

Bases - A&U C&G

Phosphodiester bonds join adjacent nucleotides

Single helix

Question 11

Contrast structure of DNA and mRNA

Answer 11

DNA - Deoxyribose
RNA - Ribose

DNA - Thymine
RNA - Uracil

DNA - Double stranded/helix
RNA - Single stranded/helix

DNA - Longer
RNA - Shorter

DNA - Has hydrogen bonds
RNA - Does not

Question 12

How does structure of DNA relate to its functions

Answer 12

2 strands - Both can act as template for semi conservative replication

Weak hydrogen bonds - Strands can be easily separated for replication

Complementary base pairing - Accurate replication

Many hydrogen bonds - Strong molecule

Double helix with sugar phosphate backbone - Protects hydrogen bonds

Long molecule - Store lots of genetic information that codes for polypeptides

Double helix (coiled) - Compact

Question 13

Suggest how you can use incomplete information about the frequency of bases on DNA strands to find the frequency of other bases

Answer 13

% of adenine in strand 1 = % of thymine in strand 2 (and vice versa)

% of guanine in strand 1 = % of cytosine in strand 2 (and vice versa)

Because of specific complementary base pairing between 2 strands

Question 14

Why is semi conservative replication important

Answer 14

Ensures genetic continuity between generations of cells

Question 15

Describe the process of semi conservative DNA replication

Answer 15

DNA helicase breaks hydrogen bonds between complementary bases unwinding the double helix

Both strands act as templates

Free DNA nucleotides attracted to exposed bases and join by specific complementary base pairing

Hydrogen bonds form between adenine-thymine and guanine-cytosine

DNA polymerase joins adjacent nucleotides on new strand by condensation reactions

Forming phosphodiester bonds

Each new DNA molecule consists of one template strand and one new strand

Question 16

Why does DNA polymerase move in opposite directions along DNA strands

Answer 16

DNA has antiparallel strands

So shapes of nucleotides on 2 ends are different

DNA polymerase is an enzyme with a specific shaped active site

So can only bind to substrate with complementary shape

Question 17

Name the two scientists who proposed models of the chemical structure of DNA and of DNA replication

Answer 17

Watson and Crick

Question 18

Describe the work of Meselson and Stahl in validating the Watson-Crick model of semi-conservative DNA replication

Answer 18

Bacteria grown in medium containing heavy nitrogen (15N) and nitrogen is incorporated into DNA bases

DNA extracted & centrifuged → settles near bottom, as all DNA molecules contain 2 ‘heavy’ strands

Bacteria transferred to medium containing light nitrogen (14N) and allowed to divide once

DNA extracted & centrifuged → settles in middle, as all DNA molecules contain 1 original ‘heavy’ and 1 new ‘light’ strand

Bacteria in light nitrogen (14N) allowed to divide again

DNA extracted & centrifuged → half settles in middle, as contains 1 original ‘heavy’ and 1 new ‘light’ strand; half settles near top, as contains 2 ‘light’ strands

Other models not supported as bands would be in different places