Lesson 10.2 - DNA structure

Question 1

Explain the following feature of DNA:
Sugar-phosphate backbone

Answer 1

This protects coding bases on the inside of the helix

Question 2

Explain the following feature of DNA:
Double stranded

Answer 2

This allows strands to act as templates in DNA replication.

Question 3

Explain the following feature of DNA:
Large molecules

Answer 3

Stores lots of information

Question 4

Explain the following feature of DNA:
Double helix

Answer 4

Ensures that the molecule is compact

Question 5

Explain the following feature of DNA:
Complementary base pairings

Answer 5

Allows accurate DNA replication

Question 6

Explain the following feature of DNA:
Weak hydrogen bonds

Answer 6

Allows stands to separate in DNA replication.

Question 7

Purines

Answer 7

Larger bases that contain a two carbon ring structure (A + G)

Question 8

Pyrimidines

Answer 8

Smaller bases containing one carbon ring structure (C +T)

Question 9

Hydrogen bonds in A +T

Answer 9

2

Question 10

Hydrogen bonds in C + G

Answer 10

3

Question 11

Why must a pyrimidine always pair with a purine?

Answer 11

Maintains a constant distance between the two sugar-phosphate backbones

Question 12

Explain DNA replication:

Answer 12

1. Enzyme DNA helicase breaks down the hydrogen bonds between complementary bases. This separates the strands and unwinds the helix.
2. Each strand acts as a template as free nucleotides attract to their complementary bases. Adenine pairs with thymine and cytosine pairs with guanine.
3. Enzyme DNA polymerase joins the nucleotides together via condensation reactions in the 5’ to 3’ direction. This means that the lagging strand is read in okazaki fragments. This forms phosphodiester bonds to create a sugar-phosphate backbone in new DNA strand.
4. Two identical copies of DNA are created. Each contains one new strand and one strand from the parent DNA. (Semi-conservative)

Question 13

Which is heavier / lighter? N14 and N15

Answer 13

N15 - Heavier (forms a ring at the bottom of the tube)
N14 - Lighter (forms a ring at the top of the tube)

Question 14

Why did Mendel-Stahl use bacteria?

Answer 14

Bacteria use nitrogen from their surroundings to replicate their DNA.

Question 15

How is DNA condensed into chromosomes?

Answer 15

• DNA is wound tightly around histones to form DNA-histone complex.
• This coils further to form chromatin.
• These coil to form chromosomes.

Each chromosome contains only one molecule of DNA.

Question 16

Genetic code

Answer 16

The sequence of bases that code for amino acids.

Question 17

Features of genetic code?

Answer 17

• Universal
• Non-overlapping
• Degenerate

Question 18

Degenerate:

Answer 18

Most amino acids are coded for by more than one triplet.

Question 19

Universal

Answer 19

Each triplet codes for the same amino acid in all organisms.

Question 20

Non-overlapping

Answer 20

Each base in the DNA sequence is only read once

Question 21

Features of mRNA?

Answer 21

• Single stranded, linear molecule
• Contains base sequence complementary to DNA sequence
• Contains codons, sets of three bases that code for amino acids
• Small enough to leave the nucleus

Question 22

Role of tRNA?

Answer 22

Transport amino acids to ribosomes to build up a polypeptide chain.

Question 23

Features of tRNA?

Answer 23

• Single stranded DNA sequence folded into a clover-leaf shape.
• Uses hydrogen bonds between complementary base pairs to hold it in place.
• Contains specific sequence of 3 amino acids known as anticodon
• Contains an amino acid bonding site at the other end

Question 24

Which of tRNA and mRNA are longer?

Answer 24

mRNA is longer

Question 25

Translation process

Answer 25

The ribosome attaches to the mRNA strand at a start codon (AUG).
A tRNA molecule, carrying a specific amino acid and with an anticodon (UAC) that is complementary to the start codon, binds to the mRNA.
A second tRNA molecule with an anticodon complementary to the next mRNA codon, and also carrying a specific amino acid, attaches to mRNA.
The amino acids carried by the first two tRNA molecules are linked together via a peptide bond using ATP.
The first tRNA molecule detaches from mRNA and is free to collect another amino acid for future use.
The ribosome moves along mRNA, allowing another tRNA molecule, which carries the next amino acid, to bind to the next codon on mRNA.
The process from step 4 to 6 is repeated, which elongates the polypeptide chain.
At any point during this process, two tRNA molecules can be attached to the ribosome.
The sequence continues until the ribosome reaches a stop codon on mRNA.
The completed polypeptide chain detaches from the ribosome.