DNA/RNA Structure (2.6, 7.1)

Question 1

What are nucleic acids and what are they made of?

Answer 1

Nucleic acids are the genetic material of the cell and are polymers of nucleotides i.e. made up of repeated nucleotide units.

Question 2

What is a nucleotide made up of?

Answer 2

A 5C sugar (either ribose or deoxyribose). Drawn as a pentagon.
A phosphate group. Drawn as a circle.
A nitrogen base (guanine, adenine, thymine, cytosine or uracil). Drawn as a rectangle.

Question 3

DNA
- Name
- Function
- Bonding
- Where it is located

Answer 3

Name: Deoxyribonucleic Acid
Function: It carries the genetic message for the functioning of the organism. A gene is a length of DNA that codes for a polypeptide which forms a protein so DNA codes for the synthesis of proteins.
Bonding: Covalent bonds within nucleotides, weak hydrogen bonds within complementary pairings.
Located: Chromosomes of eukaryotes, nucleoid region of prokaryotes, the mitochondria and in chloroplasts.

Question 4

What does complementary base pairing refer to?

Answer 4

Adenine will always pair with Thymine (A=T) and Guanine with Cytosine (C≡G).

Question 5

How was the structure of DNA discovered?

Answer 5

Rosalind Franklin and Maurice Wilkins used X-ray diffraction techniques to identify key properties of the DNA molecule
Wilkins shared this data (without Rosalind’s permission) with James Watson and Francis Crick. Watson and Crick used this data to help construct an accurate model of DNA structure (as a double helix) using paper shapes.

Question 6

What experiment provided evidence that DNA not protein was the genetic material of the cell?

Answer 6

Alfred Hershey and Martha Chase took viruses and radiolabelled the phosphorus in one and the sulphur in the other. (Phosphorus is present in DNA and sulphur in proteins). These viruses were incubated separately with bacterium. Radioactivity was seen in the bacteria infected with radiolabelled phosphorus (DNA) proving that DNA was the carrier of genetic information not protein.

Question 7

What does the term ‘anti-parallel’ refer to?

Answer 7

The strands of DNA run in opposite directions. 5’ to 3’ down one strand and 3’ to 5’ down the other.

Question 8

How does DNA carry genetic information?

Answer 8

DNA carries genetic information in the bases. Three bases (a triplet) codes for an amino acid. As amino acids are the building blocks of proteins the order of the bases codes for proteins.

Question 9

Give the order of increasing size of DNA:
Yeast
Fruit Fly
Viruses
Bacteria
Human

Answer 9

Viruses
Bacteria
Yeast
Fruit fly
Human

Question 10

Organisation of Eukaryotic DNA

Answer 10

DNA is wound around eight proteins called histones, forming a nucleosome. The nucleosomes are linked with an additional histone to form a string of chromatosomes. These coil to form a solenoid structure which condenses, forming a 30nm fibre. The fibres form loops, compress and fold around a protein scaffold to form chromatin. Chromatin supercoils during cell division to form chromosomes.

Question 11

Organisation of Prokaryotic DNA

Answer 11

The DNA is a single, double stranded molecule in the form of a loop or circle.

Question 12

What are the regions of a DNA strand?

Answer 12

Coding regions (exons) and non-coding regions. Only 1% of the human genome are exons.

Question 13

What are the regions of DNA that do not code for proteins?

Answer 13

STING

Satellite DNA: repeating sequences of DNA. Highly variable so used in DNA profiling. Called tandem repeats or STRs.

Telomeres: regions of repetitive DNA at the end of chromosomes that protect chromosomes from being damaged during DNA replication. As the cell divides, the telomere shortens until it is too short for cell division to occur.

Introns: non-coding regions within genes.

Non-coding RNA genes: codes for RNA that are not translated to proteins but other molecules such as tRNA.

Gene regulatory sequences: sequences that are involved in transcription.

Question 14

Describe the differences between RNA and DNA

Answer 14

RNA has…
- Ribose sugar instead of deoxyribose sugar
- Uracil instead of thymine
- Single strand instead of double helix
- Less stable
- Much smaller

Question 15

What are the three kinds of the RNA molecule?

Answer 15

mRNA (messenger RNA). Nucleus.
- Carries genetic information from the nucleus to the ribosome. Three bases is called a codon.

tRNA (transfer RNA). Cytoplasm.
- Clover leaf shaped molecule. At one end it has a three base code called an anticodon which is complementary to the codon on mRNA and at the other end is an amino acid.

rRNA (ribosomal RNA). Ribosomes.
- Part of the structure of ribosomes (where protein synthesis occurs).

Question 16

How is DNA replicated?

Answer 16

Helicase unwinds and separates the DNA helix by breaking the weak hydrogen bonds between bases. DNA polymerase can only add nucleotides from the 5’ to 3’ direction. So one strand is built continuously (the leading strand) and one is built in small reverse sections (the lagging strand).

Free nucleotide units found in the nucleus are joined to the leading strand by DNA polymerase III from the 3’ end (i.e. it creates 5’ to 3’).

The lagging strand is constructed using prefabricated fragments called Okasaki fragments. RNA polymerase makes RNA primers which are used as identifiers for the DNA polymerase to join Okasaki fragments. DNA polymerase I removes the primers and acts as a spell checker for errors in the base pairing. The fragments are sealed by DNA ligase to form a continuous strand.

The two new strands of DNA coil into a helix. DNA replication is semi conservative as each of the new chromatids contains a strand of the original DNA.

Question 17

What are dideoxynucleotides?

Answer 17

Dideoxynucleotides (ddNTPs) lack the 3’-hydroxyl group used for bonding to phosphate groups. So, ddNTPs prevent further elongation of a nucleotide chain and effectively terminate replication.

Question 18

How was the semi-conservative nature of DNA confirmed?

Answer 18

By the Meselson-Stahl experiment in 1958. Nitrogen is a key component of DNA and can exist as a heavier ¹⁵N or a lighter ¹⁴N. DNA molecules were prepared using the heavier ¹⁵N and then induced to replicate in the presence of the lighter ¹⁴N then separated via centrifugation. After replication the DNA molecules were found to contain a mix of ¹⁵N and ¹⁴N.