Unit 7

Question 1

Describe the structure of DNA

Answer 1

• Carbon atoms in deoxyribose are numbered; nitrogenous bases attached to C1, and the phosphate group is attached to C5
• Nucleotides are joined by a covalent bond between the C5 phosphate group and the C3 hydroxyl group
• One nucleotide strand runs 5’ - 3.’
• The nitrogenous bases interact via hydrogen bonding (complementary base pairing)
• One strand runs antiparallel to the other (this antiparallel strand runs 3’ - 5’)
• Double stranded DNA forms a double helix

Question 2

What is an intron?

Answer 2

A non-coding sequence of DNA within a gene (intervening sequence) that is cut out by enzymes when RNA is made into mature mRNA

Question 3

What is an extron?

Answer 3

The part of the gene which codes for a protein (expressing sequence)
Eukaryotic DNA contains introns but prokaryotic DNA does not

Question 4

Direction of Replication

Answer 4

5’ to 3’
- This means that the DNA polymerase enzyme responsible for adding new nucleotides moves along the original template strand in a 3’ - 5’ direction


Question 5

Semi-Conservative

Answer 5

DNA replication is semi-conservative, meaning that a new strand is synthesized from an original template strand.

Question 6

DNA replication - First step

Answer 6

• Occurs during the S phase of interphase
• Helicase unwinds and separates the double stranded DNA by breaking the hydrogen bonds between base pairs; forms the replication fork

Question 7

DNA replication- Leading Strand

Answer 7

• RNA primase adds a short RNA primer to the template DNA strand
• DNA polymerase III can now bind to the DNA
• DNA polymerase III then adds deoxynucleoside triphosphates (dNTP’s) to the existing strand of DNA in a 5’-3’ direction

Question 8

DNA Replication- Lagging strand

Answer 8

• RNA primase adds a short RNA primer near the replication fork and at intervals all along the lagging strand.
• DNA polymerase III starts replication by attaching at this 3’ end of RNA primer and adding dNTPs in a 5’-3’ direction.
• It moves away from the replication fork on this strand.
• DNA polymerase I now remove the RNA primer and replaces it with DNA using dNTPs. Short lengths of new DNA called Okazaki fragments are formed from each primer.
• Finally, DNA ligase seals up each break between the Okazaki fragments by making sugar phosphate bonds.

Question 9

When/where DNA replication is initiated?

Answer 9

Because eukaryotic genomes are (typically) much larger than prokaryotic genomes, DNA replication is initiated at many points simultaneously in order to limit the time required for DNA replication to occur
Occurs at sites called origins of replication and form replication bubbles
As replication bubbles expand in both directions, they eventually fuse together, two generate two separate semi-conservative double strands of DNA

Question 10

Transcription - facts

Answer 10

• 5’ to 3’ direction
• Occurs in the nucleus
• Produces messenger RNA which takes “message” to the cytoplasm/ribosomes

Question 11

Sense vs Anti-sense Strand

Answer 11

• The antisense strand is transcribed into RNA; Its sequence will be complementary to the RNA sequence and will be the “DNA version” of the tRNA anticodon sequence
• The sense strand is not transcribed into RNA
  Its sequence will be the “DNA; version” of the RNA sequence (identical except for T instead of U)