How DNA Replicates

Question 1

What is DNA polymerase?

Answer 1

Enzyme catalysing formation of DNA from activated deoxyribose nucleotides, using single-stranded DNA as template

Question 2

What is helicase?

Answer 2

Enzyme catalysing breaking of hydrogen bonds between nitrogenous pairs of bases in DNA molecule

Question 3

What is meant by semi-cpnservative replication?

Answer 3

How DNA replicates, resulting in 2 new molecules, each containing 1 old strand and 1 new strand with 1 old strand conserved in each new molecule

Question 4

How is DNA a self-replicating molecule?

Answer 4

• All DNA in every cell of organism carries coded instructions to make and maintain it
• Every time cell divides, DNA has to be copied so each new daughter cell receives full set of instructions
• Each DNA molecule replicates during interphase, before cell actually divides, resulting in each chromosome having identical copy of itself in eukaryotes, forming two sister chromatids

Question 5

How are sister chromatids formed?

Answer 5

Chromosomes joining at same centromere

Question 6

When does DNA replicate in mitochondria and chloroplasts?

Answer 6

Each time these organelles divide, just before the cell divides

Question 7

In semi-conservative replication, how does DNA start to make a copy of itself?

Answer 7

• Unwinds so double-helix is untwisted, a bit at a time, catalysed by gyrase enzyme
• Unzips so hydrogen bonds between nucleotide bases are broken, catalysed by DNA helicase and results in two single strands of DNA with exposed nucleotide bases

Question 8

After unwinding and unzipping, what do DNA molecules do to make a new copy of itself in semi-conservatiev replication?

Answer 8

• Free phosphorylated nucleotides, present in nucleoplasm in nucleus bond to exposed bases
• DNA polymerase catalyses addition of new nucleotide bases, in 5’ to 3’ direction, to single DNA strand using each single strand of unzipped DNA as template

Question 9

After nucleotide bonding and DNA polymerase, what do DNA molecules do to make a new copy of itself in semi-conservatiev replication?

Answer 9

• Leading strand is synthesised continuously, whereas lagging strand is in fragments later joined, catalysed by ligase enzymes
• Hydrolysis of activated nucleotides, to release extra phosphate groups, supplies energy to make phosphodiester bonds between sugar residue of 1 nucleotide and phosphate group of next nucleotide

Question 10

What is the product of DNA replication?

Answer 10

• 2 DNA molecules, identical to each other and to parent molecule
• Each molecule contains 1 old strand and 1 new strand

Question 11

How does DNA replicate in prokaryotes, mitochondria and chloroplasts?

Answer 11

Bubble sprouts from DNA loop and unwinds and unzips, then complementary nucleotides join to exposed nucleotides until eventually, whole loop is copied

Question 12

What evidence was there for semi-conservative replication in the 1950’s?

Answer 12

• Scientists knew DNA was self-replicating molecule but didn’t know how it made copies of itself
• There were 3 theories:
– Conservative: original molecule acts as template and new molecule is made
– Dispersive: original molecule breaks up into nucleotides, each one joins to complementary nucleotide and new ones join up again
– Semi-conservative: new molecule consists of 1 original strand and 1 newly formed strand

Question 13

What did Meselson and Stahl experiment in 1958 to show DNA replication is semi-conservative?

Answer 13

• Grew bacteria (E. coli) for 14 generations in medium containing heavy isotope nitrogen-15
• After 14 generations, most DNA in bacteria would be heavy
• Then transferred some bacteria into medium containing normal nitrogen-14 isotope and left them for long enough to undergo 1 replication
• DNA from bacteria after 1 division was found to be hybrid DNA, showing DNA doesn’t replicate conservatively, as that would have produced two bands of DNA, 1 heavy and 1 light
• Bacteria were allowed to divide once more and DNA was extracted and centrifuged, producing 2 DNA bands, 1 hybrid and 1 light, showing replication is semi-conservative and not dispersive

Question 14

What is meant by mutations?

Answer 14

• During DNA replication, errors may occur and wrong nucleotide may be inserted, estimated to occur in 1 in 10⁸ base pairs
• Could change genetic code
• During replication there are enzymes that can proofread and edit out such incorrect nucleotides, reducing rate mutations are produced
• However, many genes have such changes to nucleotide sequence

Question 15

What are different versions of particular genes called?

Answer 15

Alleles or gene variants

Question 16

What is meant by dome mutations not being harmful?

Answer 16

• Some appear to give neither advantage nor disadvantage
• Some can be advantageous, a white coat in an animal during winter when snow is on ground

Question 17

What is the useful mutations FOXP2?

Answer 17

• Allowed humans to speak
• Encodes protein called forkhead box protein, involved in human speech and language as well as in birdsong and mouse vocalisation
• Change to gene probably occurred about 200,000 years ago and has been selected for, as speech gave humans advantage as it enabled greater cultural evolution
• Protein product of this gene is transcription factor

Question 18

What is an important tip to remember about DNA replication?

Answer 18

During DNA replication, whole DNA molecule unwinds and unzips, but one length at a time, not all at once, replication fork moves along molecule