DNA Structure
State the two complementary base pairs in DNA.
A–T and C–G.
What does antiparallel mean for the two strands of DNA?
Opposite directions: 5′→3′ and 3′→5′.
When must DNA replication occur in relation to mitosis?
Before mitosis, in interphase.
Define semi-conservative DNA replication.
Each new DNA molecule has one original strand and one new strand.
Which process copies DNA, and which separates chromosome copies into two nuclei?
DNA replication copies DNA; mitosis separates chromosome copies.
Name the three components of a DNA nucleotide.
Deoxyribose sugar, a phosphate group and a nitrogenous base.
A double-stranded DNA molecule contains 20% adenine. What percentage is cytosine?
30%. A = T, leaving 60% for C and G equally.
What happens to the two DNA strands at the start of replication? Include the bonds involved.
Enzymes break hydrogen bonds, separating the strands.
Why can an original DNA strand act as a template?
Its base sequence determines the new strand through A–T and C–G pairing.
A cell has 4 chromosomes before DNA replication. Immediately after replication, before mitosis, what happens to chromosome number and DNA amount?
Still 4 chromosomes, each with two sister chromatids. The DNA amount doubles.
Name the bond between complementary bases and the components of the DNA backbone.
Hydrogen bonds between bases; alternating sugar and phosphate groups in each backbone.
State the complementary strand to 5′–A T G C C A–3′, including both end labels.
3′–T A C G G T–5′.
Once the DNA strands separate, how are new strands built?
Free nucleotides pair A–T and C–G with each template; enzymes join them into new strands.
One DNA molecule replicates twice, producing four DNA molecules. How many contain a strand from the starting molecule? Explain.
Two. The two starting strands separate; each remains in one molecule after both rounds.
A student writes the follwoing steps:
New nucleotides pair → DNA strands separate → new nucleotides join. Rewrite the steps in the correct order and explain what happens in the first step.
DNA strands separate. New nucleotides pair with the exposed bases on each strand. New nucleotides join together to form new DNA strands.
Explain how the sequence of bases differs from the sugar–phosphate backbone in its role.
The base sequence carries genetic information. The backbone supports the strand.
A student says each individual DNA strand must contain equal amounts of A and T. Explain why this is incorrect.
A pairs with T on the other strand. A = T applies to the whole double-stranded molecule, not necessarily each strand.
Explain how complementary base pairing helps preserve genetic information during DNA replication.
A–T and C–G pairing lets each template guide a copy, normally preserving the original DNA sequence.
A student says semi-conservative replication gives each new DNA molecule only half the original genetic information. Explain why this is wrong.
Half the strands are original, not half the information. Each new DNA molecule normally carries the complete original base sequence.
A cell does not have enough free DNA nucleotides available. Explain how this could affect DNA replication and what impact this may have on cell division.
Fewer building blocks slow or prevent DNA copying, delaying or disrupting normal cell division.
Explain how two features of DNA structure allow it to be copied.
Hydrogen bonds break so the strands separate. Complementary pairing lets each original strand guide an accurate copy.
One strand of a DNA molecule contains 30% adenine (A) and 10% thymine (T). What percentage of the entire double-stranded DNA molecule is adenine (A) Use what you know about base-pairing rules to explain your answer.
20%. A pairs with T, so the other strand has 10% A. (30% + 10%) ÷ 2 = 20%.
Explain the process that produces two DNA molecules from one. Include enzymes, templates and base pairing.
Enzymes break hydrogen bonds, separating strands. Each templates A–T/C–G pairing; enzymes join new nucleotides. Two DNA molecules form, each with one old and one new strand.
Why does the presence of DNA molecules containing one old strand and one new strand support the semi-conservative model of replication?
Each new DNA molecule contains one original (old) strand and one newly synthesized strand. This results in DNA molecules that are half old and half new.
A copying error becomes a permanent DNA base change before mitosis. Could daughter cells inherit it, and must it affect their function? Explain.
Yes, daughter cells receiving the altered DNA inherit the mutation. Its effect depends on the location and change; some mutations do not affect cell function.