DNA structure and replication - TCE Biology (Tasmania)
“DNA structure and replication: DNA is a double-stranded molecule in the nucleus of eukaryotic cells; nucleotide composition and complementary base pairing (A-T, C-G); the process of DNA replication including the enzymes helicase, primase, DNA polymerase and ligase; the significance of semi-conservative replication; genome sequencing”
DNA is a double helix of two strands of nucleotides. Each nucleotide has a deoxyribose sugar, a phosphate group and one of four bases. Bases pair by complementary base pairing, adenine with thymine and cytosine with guanine, held by hydrogen bonds. Before a cell divides, DNA is replicated: helicase unwinds and separates the strands, primase adds a primer, DNA polymerase adds complementary nucleotides, and ligase joins the new sections. Replication is semi-conservative: each new molecule has one old strand and one new strand, which keeps the sequence accurate.
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What this dot point is asking
DNA structure and replication is examined under criterion 5 (Section B of the TASC exam). The course limits the enzymes to helicase, primase, DNA polymerase and ligase, and excludes continuous and discontinuous replication, Okazaki fragments, the 5 prime to 3 prime direction, and purines and pyrimidines.
DNA structure
- Nucleotide: a deoxyribose sugar, a phosphate group and a nitrogenous base.
- Four bases: adenine (A), thymine (T), cytosine (C) and guanine (G).
- Strand: nucleotides linked in a chain by bonds between the sugar of one and the phosphate of the next, forming a sugar-phosphate backbone.
- Double helix: two strands twisted around each other, with bases pointing inwards.
- Complementary base pairing: A pairs with T, and C pairs with G, joined by weak hydrogen bonds. The sequence of one strand therefore determines the sequence of the other.
- Location: in eukaryotic cells DNA is double-stranded and found in the nucleus, packaged into chromosomes.
The hydrogen bonds between bases are weak, so the two strands can be separated for replication and transcription, while the strong bonds of the sugar-phosphate backbone keep each strand intact.
DNA replication
DNA is copied during the S phase of the cell cycle, so each daughter cell receives a complete copy.
- Helicase unwinds the double helix and separates the two strands by breaking the hydrogen bonds between bases.
- Primase adds a short primer to each exposed strand, giving DNA polymerase a place to start.
- DNA polymerase moves along each template strand and adds free nucleotides that are complementary to it (A with T, C with G).
- Ligase joins the separate sections of new DNA into a continuous strand.
The result is two DNA molecules with identical base sequences.
Semi-conservative replication
Each new DNA molecule is made of one original (parent) strand and one newly synthesised strand. Half of the parent molecule is conserved in each copy. Because each old strand acts as a template, the new sequence is complementary and the copy is accurate.
The semi-conservative model was confirmed by Meselson and Stahl in 1958, who grew bacteria with heavy nitrogen, moved them to light nitrogen and measured the density of the DNA after each generation. The result ruled out the competing conservative and dispersive models, an example of a model being tested and accepted on evidence.
Enzymes that manipulate DNA and genome sequencing
The course also asks you to apply the use of enzymes to manipulate DNA:
- DNA polymerase to synthesise DNA;
- ligase to join DNA fragments;
- endonucleases (restriction enzymes) to cut DNA at specific base sequences.
Genome sequencing determines the order of bases in an organism's DNA. Only a general understanding is needed: sequencing lets scientists compare genomes, identify genes and mutations, and study relationships between species. See biotechnology for background.
Template to new strand
Template strand: A T G C C G T A
New strand (DNA polymerase adds complementary nucleotides): T A C G G C A T
Check: A with T, T with A, G with C, C with G. The new molecule has one original strand and one new strand: semi-conservative.
Marker's note: in DNA the partner of A is T; U only appears in RNA.
- Mixing up the enzymes
- Helicase unzips, primase primes, DNA polymerase builds, ligase joins.
- Writing U in a DNA strand
- Uracil is only in RNA.
- Saying replication makes one entirely new molecule and keeps the old one
- That is the conservative model, which the evidence rejected.
- Spending time on excluded detail
- Leading and lagging strands and Okazaki fragments are not required for the TASC exam.
Exam-style questions
Questions in the style of TASC exam questions on this dot point, each with a worked answer. They are written by ExamExplained unless tagged "Past paper"; the year shows the paper a question is modelled on.
Original5 marksA DNA sample contains 30 percent adenine. Calculate the percentage of thymine, guanine and cytosine, and explain the rule you used.Show worked answer →
Calculation (3 marks). Adenine pairs with thymine, so thymine = 30 percent. A + T = 60 percent, leaving 40 percent for G + C. Guanine pairs with cytosine, so guanine = 20 percent and cytosine = 20 percent.
Rule (2 marks). Complementary base pairing: in double-stranded DNA, A always pairs with T and C always pairs with G, held by hydrogen bonds, so the amount of A equals the amount of T and the amount of C equals the amount of G.
Original6 marksDescribe the roles of helicase, primase, DNA polymerase and ligase in DNA replication, and explain why the process is described as semi-conservative.Show worked answer →
One mark for each enzyme and two marks for semi-conservative replication.
- Helicase
- Unwinds the double helix and separates (unzips) the two strands by breaking the hydrogen bonds between bases.
- Primase
- Adds a short primer to each exposed template strand, giving DNA polymerase a starting point.
- DNA polymerase
- Adds free nucleotides that are complementary to the template strand (A with T, C with G), building the new strand.
- Ligase
- Joins separate sections of newly made DNA to form a continuous strand.
- Semi-conservative
- Each new DNA molecule contains one original (parent) strand and one newly synthesised strand, so half of the original molecule is conserved in each copy. This allows the base sequence to be copied accurately, because each old strand is a template.