Topic 7 : Nucleic Acids and Proteins

7.1: DNA Structure and 7.2: DNA Replication

7.1.1: Describe the structure of DNA, including the antiparallel strands, 3’–5’ linkages and hydrogen bonding between purines and pyrimidines.

IB Question: Outline the structure of part of a double stranded DNA molecule, using a simplified diagram. [5]

IB Question: Outline the structure of part of a double stranded DNA molecule, using a simplified diagram. [5] N06/4/BIOLO/HP2/ENG/TZ0/XX two strands of DNA; anti-parallel; 3′ to 5′ linkages; purine / pyrimidine; A-T / G-C base pairing; hydrogen bonds; sugar-phosphate backbone; [5 max] Award [3 max] if answer does not include a diagram.

IB question: Outline the structure of DNA. [5]

IB question: Outline the structure of DNA. [5] double helix; two chains of nucleotides / composed of nucleotides; nucleotides consist of base, deoxyribose (sugar)and phosphate; bases are adenine, cytosine, guanine and thymine; anti-parallel / strands; 3′-5′ links between nucleotides; hydrogen bonds between base pairs / purine and pyrimidine on opposite chains; only A-T and G-C / complementary base pairs are A-T and G-C ; two bonds between A-T and 3 between G-C; [5 max] Credit can be given for any of these points shown on a correctly drawn and labelled diagram.

7.1.1:Purines and pyrimidines URACIL “U”

7.1.2: Outline the structure of nucleosomes. 7.1.3: STATE: Nucleosomes help to supercoil chromosomes and help to regulate transcription

IB QUESTION: Outline the structure of nucleosomes. [2]

IB QUESTION: Outline the structure of nucleosomes. [2] M09/4/BIOLO/HP2/ENG/TZ2/XX (eight) histone (proteins); DNA wrapped around histones/nucleosome; further histone holding these together; [2 max] Do not allow histone wrapped around DNA.

7.1.4: Distinguish between unique or single-copy genes and highly repetitive sequences in nuclear DNA.

Satellite DNA (highly repetitive sequences) Single-copy genes Satellite DNA (highly repetitive sequences) A single-copy gene has one locatable region on a DNA molecule. Satellite DNA consists of highly repetitive sequences that can repeat up to 100,000 times in various places on a DNA molecule. Single-copy genes make up 1–2% of the human genome. Satellite DNA constitutes more than 5% - 45% of the human genome. Single-copy genes are transcribed to make RNA, which in turn is translated to make a protein. Satellite DNA is not transcribed, i.e .they do not code for protein. A single-copy gene corresponds to a unit of inheritance (i.e., a protein). Satellite DNA is not involved with inheritance. Single-copy genes are usually thousands of base pairs in length. Satellite DNA is typically between 5 and 300 base pairs per repeat. Single-copy genes are less useful for DNA profiling. Satellite DNA has a high rate of mutation making it useful for DNA profiling.

IB QUESTION: Most of the DNA of a human cell is contained in the nucleus. Distinguish between unique and highly repetitive sequences in nuclear DNA. [5]

IB QUESTION: Most of the DNA of a human cell is contained in the nucleus. Distinguish between unique and highly repetitive sequences in nuclear DNA. [5] M10/4/BIOLO/HP2/ENG/TZ1/XX

7.1.5 : STATE: Eukaryotic genes contain exons and introns

7.2 D.N.A. Replication in prokaryotes 7.2.1: State that DNA replication occurs in a 5’→ 3’ direction.

7.2.2: Explain the process of DNA replication in prokaryotes, including the role of enzymes (helicase, DNA polymerase, RNA primase and DNA ligase), Okazaki fragments and deoxynucleoside triphosphates. Helicase

RNA Primase

RNA Primase

DNA Polymerase III

7.2.1 DNA replication occurs in a 5’ to 3’ direction.

Nucleotides are added in the form of deoxyribonucleoside triphosphates

DNA Polymerase I

DNA Ligase

Okazaki Fragments DNA pol III only synthesises DNA in a 5’ to 3’ direction Complementary strands of DNA run anti-parallel to each other. Lagging strand is synthesised discontinuously Leading strand is synthesised continuously

IB QUESTION: Explain the process of DNA replication. [8]

IB QUESTION: Explain the process of DNA replication. [8] occurs during (S phase of ) interphase/in preparation for mitosis/cell division; DNA replication is semi-conservative; unwinding of double helix / separation of strands by helicase (at replication origin); hydrogen bonds between two strands are broken; each strand of parent DNA used as template for synthesis; synthesis continuous on leading strand but not continuous on lagging strand; leading to formation of Okazaki fragments (on lagging strand); synthesis occurs in direction; 5 3 RNA primer synthesized on parent DNA using RNA primase; DNA polymerase III adds the nucleotides (to the end) 3 added according to complementary base pairing; adenine pairs with thymine and cytosine pairs with guanine; (Both pairings required. Do not accept letters alone.) DNA polymerase I removes the RNA primers and replaces them with DNA; DNA ligase joins Okazaki fragments; as deoxynucleoside triphosphate joins with growing DNA chain, two phosphates broken off releasing energy to form bond; [8 max] Accept any of the points above shown on an annotated diagram. (

IB QUESTION: Explain prokaryotic DNA replication. [8]

IB QUESTION: Explain prokaryotic DNA replication. [8] DNA replication is semi-conservative / each strand of DNA acts as template; (DNA) helicase separates two strands/forms a replication fork; new strand built / nucleotides added in a to direction; 5 3 (deoxy)nucleoside triphosphates hydrolysed to provide energy for nucleotide formation/base pairing; on one strand DNA polymerase III builds continuous strand; on other strand short chains of DNA/Okazaki fragments are formed; each short chain starts with RNA primer; added by RNA primase; then remainder of chain of DNA built by DNA polymerase III; DNA polymerase I removes RNA primer and replaces it by DNA; DNA ligase joins DNA fragments together forming complete strand; replication only occurs at a single replication fork; [8 max] Award credit for any of the above points clearly drawn and accurately labelled.

IB Question: State a role for each of four different named enzymes in DNA replication. [6]

IB Question: State a role for each of four different named enzymes in DNA replication. [6] Award [1] for any two of the following up to [2 max]. helicase; DNA polymerase / DNA polymerase III; RNA primase; DNA polymerase I; (DNA) ligase; 2 max Award [1] for one function for each of the named enzymes. helicase: splits / breaks hydrogen bonds / uncoils DNA / unwinds DNA; (DNA) polymerase III: adds nucleotides (in 5' to 3' direction) / proof reads DNA; (RNA) primase: synthesizes a short RNA primer (which is later removed) on DNA; (DNA) polymerase I: replaces RNA primer with DNA; (DNA) ligase: joins Okazaki fragments / fragments on lagging strand / makes sugar-phosphate bonds between fragments; 4 max [6]

7.2.3. STATE: DNA replication is initiated at many points in a eukaryotic chromosome