The basics of DNA structure and DNA replication were covered in the SL topic so this topic looks at some extra details including, the 3' and 5' ends of the two antiparallel strands and further details about the enzymes responsible for DNA replication and some of the DNA regions which don't code for proteins but that regulate gene expression, form introns, telomeres or genes for tRNA. To understand how modern sequencing machines work a clear understanding of PCR and gel electrophoresis is also required.Learn and test your biological vocabulary for 7.1 Nucleic acids using these flashcards.
Summary list for topic 7.3 Translation
DNA structure
- Part of DNA supercoiling are structures called Nucleosomes.
- DNA structure gives a clue to the mechanism of DNA replication.
- Non-coding regions of DNA have other important functions, limited to regulators of gene expression, introns, telomeres and genes for tRNAs.
DNA replication (in prokaryotes only)
- DNA polymerase enzymes can only add nucleotides to the 3’ end of a primer.
- Continuous DNA replication occurs on the leading strand and discontinuous on the lagging strand.
- A complex group of enzymes do DNA replication including; helicase, DNA gyrase, single strand binding proteins, DNA primase and DNA polymerases I and III.
Crossing over
- DNA replication makes a second chromatid in each chromosome in interphase before meiosis.
- Crossing over exchanges pieces of DNA between non-sister homologous chromatids and forms new combinations of alleles on the chromosomes formed in meiosis.
Skills (can you ...)
- See that Rosalind Franklin’s and Maurice Wilkins’ X-ray diffraction work gave evidence for helix and two strands in DNA structure.
- Awareness that the Sanger method of base sequencing uses nucleotides containing dideoxyribonucleic acid (DNA with deoxyribose missing 2 oxygen molecules) to stop DNA replication at a specific base which allows sequencing using fluorescent markers and computers. (Sanger chain termination. Video here).
- Awareness that in DNA profiling Tandem repeats are used as these vary greatly from person to person.
- Ability to analyse of results of the Hershey and Chase experiment providing evidence that DNA is the genetic material.(This is a nice graphic).
- Analyse of molecular visualisations of the association between protein and DNA in a nucleosome.
How to prepare to answer a difficult IB question about DNA replication.
From your revision notes create between 8 and 10 quick questions.
Test yourself. Can you remember the answers?
1. What does helicase do?
2. Which proteins keep the DNA strands separate?
3. What is the role of the enzyme primase?
4. How is DNA polymerase III different from primase?
5. Which direction does DNA polymerase III along the template stand?
6. What is the leading strand?
7. What is the lagging strand?
8. Why are okazaki fragments produced on the lagging strand?
9. What is the role of the enzyme DNA polymerase I ?
10. What is the role of DNA ligase enzyme?
Model answers
This quiz contains multiple choice questions covering the understandings and skills in this topic.
START QUIZ!
It is estimated that a large percentage of human DNA is 'non-coding'.
Which of the following answers lists important functions of these non-coding regions?
Non-coding regions of DNA have other important functions, limited to regulators of gene expression, introns, telomeres and genes for tRNAs.
Which of the following is a role of nucleosomes in eukaryotes?
Supercoiling of DNA involves many steps, but the formation of nucleosomes is an impotant part of this. DNA is wrapped two times around a group of histone proteins.
Also: Transcription is partly regulated by Nucleosomes in eukaryotes.
To which part of an RNA primer can DNA polymerase III enzymes add nucleotides?
DNA polymerase enzymes can only add nucleotides to the 3’ end of a primer.
This diagram shows a simplified diagram of DNA replication.
Which statement best describes what is shown by the diagram?
Continuous DNA replication occurs on the leading strand and discontinuous on the lagging strand.
Which of the following statements give a logical order in the action of just three of the enzymes of the complex group of enzymes which carry out DNA replication?
A complex group of enzymes do DNA replication including; helicase, DNA gyrase, single strand binding proteins, DNA primase and DNA polymerases I and III. (also DNA ligase)
DNA gyrase and helicase uncoil and unzip the DNA strand, so that DNA primase can attach a primer.
Then DNA poymerase III adds DNA nucleotides to the 3' end of the primer. The RNA nucleotides of the primer are replaced by DNA nucleotides by DNA polymerase I.
Which is the best description of 'crossing-over'
DNA replication makes a second chromatid in each chromosome in interphase before meiosis.
Crossing over exchanges pieces of DNA between non-sister homologous chromatids and forms new combinations of alleles on the chromosomes formed in meiosis.
The diagram shows two bacteriophage virus particles used in a famous experiment by Hershey and Chase.
Why did they use labelled DNA in one and labelled protein in another?
The Hershey and Chase experiment provided evidence that DNA is the genetic material.
In their experiments they showed that the radioactive label was passed to the DNA in the next generation of bacteriophages.
Everyone needs a bit of fun while they revise. Try this Nucleic acids card match game.
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