Topic 7: Nucleic acids (Higher level)
This page lists the understandings and skills expected for topic two.
Detailed revison notes, activities and questions can be found on each of the sub-topic pages
- 7.1 DNA structure and replication
- 7.2 Transcription and gene expression
- 7.3 Translation
7.1 DNA structure and replication
- 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.
- 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.
- Say how Rosalind Franklin’s and Maurice Wilkins’ X-ray diffraction work gave insights into the structue of DNA.
- Know that the Sanger method of base sequencing uses nucleotides containing dideoxyribonucleic acid (DNA with deoxyribose missing 2 oxygen molecules) that stops DNA replication at a specific base.
It allows sequencing using fluorescent markers and computers. (Sanger chain termination. Video here)
- Say that in DNA profiling Tandem repeats are used as these vary greatly from person to person.
- Analyse results of the Hershey and Chase experiment providing evidence that DNA is the genetic material. .Graphic
- Use of molecular visualization software to analyse the association between protein and DNA within a nucleosome
- The direction of Transcription is in a 5’ to 3’ direction as RNA polymerase adds the 5´ end (phosphate) of the free RNA nucleotide to the 3´ end of the growing mRNA molecule.
- Transcription is partly regulated by Nucleosomes in eukaryotes.
- Eukaryotes modify mRNA after transcription.
- Splicing of mRNA increases the number of different proteins an organism can produce.
- Gene expression is regulated by proteins that bind to specific base sequences in DNA. - eg. methylation
- Gene expression is affected by the environment of a cell and of an organism.
- Awareness that the promoter region is an example of non-coding DNA.
- The skill to analyse changes in the DNA methylation patterns in connection with gene expression
Three stages of translation
- Initiation is the assembly of the components (large and small ribosome subunits, mRNA and tRNA molecules) that carry out the process.
- Synthesis of the polypeptide involves a repeated cycle on a ribosome where tRNA binds to the A (aminoacyl), P (peptidyl) and E (exit) sites in turn. Polypeptide molecule is produced.
- (examples of start and stop codons not needed)
- Termination of translation is followed by disassembly of the components.
- Free ribosomes synthesize proteins for use primarily within the cell.
- Bound ribosomes synthesize proteins primarily for secretion or for use in lysosomes.
- Translation can occur immediately after transcription in prokaryotes due to the absence of a nuclear membrane.
The sequence and number of amino acids in the polypeptide is the primary structure.
- The secondary structure is the formation of alpha helices and beta pleated sheets stabilised by hydrogen bonding.
- The tertiary structure is the further folding of the polypeptide stabilised by interactions between R groups. (Polar and non-polar amino acids are relevant to the bonds formed between R groups.)
- The quaternary structure exists in proteins with more than one polypeptide chain. and may involve the binding of a prosthetic group to form a conjugated protein.