5.2 DNA Replication (not just cell division) Flashcards
Early ideas about heredity
1869: Friederich Miescher
- Discovered nucleus of cells contained acidic substance called nuclein
1927: Nikolai Koltsov
Erwin Chargaff
1950
- Found that amounts of A and T are roughly the same, and C and G are also roughly the same
- Essentially discovered base pairs, but didn’t identify them as such
Watson, Crick, Wilkins, Franklin
- James Watson, Francis Crick, Maurice Wilkins, and Rosalind Franklin together discovered the double helix structure of the DNA molecule in 1953
- Photo 51 showed that strands of DNA are not diametrically opposite, but that the strand is askew
Could be a 5 mark question on “to what extent did the quality of collaboration and communication between these scientists impact on their field of scientific research?”
DNA structure overview
- Base structure is a nucleotide
- Consists of a phsophate and sugar, with one of four nitrogenous bases (A, T, C, or G) attached to the sugar
- Nucleotides polymerise to form the whole DNA molecules. Phosphate and sugar form the backbone of the molecule and the base pairs are A-T and C-G
- Spontaneously winds itself into a double helix structure
Purine
2 ring structure, cytosine and guanine
,
Pyrmidine
One ring structure, adenine and thymine
Genes
A section of DNA coding for a specific function or trait
Ms Kaur’s “fun” fact: humans have 19,000-20,000 genes according to the Human Genome Research Project
DNA directionality
- A single strand of DNA has an end-to-end orientation, 5’-end and 3’-end
- This provides a direction by which enzymes read the code during DNA replication
- Base is attached at the first carbon (1’)
- On one strand of DNA, the nucleotide is attached at the 5th carbon (5’-end), on the other it is attached the 3rd carbon (3’-end)
’ means prime
Need to know 5’ and 3’, but don’t need to know what prime ends are
Advantages of double helix structure
- Nitorgen bases are inside the sugar-phosphate couples
- Keeping nitrogen bases inside the double helix protects them from UV radiation and damage, reducing chances of mutations (damage to nuclear structure)
- DNA bases are also hydrophilic (they can dissolve in water). The double helix shape protects them from water in the cell, as the sugar phosphate backbone and the DNA molecule as a whole are hydrophobic
- Second strand also acts as a backup of information from the frst strand. If one strand is unnaturally mutated, the other can help repair the damage
Chromosomal Theory of inheritance (1902)
- Proposed by Sutton and Boveri, independently reached the conclusion themselves
- Chromosomes are the vehicles of genetic heredity
- Sutton observed 3 new feautres of the nucleus when observing them during meiosis in grasshopper cells
- Boveri experimenting on sea urchins, discovered that not all chromosomes are idnetical and that a definite set of chromosomes is required to produce normal development. Also found that egg and sperm cells contribute equal amounts to the chromosome composition of offspring
DNA replication
Two identical strands of DNA formed from a single one
- Topoisomerase relaxes DNA from its coiled structure
- Initiator proteins bind the origins of replication
- DNA Helicase unzips the double helix by breaking hydrogen bonds between the base pairs → replication fork structure with 2 branching prongs of open DNA strands with antiparallel direcitonality
- Primase builds an RNA primer, starting from the 5’ end of the new DNA strand (using the 3’ end of the parent strand)
- Single-stranded DNA Binding protein (SSB) prevents DNA from re-ravelling back into the double helix until replication is completed
- DNA is synthesised on both open strands in 5’ to 3’ direction by DNA Polymerase.
- Exonuclease cuts out the RNA nucleotides that were used by Primase and Polymerase replaces them with DNA
- Ligase “glues” the strands together, forming two semi-conserved DNA structures
Leading vs Lagging strands
- Leading strand: DNA synthesised continuously in the same direction as the replication fork.
- Lagging strand synthesised in the opposite direction to the replication fork, in short fragments known as Okazaki fragments
- DNA Ligase seals the DNA fragments together
Primer
Made by RNA Primase
- A short strand of RNA that marks the starting point of replication (like a margin in a book)
- In lagging strand synthesis, primers are added on the lagging strand, and polymerase creates Okazaki fragments filling in the space between primer strands.
DNA semi-conservative replication
Semi-conservative because each new DNA is made up of one parent strand and one daughter strand
Relationship between DNA replication and evolution (5 marks)
- Mutations in DNA replication (sexual and asexual reproduction)
- Initial step for Meiosis: crossing over (DNA recombination) and independent assortment (sexual and asexual reproduction)
- Variants lead to natural selection, which also causes new changes in gene frequencies
- Include examples
- Beneficial traits can be inherited
