Structure of DNA Flashcards
Who discovered that the % of adenine and thymine were always similar, and the % of guanine and cytosine were always similar (complementary base pairs)? What date?
Erwin Chargaff 1952
Who used X-ray crystallography to deflect rays and give an idea of DNA structure?
Rosalind Franklin
Who suggested the model for DNA as a double stranded, right handed, anti-parallel helix?
Watson & Crick 1953
What does anti-parallel mean in terms of DNA structure?
The two DNA strands run alongside each other in different directions, the 5 prime end of one strand aligns with the 3 prime end of another
Which functional groups are at the 3’ end and 5’ ends of a DNA strand?
3’ end = OH attached to carbon on deoxyribose sugar (loses the OH in a condensation reaction)
5’ end = Phosphate group (OH is lost)
In which reaction do DNA nucleotides join, and what bond is formed between which groups?
Deoxyribose sugar links to phosphate group in a condensation reaction, forming a phosphodiester bond and eliminating water.
Which proteins coil DNA into a chromosome? In an active gene is it more loosely or tightly coiled?
Histones
More loosely
What is the structure of chromatin in interphase?
Disordered and uncondensed
What is the structure of chromatin in the cell cycle? At which phase does this first occur?
Condensed into ‘classic’ chromosome structure
Prophase
Which two people used rats and streptococcus pneumonia to confirm that DNA was a transforming agent? Give a brief description of this
Griffith 1928 and Avery 1944 A rough (non-virulent) and heat-killed smooth (virulent) strain were given to mice, which died. This proved that something in the heat killed smooth strain could transform the rough strain to the virulent strain. It was proven to be DNA because it tested positive to chemical DNA tests, had similar elemental composition to DNA, nothing else (RNA, lipids, proteins etc) caused virulence in the rough strain, and enzymes that degraded DNA degraded the transforming agent.
What did Hershey & Chase discover in 1952? How?
That DNA carries the genetic information of the cell, they grew two samples of bacteriophages, one with sulphur isotope (NOT found in DNA) and one with a phosphorous isotope (NOT found in proteins). Bacteria were infected with the phage, they were grown and centrifuged. Phosphorous was found in bacteria cells, sulphur was found outside cells; DNA must be the genetic information.
What is a bacteriophage?
A virus that infects and replicates WITHIN a bacterium
What model of DNA from the Meselson Stahl experiment proved to be correct? Why?
Semi-conservative
after the second test, there was 50% heavy and 50% light
Which enzyme unwinds DNA strands in DNA replication, and where?
DNA helices
Replication fork
What prevents separated DNA strands coming back together?
single stranded bonding proteins at the replication fork
Why is an RNA primer needed in replication? Which enzyme creates this and what does it do?
DNA polymerase can only bind to the 3’ end of the DNA, so a primer is needed to bind to the 5’ end of one strand
RNA primase creates a short strand of complementary RNA
What is a lagging DNA strand in replication?
DNA polymerase can only make DNA in the 5’ to 3’ direction. Leading strand runs 5’ to 3’ towards the replication fork and moves continuously. The lagging strand must be made in small sections, moving away from the fork.
Which enzymes are needed for DNA replication
DNA polymerase |||
RNA primase
What are Okazaki fragments?
Short stretches of DNA bases on the lagging strand, formed when DNA polymerase ||| attaches and reattaches to make DNA in the 5’ to 3’ direction
What is the role of DNA ligase in DNA replication?
Ligase fills in the gaps left between Okazaki fragments
What is the role of DNA polymerase | in replication?
Removes RNA primers and replaces them with DNA
What is the error frequency in DNA replication?
1 x 10^9
Name 3 ways error frequency in replication is reduced
DNA Polymerases have a proof reading ability
Base excision repair molecules detect and remove damaged bases
Nucleotide excision repair proteins cut out whole sections of damaged DNA
Name 3 ways mutation might occur
spontaneous
DNA damage
chemicals (mutagens)
Give 4 types of gene mutation
Base substitution
Deletion
Insertion
Rearrangement
What is the spontaneous deanimation of cytosine?
H2O is added and NH3 is lost (occurs slowly in aqueous solution) to form uracil in DNA.
Which enzyme recognises the spontaneous reanimation of cytosine to uracil and cuts it out? What is this an example of?
uracil N-glycosylase
base excision repair
What is a genome?
the complete set of genes in an organism
Which cells do not have 23 chromosome pairs?
RBCs lose their nucleus to fit into capillaries
Liver cells replicate chromosomes without dividing the cell
What is X inactivation?
Women inactivate one copy of the X chromosome into the Barr Body to ensure gene dosage is the same in each gender (random process during embryonic development)
What is synteny?
Long DNA sequences (genes) are present in the SAME ORDER across different species
What is translocation?
Genetic material is shared between different chromosomes. They break and reform. May cause disease, or may have no effect.
What causes X inactivation?
Xist (x inactivation specific transcript), a regulatory RNA that switches off a copy
What is the difference between the huntington gene in humans and in puffer fish?
It is much more compact in Fugu, the exons are a similar size and position, but the introns are smaller so there is less wasted space
What is intergenic DNA?
large regions of trash DNA between exons (gene desert)
what is the most common protein sequence in the human genome?
Reverse transcriptase from viral RNA
Define pseudo gene
Name two ways this can be generated
Stretches of DNA that are similar to normal sense but non-functional
Mutation during gene duplication which makes the gene inactive ; reverse transcriptase changes mRNA back to DNA, so no promoter gene are present
Define pseudopseudo gene
a pseudogene that unexpectedly produces an active protein
What are variable number tandem repeats (VNTR)?
areas of DNA with repeated sequences on after the other
Why do VNTRs occur?
DNA polymerase try to replicate highly repeated sequences, causing slippage where too many copies are inserted
Give a brief explanation of PCR?
Used to amplify small sections of DNA Heat and strands separate cool and primers attach at start of VNTR heated to 70 and polymerase attaches and nucleotides are added DNTR is replicated visualised using gel electrophoresis
What are telomeres?
protect the end of chromosomes and regulate the number of divisions a cell can make; they shorten with each division until they are too short and the cell can no longer divide
What builds telomeres?
the enzyme telomerase
What is special about telomeres in cancer cells and stem cells
Stem cells can divide indefinitely
cancer cells switch telomerase back on to keep dividing
What is a single nucleotide polymorphism (SNP)? Does this cause a change in phenotype?
nucleotides can vary at multiple positions; e..g one person may have a T where one person has an A. This may cause phenotypic change, or no change?
