Lecture 1.1 & 1.2: DNA + Lecture 3.2: Mutations Flashcards
Nucleic Acids
DNA
RNA
How many chromosomes do somatic human cells have?
23 chromosome pairs
46 chromosomes
How is DNA packaged in the nucleus? (4 steps)
DNA is wrapped around histones to form nucleosomes
Nucleosomes then tightly pack into solenoid structures forming 30nm fibres
These fibres compact into several heirarchical loops
These create highly condensed structures called chromosomes
Structure of a DNA molecule
Double helix
Polynucleotide
Antiparallel strands
Phosphodiester bonds between adjacent nucleotides
Hydrogen bonds between parallel nucleotides (A&T have 2, G&C have 3)
Structure of nucleotide (DNA)
Deoxyribose Sugar
Phosphate Group
Nitrogenous Base
Covalent bond between base and sugar at C1
Purines (2 rings)
Guanine
Adenine
Pyrimidines (1 ring)
Cytosine
Thymine
Uracil
Structure of RNA
Single strand
Polynucleotide
Ribose Sugar
Phosphate Group
Bases (G,C,A,U)
Cell Cycle (4)
G1: Cell grows and prepares for DNA replication, organelles are duplicated
S: The DNA within the cell is replicated
G2: The cell prepares for cell division (by growing more)
M: Mitosis occurs
When are checkpoints in the cell cycle?
G1 and G2
DNA is checked and double checked to ensure that cells with faulty DNA are not replicated
Mitosis (4/5 steps)
Prophase: DNA condenses into chromosomes
Metaphase: Chromosomes line up in centre of cell above/ below each other
Anaphase: Spindle fibres form from centrosome and pull the chromatids apart at the centromere to opposite poles of the cell
Telophase: The nuclear envelope begins to reform around chromosomes at each pole of cell and cell membrane begins to pinch in between the 2 sides of the cell
[Cytokenesis: the cell membrane pinches in, cytoplasm divides, and 2 new separate, identical daughter cells are formed]
Gametes
Sex cells
Halpoid (only 23 chromosomes not 46)
Produced via meiosis
Meiosis (8 steps)
P1: DNA condenses into chromosomes
M1: Homologous chromosomes line up in centre of cell next to each other, crossing over occurs at the chiasmata (increases variation)
A1: Spindle fibres form from centrosome and pull the chromosomes (randomly arranged so independent segregation) to opposite poles of cell
T1: The nuclear envelope reforms around chromosomes and cell membrane pinches in between the 2 poles of the cell
P2: DNA condenses into chromosomes again
M2: Chromosomes line up in centre of cell above/ below each other
A2: Spindle fibres form from centrosome and pull the chromatids apart at the centromere to opposite poles of the cell
T2: The nuclear envelope reforms around chromosomes and cell membrane pinches in between the 2 poles of the cell
Thus 4 non-identical, haploid daughter cells form
Non-disjunction
Is when the chromosomes in a cell do not separate properly during anaphase, this produces daughter cells with an abnormal number of chromosomes
What is meiotic arrest?
Is when meiotic process is stopped before the gametes (sperm/ova) can fully mature
This means that gametes are not produced (as they are destroyed/ aborted)
What process being impaired could lead to meiotic arrest?
Example: If the spindle fibres do not form during metaphase, the chromosomes cannot be separated, thus meiosis will be halted
What process being impaired could lead to meiotic arrest?
Example: If the spindle fibres do not form during metaphase, the chromosomes cannot be separated, thus meiosis will be halted
Drugs used to treat cancer that interfere with elongation stage of DNA replication
Cisplatin and BCNU
How does Genetic Variation arise?
Mutations: Permanent alteration to DNA, Leads to new alleles
Recombination/ Crossing Over during meiosis
Independent Segregation during meiosis
Types of mutations
Point
Chromosomal
Copy Number Variation
Point Mutations: Types (3 + 2)
Substitution: When a base is substituted for another
Insertion: When an extra base is inserted, causes frameshift
Deletion: When a base is removed from the sequence, causes frameshift
Transition: Purine to Purine/ Pyrimidine to Pyrimidine (A to G or T to C)
Transversion: Purine to Pyrimidine (e.g. G to T)
Point Mutation: Effects (3)
Silent: No effect on the amino acid sequence, thus no change in functional protein produced
Missense: Base change results in different AA being produced [K-Ras proto-oncogene is activated by missense mutation]
Nonsense: Base change results in generation of a stop codon
Chromosomal Mutation: Types (4)
Inversion
Deletion
Duplication
Translocation
Copy Number Variation: Types (2)
Gene Amplification
Expanding Trinucleotide Repeat
Where can mutations in DNA occur? (5)
- Protein-encoding sequences
- In promoter or enhancer sequences of a gene
- In termination signals
- In splice donor and acceptor sites
- In ribosome binding sites
Causes of Mutation (3)
Radiation/ UV Rays
Chemical Damage
Errors in DNA replication
Tautomeric Shift
Purine and Pyrimidine bases in DNA exist indifferent chemical forms, or tautomers, in which the H occupy different positions in the molecule
T and G are normally in keto forms, but when in the rare enol forms they can join by 3 H-Bonds with keto forms of G or T respectively
Cytosine and A are normally in amino forms, but when in the rare imino forms they can join by 2 H-Bonds with amino forms of A or C respectively
Slippage During DNA replication
Replication slippage or slipped-strand mispairing involves the misalignment of DNA strands during the replication of repeated DNA sequences
This can lead to genetic rearrangement
Chemical Mutagens
A physical or chemical agent that permanently changes DNA
Alkylating Agents add an alkyl group to G, this induces cross-linking between strands of DNA and the loss of a basic component (purine) or the breaking of the nucleic acid
Radiation: Ionizing radiation (alpha, beta, gamma, neutrons and X-rays)
Directly affects DNA structure by inducing DNA breaks, particularly DSBs
Secondary effects are the generation of reactive oxygen species (ROS) that oxidize proteins and lipids, and induce damage to DNA, like generation of abasic sites and single strand breaks (SSBs) and DSBs
Radiation: UV radiation
Formation of pyrimidine dimers, in which adjacent pyrimidines on the same strand of DNA are joined by the formation of a cyclobutane ring
This distorts the structure of the DNA chain and blocks transcription or replication past the site of damage
But T can be repaired by photoreactivation; energy from visible light is used
to split the bonds forming the cyclobutane ring
DNA Repair: Base Excision Repair (BER)
Corrects small base lesions that do not significantly distort the DNA helix structure, such as DNA damage from oxidation, deamination and alkylation
Enzymes involved: DNA glycosylase (leaves sugar with no base, at AP site), AP endonuclease (cleaves DNA), Deoxyribosephosphodiesterase (removes sugar), DNA polymerase (fills gap)
DNA Repair: Nucleotide-Excision Repair (NER)
Recognize a wide variety of damaged bases that distort DNA, including pyrimidine dimers and bulky groups added to bases as a result of the reaction of many carcinogens with DNA
Damaged DNA recognised + cleaved on both sides of a thymine dimer by 3′ and 5′ nucleases. Helicase results in excision of oligonucleotide with damaged bases. DNA polymerase fils gap, ligase seals.
DNA Repair: Mismatch repair (MMR)
Recognises mismatched bases that are incorporated during DNA
replication
Enzymes of this repair system are able to identify and excise the
mismatched base specifically from the newly replicated DNA strand
DNA Repair: HR and NHEJ
Homologous Recombination, Non-Homologous End Joining
Provide a mechanism for the accurate repair of DNA double-strand breaks, protecting cells from chromosomal aberrations
Breast Cancer
Can be familial or sporadic
If familial, is associated with BRCA1 (55-65% contract) & BRCA2 (45% contract)
oncogenes, can be tested through BRCA testing
5-10% women inherit BRCA genes
Knudson’s 2 hit hypothesis (both alleles on chromosomes must have mutated BRCA to develop cancer)
Sporadic/Normal BRCA 12% contract
