Lecture 1.1 & 1.2: DNA + Lecture 3.2: Mutations

Question 1

Nucleic Acids

Answer 1

DNA
RNA

Question 2

How many chromosomes do somatic human cells have?

Answer 2

23 chromosome pairs
46 chromosomes

Question 3

How is DNA packaged in the nucleus? (4 steps)

Answer 3

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

Question 4

Structure of a DNA molecule

Answer 4

Double helix
Polynucleotide
Antiparallel strands
Phosphodiester bonds between adjacent nucleotides
Hydrogen bonds between parallel nucleotides (A&T have 2, G&C have 3)

Question 5

Structure of nucleotide (DNA)

Answer 5

Deoxyribose Sugar
Phosphate Group
Nitrogenous Base
Covalent bond between base and sugar at C1

Question 6

Purines (2 rings)

Answer 6

Guanine
Adenine

Question 7

Pyrimidines (1 ring)

Answer 7

Cytosine
Thymine
Uracil

Question 8

Structure of RNA

Answer 8

Single strand
Polynucleotide
Ribose Sugar
Phosphate Group
Bases (G,C,A,U)

Question 9

Cell Cycle (4)

Answer 9

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

Question 10

When are checkpoints in the cell cycle?

Answer 10

G1 and G2
DNA is checked and double checked to ensure that cells with faulty DNA are not replicated

Question 11

Mitosis (4/5 steps)

Answer 11

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]

Question 12

Gametes

Answer 12

Sex cells
Halpoid (only 23 chromosomes not 46)
Produced via meiosis

Question 13

Meiosis (8 steps)

Answer 13

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

Question 14

Non-disjunction

Answer 14

Is when the chromosomes in a cell do not separate properly during anaphase, this produces daughter cells with an abnormal number of chromosomes

Question 15

What is meiotic arrest?

Answer 15

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)

Question 16

What process being impaired could lead to meiotic arrest?

Answer 16

Example: If the spindle fibres do not form during metaphase, the chromosomes cannot be separated, thus meiosis will be halted

Question 16

What process being impaired could lead to meiotic arrest?

Answer 16

Example: If the spindle fibres do not form during metaphase, the chromosomes cannot be separated, thus meiosis will be halted

Question 17

Drugs used to treat cancer that interfere with elongation stage of DNA replication

Answer 17

Cisplatin and BCNU

Question 18

How does Genetic Variation arise?

Answer 18

Mutations: Permanent alteration to DNA, Leads to new alleles

Recombination/ Crossing Over during meiosis

Independent Segregation during meiosis

Question 19

Types of mutations

Answer 19

Point

Chromosomal

Copy Number Variation

Question 20

Point Mutations: Types (3 + 2)

Answer 20

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)

Question 21

Point Mutation: Effects (3)

Answer 21

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

Question 22

Chromosomal Mutation: Types (4)

Answer 22

Inversion
Deletion
Duplication
Translocation

Question 23

Copy Number Variation: Types (2)

Answer 23

Gene Amplification
Expanding Trinucleotide Repeat

Question 24

Where can mutations in DNA occur? (5)

Answer 24

- Protein-encoding sequences - In promoter or enhancer sequences of a gene - In termination signals - In splice donor and acceptor sites - In ribosome binding sites

Question 25

Causes of Mutation (3)

Answer 25

Radiation/ UV Rays Chemical Damage Errors in DNA replication

Question 26

Tautomeric Shift

Answer 26

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

Question 27

Slippage During DNA replication

Answer 27

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

Question 28

Chemical Mutagens

Answer 28

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

Question 29

Radiation: Ionizing radiation (alpha, beta, gamma, neutrons and X-rays)

Answer 29

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

Question 30

Radiation: UV radiation

Answer 30

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

Question 31

DNA Repair: Base Excision Repair (BER)

Answer 31

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)

Question 32

DNA Repair: Nucleotide-Excision Repair (NER)

Answer 32

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.

Question 33

DNA Repair: Mismatch repair (MMR)

Answer 33

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

Question 34

DNA Repair: HR and NHEJ

Answer 34

Homologous Recombination, Non-Homologous End Joining Provide a mechanism for the accurate repair of DNA double-strand breaks, protecting cells from chromosomal aberrations

Question 35

Breast Cancer

Answer 35

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