Cellular Genetics and Cell Cycle Flashcards
What are examples of monogenic disorders?
Cystic fibrosis, sickle cell anemia
What are examples of polygenic disorders?
type II diabetes, rheumatoid arthritis, schizophrenia
What is the function of herceptin?
Block HER2 receptors in tumors that overexpress it
What is cytogenetics?
the study of the genetic components of a cell through the visualisation and analysis of chromosomes
In which categories does the location of the centromere organise chromosomes?
metacentric, submetacentric, acrocentric
What is the structure of a Y chromosome?
Pseudoautosomal regions (PARs) at both ends Male specific region (MSY): contains sex determining region of Y (SRY)
What does SRY encode?
A protein that acts as a testis determining factor (TDF)
What are trisomy 21, 13, 18
Trisomy 21: Down syndrome
Trisomy 13: Patau syndrome
Trisomy 18: Edward’s syndrome
Which syndrome is XXY, X and XYY?
XXY: Klinefelter’s syndrome (infertility)
X: Turner’s syndrome (monosomy), infertility, short stature and amenorrhea
XYY: asymptomatic
What causes aneuploidy?
Non-disjunction: failure of homologous chromosomes to separate properly during meiosis
What is the timeline of oogenesis?
Before birth: oogonium undergoes mitosis to become primary oocyte arrested in prophase I
After birth: meiosis continues and secondary oocyte is arrested in metaphase II
After fertilization: meiosis II is completed
What type of disorder is cystic fibrosis?
Autosomal recessive; CFTR on chromosome 7
What defines pedigrees of autosomal recessive diseases?
Trait is rare, often skips generations, affects females and males equally, transmitted by either sex
What type of disorder is Huntington’s?
Autosomal dominant, HD locus has > 36 repeats of CAG codon as compared to 28
What defines pedigrees of autosomal dominant diseases?
trait is frequent, in each generation, males and females are affected equally, transmitted by either sex
What type of disorder is haemophilia A?
X-linked recessive, mutation of gene for blood clotting factor VIII on X chromosome
What defines pedigrees of X-linked recessive diseases?
Usually affects males, cannot be passed from father to son, all daughters of affected fathers are carriers, half the sons of a carrier will be affected and half her daughters will be carriers.
What are multifactorial traits?
Polygenic and influenced by the environment (cardiovascular diseases, obesity, diabetes mellitus, mental illness)
Which nucleic acids are purines and which are pyrimidines?
Purines: G, A
Pyrimidines: C, T
What are the side groups of nucleic acids?
Amine, methyl or carbonyl
What is a nucleoside?
Sugar + base
What bonds are present in DNA?
Phosphodiester bonds
In which direction does DNA replication occur?
5’ to 3’ end
How many nucleotides are needed for one turn of DNA, and what is the diameter of the helix?
10; 2 nm
What is the composition of protein coding genes in the human genome?
1.1%
What are transposon based repeats?
Retrotransposable elements; move to randomly selected new location through mechanism involving RNA intermediate.
LINEs (e.g. haemophilia) and SINEs
What is heterochromatin?
satellite DNA, transcriptionally inactive, composed of long arrays of high copy number tandemly repeated sequences –> genetic fingerprint
What are message processing non-coding RNAs?
Small nuclear RNAs (snRNAs); forms complex with protein –> small ribonucleoprotein particles (snRNPs) required for splicing of pre-mRNAs
What is decoding RNA?
rRNA and tRNA
What is Xist?
Long non-coding RNA that controls mammalian X inactivation
What are miRNA
micro RNA; control gene expression
hairpin stem-loop precursor of miRNA –> exported to cytoplasm and processed by nucleases –> translational repression and deadenylation (mRNA instability)
What does mtDNA encode?
13 polypeptides + rRNA and tRNA
What are examples of mitochondrial cytopathies?
MELAS: myopathy, encephalopathy, lactic acidosis, stroke-like episodes
LHON: leber’s hereditary optic neuropathy
How does DNA replication happen?
DNA helix is unwound by helicase;
single strand binding proteins prevent re-formation;
topoisomerase breaks phosphodiester bond ahead of replication fork to allow freedom of movement;
DNA polymerase synthesises new strand in 5’ to 3’ direction from existing RNA primer synthesised by primase (RNA polymerase)
*Lagging strand: exonuclease degrades primer, DNA polymerase continues synthesis, DNA ligase forms last phosphodiester bond between okazaki fragments
What is the distance between origins of replications in humans
around 150 kb
What are telomeres and how are they formed?
hundreds of copies of 5’ TTAGGG 3’
Formed by telomerase (carries its own template, a stretch of RNA)
What are the different types of mutations?
Point mutations: silent, missense, nonsense
Indels: CFTR (multiple of 3), HD
Inversions
Deletions: cri du chat (chromosome 5)
Duplications
Translocations: chronic myelogenous leukemia (pieces on chromosome 9 and 22 swap)
How do mutations appear in the genome?
SPONTANEOUS:
Errors in DNA replication
Replication slippage: reverse or forward
Deamination
INDUCED MUTATIONS:
Physical: ionising radiation (DNA single or double strand breaks), UV light (UV-B, thymine dimers)
Chemical: nitrous acid, alkylating agents, free radicals
How is DNA damage repaired?
Direct repair: dealkylation enzymes
Removal of damaged region followed by re-synthesis: nucleotide excision repair (NER)
What are the three steps of transcription?
Initiation, elongation and termination
How does transcription in prokaryotes occur?
initiation: sigma factor binds to prinbow box (TATA box) and guides RNA polymerase to transcription start site
elongation: 5’ UTR region, protein coding region, 3’ UTR region
termination: reaching of transcriptional termination site (GC rich stem loop + AU weak pairing)
What are the types of mRNA processing in eukaryotes?
Splicing
Capping: formation of 7-methylguanosine cap at 5’ end
Adenylation: addition of adenosine tail to 3’ end of RNA
What are the functions of RNA polymerase I, II and III?
polymerase I: rRNAs (long)
polymerase II: mRNAs and microRNAs
polymerase III: rRNA (5S) and tRNA
When is transcription of chromatin possible?
active open chromatin, unmethylated cytosines, acetylated histones
What is Rett syndrome?
X-linked dominant; mutation in methyl CpG binding protein 2 –> responsible for silencing gene expression especially in neurons
What is the structure of tRNA?
75-90 nucleotides; CCA-OH sequence at 3’ end; anticodon on central loop, wobble mechanism of pairing;
aminoacyl-tRNA-synthetases load tRNA molecules with amino acids (require ATP)
What are the components of prokaryotic ribosomes?
70S complex: Large subunit (50S): 23S rRNA + 5S rRNA + 34 proteins Small subunit (30S): 16S rRNA + 21 proteins
What is the initiation step of translation in prokaryotes?
formation of initiation complex comprising ribosome, mRNA and initiator tRNA (met); IF1,2,3 are required; IF2 is activated by binding to GTP
- 30S subunit binds to IF1, 3 and mRNA
- fmet-tRNA binds GTP-IF2 and enters P site of 30S
- Alignment of anticodon with mRNA codon
- Large subunit binds and releases GDP + Pi
* Shine-dalgarno sequence ensures in-frame translation
What is the elongation step of translation in prokaryotes?
- aminoacyl-tRNA binds to EF-Tu-GTP and enters A site
- proof reading through GTP hydrolysis
- peptidyl-transferase forms dipeptidyl-tRNA
- translocation through Ef-G-GTP hydrolysis
- release of discharged tRNA
What is the termination step of translation in prokaryotes?
- Stop codon presented to A site
- release factor (RF1, 2) binds
- hydrolysis of protein from tRNA
- ribosome disassembles, requires IF3, ribosomal recycling factor and GTP hydrolysis
What are the components of eukaryotic ribosomes?
40S + 60S = 80S
What are the differences between prokaryotic and eukaryotic translation?
Eukaryotes: interaction btw 5' cap and ribosome initiator tRNA is met-tRNA mRNA codes for one protein (non-polycistronic) polyribosomes are free or bound
Why can prokaryotic antibiotics be used without harming the eukaryotic host despite the interference with mitochondrial protein synthesis?
Few drugs can cross the inner mitochondrial membrane
Mitochondria have a very low rate of transcription
Actinomycin
Binds to transcription initiation complex and prevents elongation by RNA polymerase
Rifamycin
Inhibition of DNA-dependent RNA synthesis through binding to prokaryotic RNA polymerase
Streptomycin
Affects initiation (30S) and causes misreading of codons
Erythromycin
Binds to 50S subunit and prevents translocation
Chloramphenicol
Inhibits peptidyl transferase by binding to 50S subunit
Tetracyclines
Inhibit binding of aminoacyl-tRNAs to ribosome
Puromycin
Premature chain termination during translation (pro and eu)
alpha-aminitin
inhibitor of RNA polymerase II (pro and eu)
Cycloheximide
inhibits protein biosynthesis in eukaryotes by interfering with translocation step
Diphteria toxin
RNA translational inhibitor by inactivating eukaryotic elongation factor-2
What are restriction enzymes?
Recognize specific nucleotide sequences in the DNA and cuts both strands of the sugar phosphate backbone
The sequence forms a palindrome
Cleavage leaves cohesive or blunt ends
What is cDNA?
copy of an mRNA produced using reverse transcriptase
How does DNA sequencing work?
DNA polymerase copies single stranded DNA using dideoxy nucleotides, gel electrophoresis is used to distinguish length, laser determines different fluorescence wavelengths
How does PCR work?
Three steps: denaturation, primer annealing and extension (thermostable DNA polymerases: Taq or Pfu are used)
Thermocycler used to amplify: doubles each cycle (30 cycles will yield 2^30)
What are DNA microarrays?
Use nucleic acid hybridisation to rapidly measure which genes are expressed in a tissue sample
What are the types of genetic variations?
Single nucleotide polymorphisms, tandem repeat polymorphisms, structural variation (copy number variant, CNV; inversions)
What are haplotypes?
Series of SNP alleles along a single chromosome
What is meant by linkage disequilibrium?
SNPs in the same genetic region may be correlated –> knowing the allele at SNP1 gives information on allele at SNP2
What is used in forensic investigations for identity matching?
Microsatellite DNA
What are examples of diseases that involve tandem repeat polymorphisms?
Huntington disease
ALS: hexanucleotide repeat on chromosome 9
What is heritability?
proportion of phenotypic variation that is due to genetic differences among individuals
Define epigenetics
The study of heritable changes in gene expression that do not involve changes to the underlying DNA sequence
