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Genetic Counseling Boards > Molecular Genetics > Flashcards

Molecular Genetics Flashcards

1
Q

Organization of DNA

A
  • 3.3 billion base pairs
  • 20,000 - 25,000 protein coding genes
  • 20,000 - 25,000 non-coding genes (produce functional RNA)
  • not random; some areas are gene-rich and some are gene-poor
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2
Q

Mitochondrial DNA

A
  • circular molecule containing 37 genes

- some mitochondrial-related DNA present in nuclear DNA

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3
Q

DNA Structure

A
  • consists of 3 key parts: nitrogenous base, deoxyribose sugar, phosphate group
  • 4 nitrogenous bases: pyrimidines - C and T, purines - A and G
  • molecule is 30 angstroms wide
  • one full turn of DNA is 34 angstroms long and consists of 10 nucleotides, each 3.4 angstroms apart
  • major groove is 24 angstroms long
  • minor groove is 10 angstroms long
  • strands of DNA are antiparallel and run in 5’ - 3’ direction
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4
Q

DNA Compaction

A
  • DNA coils around histone octamer to form a nucleosome; “beads on a string”
  • nucleosomes coil around one another to form a solenoid structure
  • solenoids from loops and coils known as chromatin (interphase DNA)
  • chromatin condenses into chromosomes during mitosis
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5
Q

DNA Replication

A
  • 3 key features: semi-conservative, semi-discontinuous, bidirectional
  • starts at replication origin and terminates at the telomeres
  • involves several proteins: helicase, single strand binding proteins, sliding clamp, topoisomerase, DNA polymerase, primase, and ligase
  • leading strand (3’ - 5’) replicated continuously
  • lagging strand (5’ - 3’) replicated discontinuously through Okazaki fragments
  • telomeres not usually preserved in somatic cells, which contributes to aging (senescence); in cells that do preserve telomeres, rely on telomerase to finish replication process and create single stranded end that protects DNA from degradation
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6
Q

Transcription

A
  • generates mRNA molecule from DNA as part of the process to form a protein
  • takes place in the nucleus and involves key proteins and regions of DNA: RNA polymerase II, transcription factors, promoter region, promoter activator sequences (enhancers)
  • initiation of transcription starts at TATA box with binding of transcription factors
  • RNA pol II binds and synthesizes RNA molecule
  • mRNA extensively processed and modified
  • 5’ end of mRNA is capped so molecule is stable, can be transported out of nucleus, and recruited to ribosomes
  • introns spliced out via lariat formation by spliceosomes
  • cleavage and polyadenylation of 3’ end to protect mRNA from degradation and help transfer it to cytoplasm
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7
Q

Translation

A
  • process that converts mRNA into protein
  • process requires ribosomes, tRNA, and amino acids
  • takes place in 3 phases: initiation, elongation, and termination
  • initiation of translation starts with recognition of start site (AUG)
  • elongation involves polymerization of polypeptide
  • termination is end of protein synthesis
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8
Q

DNA Regulation/Modification

A
  • epigenetics (reversible changes that are heritable)
  • DNA methylation
  • imprinting
  • histone modifications/variants
  • environment and lifestyle
  • RNA interference (form of mRNA degradation)
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9
Q

Mutations

A
  • most mutations found within protein coding regions
  • mutations affecting splicing, transcription, and other regulatory processes account for 10-20% of single gene mutations
  • originate from errors introduced during DNA replication or from failure to properly repair DNA damage
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10
Q

DNA Repair

A
  • types of problems to be fixed: wrong base inserted, base removed from backbone, damage to base (oxidation, deamination), cross-links (T-T dimers), double stranded breaks
  • 6 repair mechanisms: 3’ - 5’ exonuclease proofreading, mismatch repair, base excision repair, nucleotide excision repair, non-homologous end joining, homologous repair
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11
Q

Primase

A

synthesizes RNA primers on template strands for DNA polymerase to attach to and continue synthesizing

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12
Q

Ligase

A

joins nucleotides together by forming phosphodiester bonds

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13
Q

Ligase

A

joins nucleotides together by forming phosphodiester bonds

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14
Q

RNA polymerase II

A

directs synthesis of RNA in 5’ - 3’ direction using DNA as a template; does not require a primer

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15
Q

Transcription factors

A

required for transcription initiation and recruitment of RNA pol II

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16
Q

Promoter region

A

provide specificity and determine how much RNA/protein made

17
Q

Enhancers

A

modulate transcription from a distance; can be upstream, within, or downstream of a gene and are bound by specific proteins to regulate gene expression

18
Q

Spliceosomes

A

large RNA-protein complex comprised of snRNPs

19
Q

Isoforms

A

different proteins formed from the same gene as a result of alternative splicing of the same mRNA molecule

20
Q

Amino acids

A
  • basic building block of proteins and contain amino group (N-terminus), carboxyl group (C-terminus), and R side chain that specifies the type
  • 20 amino acids in total
21
Q

tRNA

A
  • intermediary molecules that transfer amino acids to growing peptide chain during translation
  • each amino acid has at least one specific tRNA
22
Q

Ribosomes

A
  • orchestrate process of translation by recognizing mRNA and initiating translation, recognizing tRNAs, and catalyzing peptide bonds
  • made up of rRNA which comes from short arms of acrocentric chromosomes and chr 1
  • contain 2 subunits: 40S and 60S
23
Q

Nonsense mediated decay

A
  • mechanism used by cells to decrease production of truncated proteins
  • does so when it encounters a premature stop codon upstream of an exon junction complex
  • cap of mRNA cleaved and mRNA molecule degraded
24
Q

Features of the Genetic Code

A
  • linear and read in non-overlapping triplets (codons)
  • degenerate because most amino acids are specified by more than one codon
  • unambiguous (AUG is always met, UUU is always phe)
  • nearly universal between eukaryotes and prokaryotes (except mitochondria)
25
Q

Silent mutation

A

alters codon but does not result in change in amino acid at that position of protein

26
Q

Missense mutation

A
  • ~50% of disease-causing mutations

- single nucleotide substitution that changes the coding strand of gene to specify a different amino acid

27
Q

Nonsense mutation

A
  • ~10% disease-causing mutations

- point mutation that causes replacement of normal codon with one of three stop codons

28
Q

Frameshift mutation

A
  • alters reading frame
  • abnormal splicing: ~10-20% of disease-causing mutations
  • insertions/deletions: ~30% of disease-causing mutations
29
Q

Dynamic mutation

A

involves amplification of a simple nucleotide repeat sequence and often leads to genetic anticipation

30
Q

Mutation effects

A
  • loss of function

- gain of function

31
Q

Mutation effects

A
  • loss of function

- gain of function

32
Q

3’ - 5’ exonuclease proofreading

A

exonuclease activity in DNA pol that proofreads the growing strand and fixes incorrectly incorporated bases

33
Q

Mismatch repair

A
  • used if proofreading does not catch a mutation

- repair enzyme complex removes mispaired base, DNA pol fills in gap, ligase seals nicks

34
Q

Base excision repair (BER)

A
  • occurs when purine or pyrimidine is damaged

- damaged based removed, DNA pol fills in gap, ligase seals nicks

35
Q

Nucleotide excision repair (NER)

A
  • occurs when bigger, bulky lesions distort double helic

- section of DNA surrounding lesion removed, DNA pol fills in gap, ligase seals nicks

36
Q

Non-homologous end joining

A
  • broken DNA ends are aligned and rejoined by DNA ligase

- can result in loss of nucleotides or lead to deletions, inversions, translocations

37
Q

Homologous repair

A
  • repair process that uses sister chromatids as templates for repair of damaged DNA strands usually after DNA replication occurs
  • involves strand displacement, ligation, branch migration, and duplex separation for accurate repair without loss of information

Genetic Counseling Boards (22 decks)

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