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Introduction to Medical Sciences > Nucleic Acids > Flashcards

Nucleic Acids Flashcards

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USMLE® Step 1 flashcards
1
Q

What did significant discovery did Frederick Griffith make and when?

A

Heat killed, virulent strains of pneumonia could transform non-virulent strains (1928)

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2
Q

What did significant discovery did Oswald Avery make and when?

A

Of the cellular biological molecules, Avery confirmed that DNA is the transforming agent (1944)

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3
Q

What did significant discovery did Hershey and Chase make and when?

A

Confirmed that DNA is the genetic material by showing that DNA was injected into bacteria by bacteriophages in order to replicate (1952)

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4
Q

What did significant discovery did Erwin Chargaff make and when?

A

Found that % of A and T in DNA is almost identical, and % of G and C is also almost identical (1952)

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5
Q

What did significant discovery did Watson and Crick make and when?

A

Discovered the double helix structure of DNA (1953)

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6
Q

Describe the backbone of DNA

A

Polymers of deoxyribose sugars bonded to phosphate groups by phosphodiester bonds

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7
Q

How many H bonds form between Adenine and Thymine?

A

2 (This leads to the theory that A and T bases are abundant in TATA box promotor regions because they require less energy to separate)

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8
Q

How many H bonds between Cytosine and Guanine?

A

3

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9
Q

Describe how DNA condenses

A

DNA wraps around histone proteins to form small nucleosomes. These condense in to chromatin. Only in non-interphase phases of the cell cycle does chromatin condense into chromosomes

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10
Q

What did the Meselson-Stahl experiment confirm?

A

That DNA replicates semi-conservatively (i.e. a new molecule is made up of one template and one new strand)

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11
Q

What is the origin of replication?

A

The region on a replicating DNA molecule that has been unwound, where replication is occurring

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12
Q

Name the two enzymes involved in unravelling the DNA strand and describe their roles

A

DNA helicase: separates the two DNA strands by breaking the hydrogen bonds

Topoisomerase: cleaves the DNA at regular intervals in order to relieve the strain caused by the action of DNA helicase

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13
Q

Describe the role of binding proteins

A

Bind to unravelled DNA to stop it recoiling

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14
Q

Describe the role of primers and name the enzyme that synthesises then

A

Primers are short segments of RNA that align with complimentary DNA on the template strand and allow DNA pol III to bind.
Synthesised by DNA primase

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15
Q

Name and describe the role of the two DNA polymerase enzymes involved in DNA replication

A

DNA polymerase III: catalyses condensation reactions between aligned nucleotides to form a new DNA strand
DNA polymerase I: synthesises a DNA strand in the place of the RNA primers

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16
Q

What is an Okazaki fragment?

A

A lagging strand of DNA

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17
Q

In what direction is a new DNA strand synthesised?

A

5’ to 3’

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18
Q

What enzyme joins the strands together within a new DNA strand

A

DNA ligase

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19
Q

What is a ‘dNTP’?

A

deoxy nucleoside triphosphate: this is how free floating nucleotides exist. They align to their complimentary nucleotide on the template strand. They provide their own energy during the synthesis of a new strand by releasing a polyphosphate molecule. The energy released causes the formation of a phosphodiester bond to another dNTP

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20
Q

What is a mutation?

A

An error in DNA replication resulting in an incorrect arrangement of bases

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21
Q

In what cell can a mutation occur and be inherited?

A

Germ cells

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22
Q

In what cell can a mutation occur and cause cancer?

A

Somatic cells

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23
Q

What is ethyl methane sulphonate (EMS)? Describe its effect

A

A mutagen. EMS alkylates a carbonyl group on guanine, so one less H bond can form between the guanine and a cytosine. This results in thymine aligning to the affected guanine, causing a GC to AT mutation

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24
Q

What do base excision repair proteins do?

A

Cut out incorrectly aligned bases. Specific to one base.

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25
Q

Describe ‘spontaneous deamination of cytosine’ and how this is corrected by the cell

A

Deamination (loss of NH3) of cytosine occurs slowly in aqueous solution. This results in the cytosine becoming uracil. A base excision repair protein, uracil N-glycosylase, recognises uracil in DNA and cuts it out

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26
Q

What do nucleotide excision repair proteins do?

A

Cut out incorrectly aligned nucleotides (whole sections of DNA). Less specific than base excision repair proteins

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27
Q

Describe exonuclease activity of DNA poll III and why this is effective

A

Exonuclease activity is the ability of DNA pol III to work backwards on the new strand (3’ to 5’) in order to remove an incorrectly added nucleotide

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28
Q

How many chromosomes do human cells have?

A

46

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29
Q

Name the two organelles that contain DNA

A

Nuclei and Mitochondria

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30
Q

What is the Xist gene and why is it necessary?

A

X inactivation specific transcript: translates regulatory RNA that inactivates one X chromosome in females. Necessary to ensure males and females have similar gene dosage per cell (X chromosomes contain much more DNA than Y chromosomes)

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31
Q

Whats is ‘synteny’?

A

Where long DNA sequences are present in the same order across species

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32
Q

What is ‘translocation of chromosomes’ and what adverse effect may this cause?

A

Rearrangement of parts between non-homologous chromosomes. This causes disease. E.g. rearrangement of chromosomes 9 and 22 causes the Philadelphia, causing Huntington’s disease.

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33
Q

What is an intron?

A

Non-coding regions within genes that are removed by splicing

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34
Q

What is an exon?

A

Coding regions in genes

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35
Q

True or False?

Humans have a compact genome in relation to other species

A

False

36
Q

What is the difference between a ‘gene cluster’ and ‘gene desert’?

A

Gene clusters are closely grouped genes. Gene deserts are regions between genes.

37
Q

What is a ‘pseudogene’?

A

A non-functional gene that has a similar sequence to a functional one. These can mutate, become active and cause disease (or may even cause evolution).

38
Q

Describe the two ways in which pseudogenes can form

A

1) Duplication of a gene, but where one becomes silenced by mutation
2) Reverse transcriptase of mature mRNA forms a DNA strand without introns that is reinserted into the genome.

39
Q

What are ‘VNTRs’?

A

Variable Number Tandem Repeats: short repeated sequences of bases between genes. One set inherited from each parent and can be used in genetic fingerprinting and paternity testing

40
Q

Describe the role of telomeres

A

Telomeres are sequences of DNA that protect the ends of chromosomes. When chromosomes replicate, the telomeres shorten until they are too short and the healthy cell can no longer divide

41
Q

What enzyme is turned on in cancerous cells that allows infinite cell division? Describe how this works

A

Telomerase
Telomerase builds telomeres. When the cell divides, the telomeres shorten. Activated telomerase maintains the length of the telomeres so that the chromosome can continuously replicate and hence the cell can infinitely divide

42
Q

What is a single nucleotide polymorphism?

A

A difference of one nucleotide between alleles of a gene. This must be present in at least 1% of the population to be classed as a SNP. These polymorphisms affect susceptibility to many diseases.

43
Q

What is a polygenic disease?

A

A disease caused by more than one gene

44
Q

What is the ENCODE controversy?

A

Controversy between the role of DNA that was previously thought of as being ‘junk’ (dark DNA)

45
Q

What is the nucleoplasm?

A

The nucleus (excluding the nucleolus) where chromosomes are found

46
Q

What is a polysome?

A

A complex where multiple ribosomes are translating one mRNA at a time

47
Q

Describe the appearance and role of mitochondrial DNA

A

A circular chromosome that codes for a small proportion of mitochondrial processes (most are coded for by nuclear DNA). This is inherited from the mother

48
Q

How many RNA polymerases are resent in mammalian cells, and which one is involved in transcription?

A

3

RNA pol II

49
Q

Name and describe the two different promotor regions present in DNA. What are their collective functions?

A

TATA boxes: short repeats of A and T bases close to the start of the gene
CPG islands: stretches of DNA where there are multiple CG repeats upstream from genes.
Collectively these act as promotor regions where DNA poll II can bind for transcription

50
Q

Describe the role of a Poly A tail and how it is added

A

Poly A tails are repeated A bases at the end of an mRNA molecule that provide stability and prevent degradation. The gene has a poly A signal, which the RNA polymerase transcribes. The enzyme detaches, a separate enzyme cleaves the end of the pre-mRNA and a poly A tail is added

51
Q

Describe the role of a capping nucleotide

A

A methylated nucleotide added to the start of an mRNA molecule that protects and prevents degradation

52
Q

What are 5’ and 3’ untranslated regions?

A

Regions on mRNA before the start codon and after the stop codon that are not translated

53
Q

What does the term ‘polycistronic’ mean?

A

Only occurs in prokaryotes. Where one mRNA molecule can code for multiple proteins.

54
Q

What are the two key sequences on mRNA that often signal splicing between them?

A

AG-GU and AG-A/G

55
Q

What is a polypyrimidine tract?

A

A sequence of heavily repeated pyrimidine (C and U) bases at the 3’ end of an intron. This signals to spliceosomes that the region to splice is approaching

56
Q

In gene technology, what is meant by ‘large constructs’ and why are they necessary?

A

large sections of DNA either side of a transgene that act as locus control elements - prevent signals from other genes interfering with the expression of the transgene. They hence allow the transgene to act as a self contained unit

57
Q

What are the 3 codon sequences that act as stop codons?

A

UAA, UAG, UGA

58
Q

What codon acts as the start codon and what amino acid is coded by it?

A

AUG - methionine

59
Q

What is the name given to the groups of enzymes that bind amino acids to their complimentary tRNA molecules?

A

aminoacyl tRNA synthase

60
Q

What does ‘wobble pairing’ mean?

A

The 3rd position on the anticodon does not have to be complimentary to the codon. This makes the code degenerate (multiple codons can code for the same amino acid)

61
Q

What binds to the small ribosomal subunit at the start of the mRNA that allows it to travel down the molecule?

A

Eukaryotic initiation factors

62
Q

At which site in a ribosome does a new tRNA align with its codon?

A

A site

63
Q

At which site in a ribosome do the tRNA molecules release?

A

E site

64
Q

Which molecules cause the release of a ribosome from a translated mRNA strand?

A

Release factors

65
Q

What is a ‘trans acting factor’?

A

A molecule that acts at a different site, such as transcription factors

66
Q

What is a ‘cis acting element’?

A

Molecules that affect the same site, such as promoter regions

67
Q

Name the 3 subsections of the Golgi apparatus (from nuclear to extracellular side)

A

cis-golgi, medial-golgi, trans-golgi

68
Q

What are ‘signal sequences’ and what are they made up of?

A

Sequences on mRNA of secretory proteins. Made of codons for hydrophobic amino acids

69
Q

What do ‘signal recognition particles’ do?

A

Bind to signal sequences on mRNA, and then cause the attached ribosome to dock to ER in order for them to be sent to the Golgi for secretion

70
Q

What is the role of the ‘transmembrane domain’ in the translation of its protein?

A

The domain is hydrophobic and is the section of protein which passes through a membrane. During translation, once the domain is transcribed it inserts itself in the membrane

71
Q

What is the C terminus?

A

The COOH end of protein - last to be transcribed

72
Q

What is ‘prenylation’ and how does it arise?

A

The addition of a lipid tail to a protein (lipid-linked proteins). These proteins have a sequence at their C terminus which causes an enzyme to cleave the end and add a lipid

73
Q

Is the import of mitochondrial proteins pre or post translational?

A

Post-translational

74
Q

What two integral proteins do mitochondrial proteins travel through for import?

A

TOM (translocase of the outer membrane) and TIM (translocase of the inner membrane)

75
Q

True or False?

Not all secretory proteins travel through the ER and Golgi for exocytosis

A

True

76
Q

What is the name of the signal that causes proteins to enter lysosomes from the Golgi?

A

Mannose phosphate

77
Q

Name the features of cancer

A

uncontrolled division, change in morphology, dedifferentiation of cells, cell migration to surrounding tissue, loss of apoptosis mechanisms, immortal cells

78
Q

What is a tumour?

A

A group of cells that grows uncontrolled

79
Q

What is a neoplasm?

A

An abnormal group of cells that will grow to form a tumour

80
Q

What does ‘malignant’ mean?

A

A tumour that tends to invade and destroy nearby tissue and spread to other parts of the body

81
Q

What does ‘benign’ mean?

A

A tumour that is not cancerous. It cannot invade or spread. It can cause issues by compressing other tissues

82
Q

What does ‘metastasis’ mean?

A

The spread of cancer from where it started to lymph nodes and/or other organs

83
Q

What does ‘adenoma’ mean?

A

A non-cancerous, benign tumour

84
Q

What does ‘carcinoma’ mean?

A

Cancer of epithelial tissue

85
Q

What is a thymine dimer? How is it caused and what risk does it present?

A

When two adjacent thymine bases form a covalent bond. Caused by energy from UV light. Can cause cancer.

86
Q

What is a DNA adduct?

A

When a chemical binds to a segment of DNA. Can cause cancer.

Introduction to Medical Sciences (2 decks)

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