Quiz 6 Flashcards
timeline of discoveries for DNA structure
- Morgan’s lab: genes located on chromosomes, 2 components (DNA and protein)
- Griffith (1928): used bacteria to show DNA is genetic material
- Chargaff (1950): DNA composition varies among species but ratios of purine and pyrimidine bases remains constant
- hereditary info encoded in DNA found in all cells; directs biochemical, anatomical, physiological, and behavioral traits
- Watson and Crick (1953): double helical DNA model, images stolen from Franklin and Wilkins
DNA structure
- nucleotide polymer (nitrogenous base, sugar, phosphate group)
- 3’ and 5’ end
- bases for Chargaff’s rules about equal A/T and G/C not understood until discovery of double helix
Watson and Crick’s discoveries
- deduced DNA is helical, helix width, spacing of nitrogenous bases, and that DNA has 2 strands in double helix
- ***models built to conform to chemistry
- Franklin had already concluded 2 outer sugar-phosphate backbones with nitrogenous bases paired in interior
- Watson-Crick model found that backbones were antiparallel (subunits run in opposite directions)
- model explains Chargaff’s rules: purine/pyramidine pairing resulted in uniform width
purines and pyramidines
purines: adenine and guanine (2 rings)
pyrimidines: thymine and cytosine (1 ring)
semi-conservative model of replication basics and how did Watson and Crick’s work provide evidence for this?
- Watson and Crick’s discovery of specific base paring suggested a possible copying mechanism (since 2 strands are complementary, each strand acts as a template for building a new strand)
- parent molecule unwinds and 2 new daughter strands are built based on base-pairing rules
- “Semi-conservative” because each daughter molecule has one parent strand and one newly made strand (proven using nitrogen isotopes)
where does DNA replication happen?
*fast and accurate process with more than a dozen enzymes
- begins at origins of replication, where 2 DNA strands separate to form a replication bubble (eukaryotic chromosomes may have thousands of these)
- replication proceeds in both directions from the site until the entire molecule is copied
- at the end, the replication bubble is a y-shaped replication fork, where new DNA strands elongate
important enzymes before replication
- helicase: untwist double helix at replication fork by breaking hydrogen bonds
- single-stranded binding proteins: bind to stabilize single-stranded DNA
- topoisomerase: corrects “overwinding” ahead of replication forks by breaking, swiveling, and rejoining DNA strands
how does DNA replication begin?
- primase starts RNA chain at 3’ end using parent DNA as a template (initial nucleotide strand is a short RNA primer 5-10 nucleotides long)
- works around 50 nucleotides/sec in humans
- nucleotides are added to the 3’ end of the growing strand (new strand elongates in 5’ –> 3’ direction due to antiparallel structure)
how does antiparallel structure impact DNA replication? (2 strands)
2 strands are synthesized differently, replication goes in both directions from the origin of replication
leading strand: synthesized continuously moving towards replication fork (where DNA is unzipped)
lagging strand: synthesized as a series of Okazaki fragments moving away from the replication fork and then shifting back towards it; fragments joined by DNA ligase
DNA replication machine
name for the large complex of proteins that participate in DNA replication
how is DNA proofread and repaired? how is it protected?
- DNA polymerases replace incorrect nucleotides
- repair enzymes correct errors in mismatched base pairing
- nucleotide excision repair: nuclease cuts out and replaces damages DNA stretches
*DNA is protected by telomeres, special nucleotide sequences at the end of chromosomes that prevent erosion of genes
what damages DNA?
- exposure to harmful chemicals
- physical agents (x-rays)
- mutations (although some good)
important enzymes during DNA replication
- DNA polymerase: catalyze elongation of new DNA at replication fork
* can’t initiate synthesis (requires a primer and template strand) and can only add nucleotides to 3’ end - DNA primase: starts RNA chain
- DNA ligase: joins together Okazaki fragment
makeup of nucleic acids
- polymer “polynucleotides” made up of nucleotide mononers
- monomers joined by covalent bonds to create backbone
- include 2 types of nitrogenous bases: purines (adenine, guanine) and pyramidines (cytosine, thymine, uracil)
nucleotides without phosphate: nucleosides
difference between DNA and RNA
DNA - 2 polynucleotide chains; antiparallel double helix; complementary base pairing for A/T and G/C
RNA - single polypeptide chains; T replaced by U; complementary base pairing can occur between 2 RNA molecules of parts of the same RNA molecule
types of RNA
mRNA (nucleus) - carries genetic info from nucleus to ribosomes in cytoplasm
tRNA (cytoplasm) - carries amino acids to mRNA in ribosomes to form polypeptides
rRNA (ribosomes) - makes up the ribosome, along with proteins
what is a gene?
region of DNA that can be expressed to produce a final functional product, either a polypeptide or a RNA molecule
Beadle and Edward’s mold experiments for the flow of genetic info
- initial “one gene one enzyme” hypothesis
- changed to “one gene one protein” hypothesis (not all proteins are enzymes)
- changed to “one gene one polypeptide” hypothesis (some proteins multiple polypeptides)
***proteins are the link between genotype and phenotype!
what is gene expression, and its basic steps
gene expression: process where DNA directs protein synthesis
- transcription: synthesis of RNA from DNA using a template strand; producing mRNA
- translation: synthesis of polypeptides at the ribosomes using mRNA base triplets (codons) to specify amino acid sequence
central dogma of gene expression
concept that there is a cellular chain of command
DNA - RNA - protein
the genetic code
universal code for assembling 20 amino acids into proteins using the 4 nucleotide bases in DNA
transcription and the role of RNA polymerase
- one of 2 DNA strands provides a template for nucleotide sequence in a RNA transcript
- RNA polymerase pries DNA apart and hooks in RNA nucleotides
transcription initiation
formation of the transcription initiation complex: RNA polymerase attaches to the promoter “TATA box” (initial DNA sequence) and transcription factors
transcription elongation
- RNA polymerase moves along DNA; untwists helix 10-20 bases at once
- 40 nucleotides/sec transcribed in eukaryotes
- several RNA polymerases can transcribe a gene at once!
transcription termination
pre-mRNA is cleaved from growing RNA chain, polymerase continues transcribing and eventually falls off DNA
translation
- mRNA base triplets (codons) specify amino acids to translate mRNA into polypeptides
- tRNA carries amino acids attached to anticodons that complement mRNA codons
- occurs at ribosomes in cytoplasm (2 subunits made of proteins and rRNA)
- enzyme mediated process–all 3 stages require protein factors
translation initiation
- mRNA, tRNA, and ribosome come together
- small subunit moves along mRNA until reaching a start codon (AUG)
- initiation factors bring in large subunit to make the translation initiation complex
translation elongation
- amino acids added by complementary mRNA codon/tRNA anticodon pairings
- each addition involves elongation factor proteins
translation termination
- stop codon in mRNA reaches ribosome; release factor proteins activated
- ***polypeptide chains often modified after translation to make functional proteins!
how does DNA give clues about evolution?
linear sequences of nucleotides passed from parents to offspring; 2 closely related species will be more similar in DNA
what are mutations? point mutations?
mutations: changes in the genetic material of a cell or virus
point mutations: chemical changes in just one base pair of a gene; can lead to the production of an abnormal protein
codon
codon: small triplet that specifies an addition of one amino acid to the polypeptide (codes for gene to protein)
- 64 codons; 61 code for amino acids and 3 “stop” codons end translation
- redundant (more than one codon may specify a certain amino acid) but not ambiguous (no codon specifies more than one amino acid)
- codons must be read in correct reading frame (correct groupings)
transcription unit
stretch of DNA being transcribed to synthesize RNA
