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Terms in this set (47)
Explain why researchers originally thought protein was the genetic material.
Proteins are macromolecules with great heterogeneity and functional specificity
studied Streptococcus pneumoniae bacterium and learned that while was strand was harmless, the other was pathogenic. he injected a mouse with heat killed pathogenic and a live pathogenic. When the mouse died, he found the pathogenic in the mouses blood. He called this transformation.
Avery, McCarthy, McLeod
They tested DNA, RNA, and proteins from heat killed pathogenic cells to identify the transforming agent. They concluded DNA transformed bacteria.
Hershey and Chase
They used radioactive isotopes to show that DNA was the genetic material of a phage that infects bacterium.
The ratio of nitrogenous bases in DNA is specific with each species. The ratio of A to T is the same. And the ratio of G to C is the same.
Used x-ray crystallography to study the structure of DNA
Watson and Crick
DIscovered the double helix shape of DNA because of Franklin
Describe the structure of DNA
DNA is helical. They are two strands of DNA to form the double helix. The sugar-phosphate backbone is on the outside of the DNA strands. Nitrogenous bases form hydrogen bonds. 2 H-bonds: A-T 3 H-bonds: G-C
Describe the semiconservative model of replication and the significance of Meselson and Stahl's experiments
one new strand, one old strand in both helices. Use heavy N15 isotope to differentiate DNA by density. When N15 and N14 cultured bacteria reproduce together, centrifuge data suggested that density of N14 + N15 pairing resulted in one strand of N14 and one strand of N15 in the double helix, thus semiconservative model is correct.
Describe the process of DNA replication
DNA transcribed into mRNA in cell nucleus; mRNA is processed & introns are snipped out, exons spliced together; mRNA moves out of the nucleus to the ribosome; tRNA matches amino acid to mRNA codons
Single Stranded Binding Proteins
bind to the single stranded DNA to prevent it from reforming the double helix (annealing).
DNA polymerase I
An Enzyme that removes RNA primers and replaces them with the appropriate nucleotides during DNA replication.
DNA polymerase III
An enzyme that catalyzes the elongation of new DNA at a replication fork by the addition of nucleotides to the existing chain.
A linking enzyme essential for DNA replication; catalyzes the covalent bonding of the 3' end of a new DNA fragment to the 5' end of a growing chain.
An enzyme that untwists the double helix of DNA at the replication forks, separating the two strands and making them available as template strands.
An enzyme that joins RNA nucleotides to make the primer.
What is the energy source that drives DNA replication?
It comes from two of the three total phosphates attached to each unincorporated base.
Define antiparallel and explain why continuous synthesis of both DNA strands is not possible
Each strand of DNA has a 5' and 3' end, each strand has the 5' on different ends. DNA can only be replicated from the 5' to 3' end, so they go in opposite directions
Distinguish between leading and lagging strands.
The leading strand is in the 3' to 5' direction and continuously synthesized (DNA polymerase synthesizes the copied strand continuously by moving in the complementary 5' to 3' direction) The lagging strand is in the 5' to 3' direction and is discontinuously synthesized.
Explain how the lagging strand is synthesized even though DNA polymerase can add nucleotides only to the 3' end.
Because there is no 3' Oh for covalent extension on the lagging strand, Telomerase, a RNA-dependent DNA polymerase adds tandem repeats of TTAGGG to the 3' end of DNA. It contains a reverse transcriptase and carries its own RNA molecule which is used as a template. Telomerase then reverse transcribes DNA onto the DNA leading strand from the RNA template. DNA pol then fills in the remaining nt on the lagging strand.
Describe the significance of Okazaki fragments.
Segments of the lagging strand synthesized discontinuously. DNA ligase welds them together
Explain the roles of DNA polymerase, mismatch repair enzymes and nuclease in DNA proofreading and repair.
The nuclease enzyme cuts the damaged DNA strands at 2 points and damaged section is removed. Repair synthesis by DNA pol fills in the missing nucleotides. DNA ligase seals the free end of the new DNA to the old DNA, making the strand complete.
Explain the reasoning that led Garrod to first suggest that genes dictate phenotypes through enzymes.
Garrod was an MD
Focused on rare and unusual birth defects; inborn errors of metabolism
Urine turns black; contains alkapton
Hypothesized a missing enzyme for brealing down alkapton
Describe Beadle and Tatum's experiments with Neurospora and explain the contribution they made to our understanding of how genes control metabolism.
Neurospora can grow on a minimal medium (MM): sucrose, salts, + biotin. Can synthesize all other biomolecules: amino acids, nitrogen bases, etc. Biomolecules are made stepwise by a series of enzymes Beadle and Tatum collected mutants that required arginine in their MM Intermediates in the pathway were known
Mutants can be grouped by which intermediate cures them
Concluded that each group of mutants lack a different enzyme in the pathway
Distinguish between the one gene - one enzyme hypothesis and the one gene-one polypeptide hypothesis and explain why the original hypothesis was changed.
One gene-one enzyme- Each gene makes an enzyme, but some proteins are not enzymes Counter examples: insulin, keratin, hemoglobin
One gene-one polypeptide- More inclusive Counter examples: RNA genes, regulatory sequences
Explain how RNA differs from DNA
1. RNA contains ribose instead of deoxyribose
2. Has the nitrogenous base Uracil instead of Thymine
3. Consists of a single strand
Briefly explain how information flows from gene to protein
The DNA strand is in the nucleus, RNA transcribes it by unwinding it and copying the complementary bases of an entire strand, the mRNA then goes to a ribosome and translates where the sets of codons code for proteins
Distinguish between transcription and translation.
Transcription transcribes the DNA to make a matching RNA copy, Translation translates the RNA copy into proteins
Compare where transcription and translation occur in prokaryotes and in eukaryotes.
Prokaryotes: Transcription and translation occur in the cytoplasm. Eukaryotes: Transcription occurs in the nucleus while translation occurs in the cytoplasm. Eukaryotic transcription and translation are separated by time because they are separated by space. They are separated by space because translation of the RNA occurs in the cytoplasm but DNA is only located in the nucleus so that is the only area where transcription can occur.
Define codon and explain the relationship between the linear sequence of codons on mRNA and the linear sequence of amino acids in a polypeptide.
mRNA base triplet. The linear sequence of the codon on the mRNA strand will code for some amino acid and thus a protein will be created.
Explain the early techniques used to identify what amino acids are specified by the triplets: UUU, AAA, GGG, CCC
Nirenberg synthesized poly-uracil to serve as an artificial mRNA
In vitro protein synthesis made only poly-phenylalanine
Explain why polypeptides begin with methionine when they are synthesized
Codon for met often signifies the starting place for the ribosome
Often removed after translation
Explain what it means to say that the genetic code is redundant and not ambiguous.
Because there are codons that specify the same amino acids but none of them ever specify any other amino acid
Explain the significance of reading frames during translation.
mRNAs are read first three, next three, next three, etc.
Shifting the start one base changes the triplets downstream
Explain the evolutionary significance of a nearly universal genetic code.
A language shared by all living things must have been operating very early in the history of life--early enough to be present in the common ancestors of all modern organisms.
Explain how RNA polymerase recognizes where transcription should begin.
RNA polymerase binds at the promoter
Promoter includes transcription start point
Determines template strand
Protein transcription factors bind to the promoter first + RNA polymerase = transcription initiation complex
In prokaryotes, proteins bound to RNA polymerase specify binding
Transcription unit extends from within the promoter past the terminator
Explain the general process of transcription.
- RNA polymerase binds to DNA at a specific sequence of nucleotides called the promoter
- The promoter contains an initiation site where transcription of the gene begins.
- RNA polymerase then unwinds DNA at the beginning of the gene.
- Only one of the DNA strands acts as a template for RNA synthesis
- RNA polymerase can only add nucleotides to the 3' side of the strand, so like DNA, RNA is synthesized in the 5' to 3' direction
- Free ribonucleotides triphospates from cytoplasm are paired with their complementary base pairs on the DNA template.
- RNA polymerase joins ribonucleosides to form an mRNA strand.
- As RNA polymerase advances, the process continues.
- RNA polymerase continues to elongate until it reaches the terminator.
- Transcription stops and mRNA polymerase and mRNA are released
Explain RNA processing in eukaryotic organisms.
modification of RNA before it leaves the nucleus.
Describe the functional and evolutionary significance of introns.
Introns may control gene activity
Alternate splicing can give >1 protein from the same gene
Exons code for functional domains of proteins
Widely spaced functional components allows recombination → new proteins
Describe the structure and functions of tRNA
Type of RNA molecule that transfers amino acids to ribosomes during protein synthesis. Loop-like structure due to hydrogen bonds.
Wobble explains why the synonymous codons for a given amino acid can differ in their third base, but usually not in their other bases
Explain how tRNA is joined to the appropriate amino acid.
tRNA is joined to the appropriate amino acid by use of a family of enzymes called aminoacyl-tRNA synthetases. the active site of these synthetases fit only a specific combination of tRNA and amino acid.
Describe the structure and functions of ribosomes.
The ribosome is made up of two subunits, a large one and a small one. Its job is to translate mRNA, once it enters the cytoplasm, and make proteins using the information in the mRNA.
Describe the process of translation.
Small subunit binds to mRNA leader or 5' cap and tRNAmet in P-site
Large subunit then binds
Protein initiation factors and energy from GTP are required
Elongation of the Polypeptide Chain
Next tRNA binds codon under A-site; uses elongation factors and GTP
Peptide bond formed
Growing polypeptide chain bound to tRNA in A-site
Translocation = ribosome shifts over one codon
Termination of Translation
Release factor binds stop codon (UAA, UAG, or UGA) under A-site
Water molecule added (hydrolysis)
New polypeptide released from last tRNA
Describe the significance of polyribosomes.
Ribosomes can sequentially attach to mRNAs
Allows many copies of a protein to be made using the same message
Explain what determines the primary structure of a protein. Describe how a polypeptide must be modified before it becomes fully functional.
DNA determines primary structure, and amino acid sequence is the primary structure; The primary structure of the protein is determined by the sequence of DNA. Every three nucleotides (sub-unit/building blocks of DNA) codes for a particular amino acid. The sequence of amino acids (which are sub-units/building blocks of protein) in a protein structure is the primary structure of the protein.
Describe what determines whether a ribosome will be free in the cytosol or attached to the rough ER.
Signal sequence, 20 amino acids found at the start of a protein being coded by the ribosome alerts the ribosome to attach itself to the ER. If the sequence is missing it will remain free.
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