Smith Molecular Genetics 2-6 Flashcards
What is DNA replication and what stage does it take place?
the process in cells that produces two identical copies of DNA from one original DNA molecule
S phase of interphase
What are three models of DNA replication that have been considered in the late 1950s?
Conservative mechanism: after the first replication of the two DNA molecules, one retained the old two parental strands. In the end 3/4 are new and 1/4 are the old strands
Semiconservative mechanism: the first replication creates two mixed strands and the second creates two new and two mixed strands
Dispersive mechanism: results in all strands having interspersed segments of both parental and replicated DNA
Out of the three considered models for DNA replication, which one is correct? And what experiment confirmed this?
Semiconservative mechanism
Meselson and Stahl’s experiment
Separation of the parental strands begins at ______ origins of replication in each chromosome. Thus, replication is _______.
several, bidirectional
What are the four deoxynucleoside triphosphates used in DNA synthesis?
dATP, dGTP, TdTP, and dCTP
Enzyme DNA ______, which synthesizes the new DNA strands, can only add nucleotides onto the __ end of a new daughter strand; the direction of synthesis is in the 5’ to 3’ direction
polymerase, 3’
The addition of nucleotides to the daughter strand requires energy. Where is this energy from?
Bonds between the phosphates is broken (when pyrophosphate is removed) releasing energy used to form a bond between the incoming nucleotide and the growing chain
At each replication fork, one strand is synthesized continuously 3’ to 5’ and the other strand is synthesized ____ from the replication fork along the _____ strand template in fragments called _________ _______.
away, lagging, Okazaki fragments
Describe the role of helicase.
Unwinds the double helix by breaking hydrogen bonds; this has the effect of twisting the double helix in front more tightly
Describe the role of Topoisomerase and Single-Binding proteins
Topoisomerase’s (gyrase’s) role is to alleviate additional coiling by breaking, swivelling, and rejoining the DNA strands ahead of the replication fork
Single-stranded binding proteins prevent separated strands from coming together again behind DNA helicase
Describe the role of primase.
associate with a bare template strand and build a complementary RNA primer consisting of 5 to 10 nucleotides
Describe the role of DNA polymerase III
removes single-stranded binding proteins and begins to build a daughter strand by adding DNA nucleotides to the daughter strand
-AT/GC rule
What is the purpose of the sliding clamp?
holds DNA polymerase III onto the template strand
What is lagging strand synthesis?
DNA polymerase III has to detach from the fragment it’s synthesizing because it encounters the RNA primer of a previously synthesized Okazaki fragment, so it releases from the fragment leaving a fragment in place.
The short Okazaki fragments are parts of what is called the lagging strand.
Therefore, synthesis of the lagging strand is by discontinuous replication
Describe the role of DNA polymerase I
Attaches to a fragment at it’s 3’ end and removes the RNA primer nucleotides of the neighbouring fragment, replacing them with DNA nucleotides in the 5’ to 3’ direction
Note: a nick remains in the sugar phosphate backbone between the fragments
Describe the role of ligase.
Enzyme is needed to complete the covalent phosphodiester bond that will repair the nick left by DNA polymerase I
In prophase, each condensed chromosome consists of one old _____ strand and one new ______ strand
parental, daughter
True or false? Smaller simpler bacterial chromosome requires only a single origin of replication called the _____.
True, ori
What is the central dogma of molecular biology?
DNA codes for RNA (transcription)
RNA codes for proteins (translation)
Proteins do not code for DNA or RNA
-flow of genetic information is one way-
What is transcription?
The synthesis of an RNA molecule complementary to a portion of a strand of DNA acting as a template
Describe initiation (the first stage of transcription) in eukaryotes.
The TATA box in the promoter unit of a gene consists of 30 base pairs of Ts and As upstream the transcription start point.
First transcriptor proteins binds to the promoter, that recruit the RNA polymerase to the beginning of the transcription unit.
This altogether forms the transcription initiation complex
Describe elongation (the second stage of transcription) in eukaryotes
As RNA polymerase reads the nucleotides on the DNA template strand, it uses complementary base-pairing to form the new RNA molecule from ribonucleotide triphosphates (AG/CU)
When calculating the complementary RNA sequence to a given DNA sequence what technique should be used?
(1) rewrite the DNA sequence so that it is antiparallel (from 5’to3’ to 3’to5’)
(2)Then write the corresponding nucleotide
(G-C, C-G, A-U, T-A)
Describe termination (the third and final stage of transcription) in eukaryotes
When RNA polymerase reaches the end of the gene, it encounters a terminator sequence of nucleotides; the RNA polymerase and the RNA transcript are released from the DNA
