40 Questions
What did Arthur Kornberg demonstrate in 1957?
The existence of DNA polymerase I
What is the function of DNA polymerase I in DNA replication?
To synthesize DNA in a 5' to 3' direction
What is the role of primase in DNA replication?
To make a short RNA primer that matches the single-stranded DNA
During DNA replication, what happens to the RNA primer?
It is removed by DNA polymerase's exonuclease function
What is the direction of DNA synthesis on the leading strand?
5' to 3'
What is the role of DNA helicase during DNA replication?
To break hydrogen bonds between DNA strands
What is the function of DNA ligase during DNA replication?
To join Okazaki fragments on the lagging strand
What is the phase of the cell cycle during which DNA replication occurs?
S phase
Where does the metabolism of carbohydrates, amino acids, and nucleotides occur?
Cytosol
What is the function of the Rough Endoplasmic Reticulum?
Membrane associated protein synthesis and secretion
What is the term for a protein as it is being formed by a ribosome before it folds into its active shape?
Nascent protein
What is the site of lipid synthesis in a cell?
Smooth Endoplasmic Reticulum
What is the function of the Golgi Apparatus in protein synthesis?
Modification and packaging of proteins and lipids
What is the characteristic feature of the Rough Endoplasmic Reticulum?
Presence of embedded ribosomes
Where do proteins move from in the Golgi Apparatus?
Convex/cis end to concave/trans end
What is the shape of the cisternae in the Golgi Apparatus?
Stacked and curved
What is the primary function of the nucleus in a eukaryotic cell?
To store and coordinate the cell's activities
Which of the following is a characteristic of heterochromatin?
It is condensed and has no active RNA synthesis
What is the function of the nuclear envelope?
To allow macromolecules to pass through the nucleus
What is the site of ribosomal RNA synthesis in a eukaryotic cell?
Nucleolus
What is the central dogma of molecular biology?
Genetic information flows from DNA to RNA to protein
What is a characteristic of prokaryotic cells?
They have a circular plasmid genome
What is the function of euchromatin?
It is involved in gene expression
What is the term for a compartment of a cell with a specific function?
Organelle
What is the primary function of DNA ligase during DNA replication?
To create phosphodiester bonds between sugar and phosphate groups
What is the main difference between exonuclease and endonuclease?
Exonuclease removes nucleotides from the ends, while endonuclease removes nucleotides within the polynucleotide molecule
What is the purpose of the exonuclease domain in DNA polymerase?
To proofread and remove incorrectly matched bases
Why is DNA polymerase I not the main replication enzyme in E. coli?
It is too slow to replicate the genome
What is the definition of processivity in DNA polymerase?
The number of nucleotides added to a growing DNA chain before the enzyme dissociates
What happens when DNA polymerase I stalls due to an incorrect nucleotide being added?
The enzyme stalls and cannot add the next nucleotide to the chain
What is the result of the semi-conservative replication model?
Two DNA molecules, each with one original DNA strand and one new DNA strand
Why are there more DNA polymerases in E. coli?
To increase the speed of DNA replication
What is the fundamental principle of the conservative model of DNA replication?
The original DNA molecule remains intact, and an entirely new DNA molecule is synthesized.
What is the purpose of density gradient centrifugation in the Meselson-Stahl experiment?
To separate DNA molecules based on density.
What is the significance of the intermediate band observed in the Meselson-Stahl experiment?
It supports the semiconservative model of DNA replication.
In the Meselson-Stahl experiment, what is the purpose of growing E. coli bacteria in a medium containing a heavy isotope of nitrogen?
To make the DNA molecules denser.
What is the outcome of the Meselson-Stahl experiment that supports the semiconservative model of DNA replication?
The formation of DNA molecules with a mix of N-15 and N-14 isotopes.
What is the main difference between the conservative and dispersive models of DNA replication?
The conservative model involves the synthesis of new DNA molecules, whereas the dispersive model involves the fragmentation of the original DNA molecules.
What is the significance of transferring the labeled E. coli bacteria to a medium containing a lighter isotope of nitrogen in the Meselson-Stahl experiment?
To allow the bacteria to replicate their DNA in the new medium.
What is the result of DNA replication according to the semiconservative model?
The synthesis of new strands complementary to the old ones.
Study Notes
Arthur Kornberg's Discovery
- In 1957, Arthur Kornberg demonstrated the existence of DNA polymerase I, which has three enzymatic activities in a single polypeptide: a 5' to 3' DNA polymerizing activity, a 3' to 5' exonuclease activity, and a 5' to 3' exonuclease activity.
Semi-Conservative DNA Replication
- DNA replication occurs in the synthesis phase (S phase) of the cell cycle.
- The process involves three stages: initiation, elongation, and termination.
- Initiation: DNA helicase untwists the DNA strands, breaking their hydrogen bonds, and primase makes a short RNA primer that matches the single-stranded DNA.
- Elongation: DNA polymerase builds a new 5' to 3' strand on the 3' to 5' strand (leading strand), and on the 5' to 3' strand (lagging strand), new DNA polymerases start due to helicase action, creating Okazaki fragments.
- Termination: DNA ligase connects the remaining Okazaki fragments and the sections where forks have met, creating phosphodiester bonds between sugar and phosphate groups.
Cellular Components
- Cytosol: involved in metabolism, protein synthesis (free ribosomes).
- Endoplasmic Reticulum (ER):
- Rough ER: translation of mRNA into membrane-associated proteins or for secretion out of the cell, with ribosomes attached to the outer surface of the membranes.
- Smooth ER: synthesis of lipids, Ca2+ storage, and steroid production, with no embedded ribosomes.
- Golgi Apparatus: modifies and packages proteins and lipids for delivery to other organelles or for secretion out of the cell.
DNA Replication Models
- Conservative Model: the original DNA molecule remains intact, and an entirely new DNA molecule is synthesized.
- Dispersive Model: the original parental DNA molecule breaks into fragments, which serve as templates for the synthesis of new DNA.
- Semi-Conservative Model: each newly formed DNA molecule consists of one parental strand and one daughter strand, supported by the Meselson-Stahl Experiment.
Meselson-Stahl Experiment
- E. coli bacteria were grown in a medium containing a heavy isotope of nitrogen (N-15), making their DNA denser.
- The labeled bacteria were transferred to a medium containing a lighter isotope of nitrogen (N-14), and allowed to replicate their DNA.
- DNA samples were collected at various time intervals and subjected to density gradient centrifugation, separating DNA molecules based on density.
- An intermediate band containing a mix of N-15 and N-14 DNA was observed, supporting the semi-conservative model.
Cell Structure
- Prokaryote Cell: no nucleus, membrane-bound organelles, genome is found as a circular plasmid DNA, and plasmid is important for gene modification.
- Eukaryotic Cell Structure: organelle is a compartment of a cell with a specific function.
- Nucleus: contains DNA, nucleoproteins, and RNA, and functions as the control center for the cell's activities, such as growth, metabolism, protein synthesis, and cell division.
- Nucleolus: site of ribosomal RNA (rRNA) synthesis, found within the nucleus.
- Nuclear Envelope: a double-layered membrane with nuclear pores, allowing macromolecules to pass through when entering or exiting the nucleus.
DNA Polymerase and Exonuclease
- Endonucleases: enzymes that remove successive nucleotides within the polynucleotide molecules.
- Exonucleases: enzymes that remove successive nucleotides from the ends of a polynucleotide molecules.
- The exonuclease activity in DNA polymerase I serves a proofreading function, removing incorrectly matched bases.
- DNA polymerase I is not the main replication enzyme due to its slow speed and moderate processivity.
Learn about Arthur Kornberg's groundbreaking discovery in 1957, where he demonstrated the existence of DNA polymerase I, an enzyme with three enzymatic activities. Discover its roles in DNA polymerization and exonuclease.
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