5+Replication+and+Proofreading+Short

Fundamental process of copying DNA to ensure genetic information is passed on during cell division.

DNA double-helix unwound in specific region (replication fork).
Template strands: Each DNA strand serves as a template for the synthesis of new complementary strands.

Nucleotide Addition: Individual deoxyribonucleoside triphosphates added sequentially to growing strands.
Base Pairing: The order of base addition determined by complementary pairing to the template strand.
The result is two new semiconservative double strands.
DNA Replication Mechanism
1. Unwinding DNA: The process begins at the replication fork, where DNA helicase unwinds and separates the double-stranded DNA, creating single-stranded regions accessible for replication.
2. Template Strand Recognition: Each separated DNA strand serves as a template. DNA polymerases recognize and bind to the single-stranded template, which provides the necessary sequence information for the synthesis of new strands.
3. Nucleotide Addition: Individual deoxyribonucleoside triphosphates are added one by one to the growing DNA strand. This addition occurs through a polymerization reaction where the incoming nucleotide's triphosphate releases two phosphates, providing energy for the bond formation.
4. Base Pairing: The order of nucleotide addition follows the complementary base pairing rules (A with T, and C with G). This specificity ensures that the new strand is an exact copy of the original strand.
5. Formation of New Strands: The result is the formation of two new double-stranded DNA molecules, each comprising one original template strand and one newly synthesized strand. This semiconservative method ensures accurate genetic replication.

Function: Unwinds and separates DNA strands, facilitating the replication process.
Mechanism: Binds and hydrolyzes ATP, leading to conformational changes that pry apart the DNA strands.

Prevents re-annealing of separated strands, ensuring accessibility for replication enzymes.

Role: Synthesizes short RNA primer (10-200 nucleotides) necessary for initiating replication.
Primer consists of ribonucleotides.

Synthesized continuously in the 5’ to 3’ direction using the 3’ to 5’ template strand.

Template strand oriented 5’ to 3’, requiring synthesis in fragments (Okazaki fragments) since replication cannot occur in the 3’ to 5’ direction.

Short, unconnected sequences formed on the lagging strand needing to be ligated together by DNA ligase.

RNA primers are initially synthesized due to the difficulty of starting DNA strands.
They allow for easy replacement and correction of errors later in replication.

Relieves tension in the DNA strand by creating a single-strand break, allowing rotational movement to relieve strain.

Functions by creating double-strand breaks, allowing another part of the helix to pass through, subsequently resealing the break.

Various proteins including DNA helicase, DNA primase, DNA polymerases, and SSBs collaborate in the replication fork to synthesize new DNA.

Changes in DNA sequences (mutations) can occasionally benefit species but also pose risks to individual survival.
Mutation Rate: Typically ~1 base per 10^9 nucleotides per replication.

Errors during replication are corrected by mechanisms before they become permanent mutations.
DNA Polymerase Proofreading: Ensures base pair fidelity through conformational changes and stalling if a mismatch occurs.
Exonucleolytic Proofreading: Processes any incorrectly paired bases, allowing polymerase to remove and replace them.

Rare forms of bases can lead to mispairing during replication, potentially causing mistakes that are corrected through proofreading.

Eukaryotic chromosomes have proteins (histones) that need to be synthesized and incorporated as DNA is replicated.
Histone Modifications: Copied during replication, maintained by reader-writer complexes.

Lagging strand cannot synthesize at the end of the DNA molecule due to lack of binding site.

Enzyme that adds repetitive nucleotide sequences to the ends of chromosomes (telomeres) using a built-in RNA template to extend the parental strand.

Longer 3' DNA strand looks like damaged DNA to repair machinery; it folds over to avoid detection.
Telomere length is regulated, and telomerase activity can be diminished leading to cellular aging or death.
Certain cancers are associated with uncontrolled telomerase activity.