Lecture 7

DNA replication is a semiconservative process where each original strand of the DNA double helix acts as a template for the synthesis of a new, complementary strand. This results in two identical DNA molecules, each containing one original strand and one newly synthesized strand.

The process of DNA replication occurs only in the 5' to 3' direction. This means that new nucleotides are only added to the 3' end of the growing DNA strand.

DNA replication requires numerous protein complexes called enzymes to carry out various functions, including unwinding the DNA helix, separating the strands, and synthesizing the new strands.

The hydrolysis of high-energy phosphate bonds, primarily from ATP, provides the energy needed for DNA replication. This energy powers the enzymatic reactions involved in unwinding DNA, separating strands, and adding new nucleotides.

The MCM 2-7 complex, a ring-shaped structure formed by six identical subunits, is a DNA helicase responsible for breaking hydrogen bonds between complementary nitrogenous bases. It uses the energy from ATP hydrolysis to unwind the DNA double helix.

Topoisomerases are enzymes that change the topology of DNA. They reduce torsional stress, a twisting tension that builds up as the DNA helix is unwound during replication. This ensures that DNA unwinding can continue without tangling or breaking.

Replication Protein A (RPA) binds to single-stranded DNA fragments created during DNA unwinding. It prevents these fragments from forming short, double-stranded structures that would hinder DNA polymerase from working. This allows DNA polymerase to access and copy the single-stranded DNA template efficiently.

Proliferating Cell Nuclear Antigen (PCNA) is a ring-shaped protein that acts as a scaffold for proteins involved in DNA replication and repair. It surrounds the DNA strand and helps to keep DNA polymerase attached to it, ensuring efficient replication.

Replication Factor C (RFC) is a protein complex responsible for loading PCNA onto the DNA strand. It opens the PCNA ring, allows DNA to pass through, and then closes the ring. This process is crucial for the initiation of DNA replication and for the assembly of the replication machinery.

DNA polymerases are enzymes that synthesize new DNA strands using a template strand as a guide. They require a primer, a short RNA or DNA sequence, to initiate synthesis and can only add nucleotides in the 5' to 3' direction.

Polymerase α/primase is a complex that synthesizes a short RNA primer, called iRNA, at the start of each new DNA strand. This primer is later removed during DNA replication, making room for the addition of DNA nucleotides.

Polymerases δ and ε are the main DNA polymerases that extend the DNA strands after the primer is synthesized. Polymerase δ is primarily involved in leading strand replication, while polymerase ε is involved in lagging strand replication.

DNA polymerases have a proofreading function, meaning they can detect and correct errors during DNA synthesis. This 3' to 5' exonuclease activity ensures accuracy in DNA replication and helps maintain the genetic integrity of the cell.

RNases are enzymes that break down RNA molecules. Endoribonucleases cleave RNA molecules within the chain, while exoribonucleases remove nucleotides from the ends of RNA molecules. During DNA replication, RNase H and FEN1 are involved in removing the RNA primer from the new DNA strand.

Ligase is an enzyme that joins fragments of DNA together. It is crucial for connecting Okazaki fragments, short segments of DNA synthesized on the lagging strand, to form a continuous strand.

The process of creating a polypeptide chain from an mRNA molecule. This process occurs in the cytoplasm or on the membranes of the rough endoplasmic reticulum.

A large molecular complex that catalyzes the formation of peptide bonds between consecutive amino acids during protein synthesis.

The smaller subunit of a ribosome that binds to mRNA during translation.

The larger subunit of a ribosome that contains the catalytic site for peptide bond formation.

The site on the ribosome where the mRNA molecule binds.

The site on the ribosome where the aminoacyl-tRNA carrying the next amino acid to be added to the polypeptide chain binds.

The site on the ribosome where the tRNA carrying the growing polypeptide chain binds.

The site on the ribosome where the tRNA that has delivered its amino acid exits the ribosome.

The first stage of translation, where the ribosome assembles on the mRNA and the first tRNA carrying methionine binds to the start codon.

The tRNA molecule that carries the first amino acid, formylmethionine, in a polypeptide chain. It binds to the start codon (AUG) on mRNA.

The process by which aminoacyl-tRNA synthetase attaches the correct amino acid to its corresponding tRNA molecule.

The second stage of translation, where the polypeptide chain is elongated by the addition of amino acids one at a time.

The third stage of translation, where the ribosome encounters a STOP codon on the mRNA and releases the completed polypeptide chain.

Modifications that occur to a protein after it has been synthesized. These modifications can include folding, phosphorylation, glycosylation or other changes.

Proteins that assist in the proper folding of other proteins.

The process of copying genetic information from a DNA molecule to an RNA molecule, based on the complementary base pairing principle, is called transcription.

The initiation of DNA replication begins with the attachment of the Origin Recognition Complex (ORC) to the specific DNA sequence called the origin of replication.

The pre-replication complex (pre-RC) is assembled at the origin of replication, consisting of the ORC, Cdt1, and Cdc6 proteins, and functions to recruit helicase, crucial for separating DNA strands.

The DNA helix is unwound by the enzyme helicase, which breaks the hydrogen bonds between complementary bases, creating two single-stranded DNA templates.

Topoisomerase is an enzyme involved in relaxing supercoils in DNA, preventing tangling and ensuring smooth replication.

The replication bubble is a region where the DNA is separated into two single strands, forming a 'bubble' shape, with the two ends of this bubble forming replication forks.

A replication fork is a Y-shaped structure formed during DNA replication, where the DNA is unwound and new strands are synthesized.

RNA polymerase II is an enzyme responsible for transcribing genes that code for messenger RNA (mRNA), which carries genetic information from DNA to ribosomes for protein synthesis.

The process of adding a modified guanine nucleotide cap to the 5' end of pre-mRNA is called capping, which protects the mRNA molecule and helps it bind to ribosomes for translation.

Introns are non-coding regions of pre-mRNA that are removed during splicing, while exons are the coding regions that are joined together to form mature mRNA.

Splicing is the process of removing introns from pre-mRNA and joining exons together, forming a mature mRNA molecule that is ready for translation into protein.

Polyadenylation is the process of adding a tail of multiple adenine nucleotides (poly-A tail) to the 3' end of pre-mRNA, which stabilizes the mRNA and helps it exit the nucleus.

The leading strand in DNA replication is synthesized continuously in the 5' to 3' direction, following the movement of the replication fork.

The lagging strand in DNA replication is synthesized discontinuously, forming short fragments called Okazaki fragments, which are later joined together.

Okazaki fragments are short segments of DNA synthesized on the lagging strand during DNA replication.

A complex of proteins that binds to the DNA origin, initiating replication, composed of six protein domains.

An enzyme that unwinds the DNA double helix, separating the two strands, breaking hydrogen bonds between complementary bases.

An enzyme that relaxes supercoils in DNA, preventing tangling and ensuring smooth replication.

An enzyme that catalyzes the synthesis of RNA from a DNA template.

Non-coding regions of pre-mRNA that are removed during splicing

Coding regions of pre-mRNA that are joined together to form mature mRNA.

DNA replication is a process where the DNA double helix is copied, resulting in two identical DNA molecules.

The DNA double helix unwinds at specific sites called origins of replication.

Helicase unwinds the DNA double helix by breaking the hydrogen bonds between complementary bases. This creates two single strands that can serve as templates for replication.

Topoisomerases relieve the torsional stress that builds up as the DNA helix unwinds, preventing tangling and ensuring accurate replication.

Replication Protein A (RPA) binds to single-stranded DNA, preventing it from re-forming double-stranded structures. This allows DNA polymerase to access and copy the template efficiently.

Proliferating Cell Nuclear Antigen (PCNA) acts as a scaffold for proteins involved in DNA replication and repair. It recruits and anchors DNA polymerase, ensuring efficient replication.

Replication Factor C (RFC) loads PCNA onto the DNA strand, allowing replication to begin. It acts like a ring that clamps PCNA around the DNA, securing it.

DNA polymerases are enzymes that synthesize new DNA strands using a template strand as a guide. They require a primer, a short DNA or RNA sequence, to initiate synthesis and can only add nucleotides in the 5' to 3' direction.

Polymerase α/primase synthesizes a short RNA primer, necessary for DNA polymerase to begin replication. The primer is later removed and replaced with DNA.

Polymerases δ and ε are the main DNA polymerases that extend the new DNA strands. δ works on the leading strand, while ε works on the lagging strand.

DNA polymerases have proofreading activity, allowing them to detect and correct errors during replication. This ensures that the new DNA strand has high fidelity.

Ligase joins the Okazaki fragments, short segments of DNA synthesized on the lagging strand, to form a continuous strand. It acts like glue for DNA pieces.

The process of copying genetic information from a DNA molecule to an RNA molecule is called transcription.