DNA Replication Flashcards
Overview
DNA replication is the process by which a cell copies it’s DNA before cell division. This ensures that each daughter cell receives an identical set of genetic information
Semi conservative Process
Each new DNA molecule consists of one original stran (tempplate strand) and one sythensised strand
Initiation
In the initiation phase of DNA replication, helicase unwinds the double-stranded DNA at the origin of replication by breaking the hydrogen bonds between complementary base pairs, creating two single-stranded DNA templates. Single-stranded binding proteins (SSBs) bind to the exposed single strands, preventing them from re-annealing or forming secondary structures. This stabilized single-stranded DNA serves as a template for the synthesis of new complementary strands.
Elongation
During elongation in DNA replication, DNA polymerase synthesizes new DNA strands in the 5’ to 3’ direction. On the leading strand, replication is continuous, as it proceeds in the same direction as the replication fork. On the lagging strand, replication is discontinuous, producing short fragments called Okazaki fragments due to its opposite orientation relative to the fork. DNA primase lays down RNA primers for each Okazaki fragment, and DNA polymerase extends them.
Termination
DNA polymerase (I) replaces ENA primers with DNA nucleotides to complete the new strands. Gaps between Okazaki fragments are sealed by DNA ligase, resulting in two continuous and complete DNA molecules
What is the significance of the semi-conservative theory
The theory explains that during DNA replication each new DNA molecule consists of one original strand and one newly synthesised one. This process ensures that genetic information is accurately passed to new cells preserving the sequences of DNA. By using the original strand as a template, the risk of replication errors is reduced ensuring stability in the genetic material while allowing room for evolutionary changes through mutations
Genome sequencing
Genome sequencing is the process of determining the complete DNA sequence of an organism’s genome, including all its genes and non-coding regions.
This is achieved through advanced technologies such as next-generation sequencing (NGS) or whole-genome sequencing (WGS), which analyze the order of nucleotide bases (adenine, thymine, cytosine, and guanine) across the entire genome.
Genome sequencing provides insights into genetic variations, evolutionary relationships, and disease mechanisms.
Exonulcease
An enzyme that breaks down nucleotides one at a time from the end of a polynucleotide chain.
Simplified: Removes RNA primers, allowing polymerase to fill the gaps with nucleotides.
DNA Ligase
DNA ligase is an essential enzyme that facilitates the joining of DNA strands by catalysing the formation of a phosphodiester bond between the 3’ - hydroxyl end of one DNA strand and the 5’ phosphate end of another.
Simplified: DNA ligase seals up gaps between the 3’ end of one and the 5’ end of another strand of DNA with a phosphodiester bond. This is because after the exonuclease removes RNA and polymerase fills the large gap with nucleotides, there is still a small gap left over (which ligase must fill).
DNA Polymerase
synthesises new DNA strands by adding nucleotides to a pre-existing strand (template) during cell division. It ensures that genetic infromation is accuratley copied and passed
Primase
DNA Primases are enzymes whose continual activity is required at the DNA replication fork. They catalyse the synthesis of short RNA molecules used as primers for DNA polymerase. It provides a free 3’ hydroxyl (-OH) group for DNA polymerase to start adding nucleotides.
Simplified: Makes a small piece of RNA called a ‘primer’ which marks the starting point for the construction of the new strand of DNA.
Where does DNA replication occur?
DNA replication occurs in the nucleus of eukaryotic cells and in the cytoplasm of prokaryotic cells. It takes place during the S phase of the cell cycle before cell division to ensure that each daughter cell receives an identical copy of the DNA.
Helicase
The function of the helicase is to unpack an organism’s genetic material. Helicases are motor proteins that move directly along two hydrolysed nucleic acid strands, separating them.
Priming
In the priming phase of DNA replication, DNA primase synthesizes short RNA primers complementary to the single-stranded DNA template. These primers provide the free 3’-hydroxyl (OH) group required for DNA polymerase to initiate DNA synthesis. Primase lays down primers on both the leading and lagging strands, ensuring that DNA synthesis can proceed in the 5’ to 3’ direction.
Okazaki fragments
An Okazaki fragment is a short segment of DNA synthesized on the lagging strand during DNA replication.
Since DNA polymerase can only synthesize DNA in the 5’ to 3’ direction, the lagging strand, which runs in the opposite direction of the replication fork’s movement, is replicated discontinuously.
Each fragment begins with an RNA primer laid down by primase, which DNA polymerase extends to form a short piece of DNA. These fragments are later joined together by DNA ligase to form a continuous strand.
Single Strand Binding proteins
SSB’s are proteins that bind to the separated single strands of DNA at the replication fork, preventing them from re-annealing (coming back together) and maintaining them as separate templates for new DNA synthesis.
Steps in DNA replication
- Initiation
- Priming
- Elongation
- Proofreading
- Ligation
Ligation
Ligation is the enzymatic process in DNA replication wherein DNA ligase catalyzes the formation of phosphodiester bonds between adjacent nucleotides, sealing nicks in the sugar-phosphate backbone. This step is essential for joining Okazaki fragments on the lagging strand and ensuring the structural integrity of the newly synthesized DNA molecule.
Lagging strand
The lagging strand is the discontinuously synthesized strand of DNA that is replicated in the opposite direction of the replication fork’s movement, forming Okazaki fragments.
Leading Strand
The leading strand is the continuously synthesized strand of DNA that is replicated in the same direction as the replication fork’s movement.
What is DNA
Deoxyribonucleic acid (DNA) is a double-stranded helical molecule composed of nucleotide sequences that store and transmit genetic information in living organisms. Each nucleotide consists of a deoxyribose sugar, a phosphate group, and a nitrogenous base (adenine, thymine, cytosine, or guanine). The complementary base pairing (A-T, C-G) enables DNA replication and the synthesis of proteins through transcription and translation. DNA is the primary carrier of hereditary information and plays a fundamental role in cellular function, development, and evolution.
Prophase I
The first phase of meiosis, where homologous chromosomes pair up through synapsis to form tetrads. Crossing over occurs at chiasmata, leading to genetic recombination. The nuclear envelope breaks down, and spindle fibers begin to form.
Metaphase I
Homologous chromosome pairs (tetrads) align along the metaphase plate. Independent assortment occurs, where maternal and paternal chromosomes are randomly distributed, increasing genetic variation.
Anaphase I
Homologous chromosomes are separated and pulled to opposite poles by spindle fibers. Unlike mitosis, sister chromatids remain attached, ensuring a reduction in chromosome number.
