DNA Replication and Chromosome Structure

DNA Replication: A biological process where DNA is duplicated to produce two identical cells of DNA from one original cell.

Helicase:
- Enzyme/protein essential for breaking hydrogen bonds between base pairs to unzip the DNA helix.
- Creates a Y-shaped structure known as the replication fork.
Primase:
- Enzyme/protein used to build RNA primers that initiate DNA replication.
- Functions during the S-phase (Synthesis phase) of the cell cycle.
DNA Polymerase:
   - Enzyme that adds DNA nucleotides to the end of an RNA primer.
   - Uses the original DNA strand as a template to build a complementary strand.
   - DNA Polymerase (I):
     - This variant can proofread DNA, removing RNA primers and replacing them with DNA nucleotides.
     - Looks for errors and replaces them to maintain fidelity in DNA replication.
DNA Ligase:
- Enzyme that seals nicks between DNA strands, connecting bases to form a continuous DNA backbone.
- Works to ensure the completeness of the double-stranded DNA.
DNA Gyrase:
- Enzyme that helps relieve the strain on DNA strands, preventing them from getting too twisted during replication.
Single-Strand Binding Proteins (SSB):
- Proteins that bind to single-stranded DNA during replication, stabilizing the separated strands.

Semi-Conservative Replication:
- Each new double-stranded DNA molecule consists of one parental strand and one newly synthesized daughter strand.

Purpose: A technique used to copy specific segments of DNA for further experimental analysis.

Denaturation (95 °C):
- DNA is heated to 95 °C to become single-stranded, replacing the need for the helicase enzyme in DNA.
Annealing (50-60 °C):
- Primers bind to each of the single DNA strands.
Extension (72 °C):
- DNA polymerase adds complementary bases to each strand, following a 5' to 3' elongation process with continuous replication.
- DNA polymerase tracks directly behind the helicase enzyme, synthesizing the complementary DNA strand as it moves toward the replication fork.

Leading Strands:
- Undergo continuous replication.
Lagging Strands:
- Do not replicate continuously; create short Okazaki fragments that are later connected by DNA ligase.
- These strands move away from the replication fork.

Autosomes:
- Human DNA has 22 pairs of autosomes, which are not sex chromosomes.
- Autosome pairs are homologous, meaning one is inherited from each parent.
Sex Chromosomes:
   - The 23rd pair of chromosomes determines biological sex in humans.
   - Denoted as:
     - XX for Female
     - XY for Male

Pairs of chromosomes that carry the same type of genetic information.
Characteristics:
- Share size, length, banding patterns, and genes located at the same positions.

Composed of a mixture of 50% DNA and 50% proteins, known as chromatin.
Identification:
- Early 1900s scientists recognized chromosomes as physical carriers of genetic information.
- Chromatin structure shows that DNA wraps around spool-like proteins called histones to form units known as nucleosomes (beads).
Function of histones:
- Act as a structural framework for DNA, assisting in packaging long DNA strands so they do not get tangled.
- They may carry different alleles (variants of genes) affecting traits such as eye and hair color.

An organized profile or map of a person's chromosomes arranged in pairs from largest to smallest.

Alleles:
- Variant forms of a gene found at the same spot on a chromosome.
Sister Chromatids:
- Identical copies of a single chromosome formed during DNA replication, joined at the centromere.
Diploid (2N):
- Cells containing two complete sets of chromosomes (one from each parent), typically somatic cells.
Haploid (N):
- Cells containing only one set of chromosomes, such as gametes (sperms and eggs).