DNA Structure and Function

• A chimera is a human with DNA (and sometimes body parts) from two individuals.
• PTCC refers to the genetics of taste, specifically a chemical that tastes bitter to some but is tasteless to others.

Genetic Material: DNA

• DNA (deoxyribonucleic acid) contains the genetic instructions for making proteins, which determine traits.
• DNA is composed of nucleotides, each with a base: Adenine (A), Thymine (T), Cytosine (C), or Guanine (G).
• Base pairing rules: A pairs with T, and C pairs with G.
• DNA is a twisted ladder (double helix), with base pairs forming the rungs and a sugar-phosphate backbone forming the sides.
• In a non-dividing cell, DNA exists as chromatin (thin and relaxed) within the nucleus.
• During cell division (mitosis), chromatin condenses into tightly packed structures called chromosomes.
• Chromatin must be thin for DNA accessibility during RNA transcription.
• Humans possess 46 chromosomes arranged as 23 pairs.
• Of these, 22 pairs are autosomes, and 1 pair are sex chromosomes (XX or XY).
• The genome is the complete set of DNA in a cell, containing the instructions to build and maintain an organism.
• The nuclear membrane protects the DNA stored in the nucleus.
• DNA cannot exit the nucleus; copies are made into RNA to be "read" by the cell.
• Somatic cells (body cells) divide via mitosis, creating exact copies with 46 chromosomes (diploid).
• Sex cells (sperm and egg) contain 23 chromosomes (haploid).
• Sperm (23) + egg (23) combine to form a zygote (46).
• The presence of a Y chromosome determines biological maleness.
• Homologous chromosomes have the same centromere position and size, with alleles coding for the same characteristics but not necessarily the same trait (e.g., eye color).
• A karyotype is a complete lineup of an individual's chromosomes.
• Haploid (n) refers to half the total chromosome number, found in gametes created by meiosis.
• Diploid (2n) refers to the full chromosome number, found as pairs in somatic cells.
• A zygote (fertilized egg) has 46 chromosomes (diploid).
• An extra chromosome on an autosome has a more significant impact than on a sex chromosome.
• A gene is a DNA section that codes for a specific protein.
• The process: DNA → RNA → Protein.

Chromosomes and Cell Division

• A chromosome contains DNA within the nucleus, composed of tightly packed DNA and protein (chromatin).
• Chromosomes become visible during cell division (mitosis).
• 2n = 4 represents a diploid cell with 4 total chromosomes.
• n = 2 represents a haploid cell with 2 total chromosomes, found in sperm or egg.
• After DNA replication in the S phase, each chromosome has two identical sister chromatids joined at the centromere.
• Sister chromatids are exact copies with the same DNA sequence.

Cell Cycle and Mitosis

• The cell cycle includes interphase and cell division.
• Interphase is the "resting" phase where the cell grows and replicates DNA.
• Cell division includes mitosis (nucleus division) and cytokinesis (cytoplasm division).
• Mitosis produces two identical daughter cells, genetically identical to the parent cell, and are diploid (2n) with 46 chromosomes.
• Mitosis starts with 46 chromosomes and ends with two daughter cells, each with 46 chromosomes.
• Mitosis proceeds through four stages: prophase, metaphase, anaphase, and telophase (PMAT).

Stages of Mitosis

• Prophase: Chromatin condenses into visible chromosomes, nuclear membrane breaks down, centrosomes move to opposite poles, and spindle fibers form (46 chromosomes).
• Metaphase: Chromosomes align at the cell's equator, and spindle fibers attach to the centromere of each chromosome (46 chromosomes). Sister chromatids are still considered one chromosome.
• Anaphase: Spindle fibers retract, separating sister chromatids to opposite poles, now considered individual chromosomes (92 chromosomes).
• Telophase: Chromosomes reach opposite poles, loosen back into chromatin, spindle fibers break down, nuclear membrane reforms around each set of chromosomes, and nucleolus forms within each nucleus (92 chromosomes).
• Cytokinesis: Specialized proteins constrict the middle of the cell, the cell membrane pinches inward, dividing the cytoplasm into two separate cells.
• Result: Two cells, each with 46 chromosomes (2n = 46).

Meiosis and Gamete Production

• Meiosis is the process of producing gametes (sex cells) with a haploid number of chromosomes.
• Meiosis occurs in the gonads: testes in males (spermatogenesis) and ovaries in females (oogenesis).
• The goals of meiosis: genetic reduction and genetic recombination.
• Genetic reduction reduces chromosome number by half (diploid → haploid).
• Genetic recombination increases genetic variation.
• Meiosis proceeds through two cycles: Meiosis I and Meiosis II (PMAT I & II).

Stages of Meiosis I

• Prophase I: Synapsis (alignment of homologous chromosomes), chromatin condenses, nuclear membrane breaks down, centrosomes move to poles, spindle fibers form, and crossing over occurs.
• Metaphase I: Homologous chromosomes line up at the equator, spindle fibers attach to centromeres, and independent assortment takes place.
• Anaphase I: Homologous chromosomes separate while sister chromatids stay attached, moving to the poles as spindle fibers shorten.
• Telophase I: Spindle fibers disappear, and the nuclear membrane/nucleolus reappear.
• Cytokinesis: The cytoplasm divides, creating two haploid cells, each with half the chromosome number (23), and chromosomes are still in the form of sister chromatids.
• Meiosis I produces 2 daughter cells with 23 double-stranded chromosomes.

Stages of Meiosis II

• Prophase II: Spindle fibers attach to the centrosomes, and the nuclear membrane disappears.
• Metaphase II: Chromosomes align single file.
• Anaphase II: Spindle fibers pull the sister chromatids to opposite sides.
• Telophase II: Four haploid gametes form (n=23), spindle fibers disappear, and the nuclear membrane reforms.
• Cytokinesis: The cytoplasm divides, creating 4 haploid gametes, each with 23 chromosomes and single chromatids. Each gamete is genetically unique.