Meiosis: Formation of Gametes

1. Meiosis

Traits are expressed through genes.
Genes are organized into chromosomes, which appear in pairs.
- Most cells in an organism have these pairs, making them special.
- Chromosomes in pairs are called diploid (two of each kind of chromosome).
- One chromosome is from the mother and the other from the father.
Gametes contain one chromosome from each pair.
- These cells have half the number of chromosomes, called haploid (one of each kind of chromosome).

Formation of Gametes: Sperm and egg.
- Gametes combine to form a zygote, which is sexual reproduction.
Phases of Meiosis: Follow the same pattern as mitosis.
- Prophase 1: Similar to mitosis.
  - Nuclear envelope dissolves, chromatin turns into chromatids, centrioles migrate, and spindles form.
  - Homologous chromosomes form (only during Prophase 1).
  - Crossing over occurs (only during Prophase 1).
- Metaphase 1: Homologous chromosomes line up on the metaphase plate.
- Anaphase 1: Homologous chromosomes are moved to opposite ends.
- Telophase 1 & Cytokinesis: Formation of 2 diploid cells.
- Prophase 2: Formation of sister chromatids.
- Metaphase 2: Sister chromatids line up on the metaphase plate.
- Anaphase 2: Sister chromatids are moved to opposite sides of the cells.
- Telophase 2 & Cytokinesis:
  - Opposite of Prophase 1.
  - Nuclear envelope returns, stays as chromatids, and spindles go away.
  - Formation of 4 haploid cells.
Genetic Recombination/Variation
- Number of possible genotypes from meiosis.
- Example: Pea Plants with 7 pairs of chromosomes can line up during metaphase in 2 different ways resulting in 128 different sperm (2^7), contributing to genetic variation.

Nucleotides: Subunit of DNA.
- DNA: Contains sugar (ribose), phosphate group, and nitrogenous bases (Adenine, Thymine, Cytosine, Guanine).
- Categories:
  - Purines: 2 ring structure (Adenine and Guanine).
  - Pyrimidines: Single ring structure (Cytosine and Thymine).
- Chargaff’s Rules: The percentage of complementary bases (A=T and C=G) is equal, known as Complementary Base Pairs.
- Phosphate groups bind to place nucleotides in the proper order.
- 1953: Watson & Crick proposed the double helix structure of DNA (notably excluding Franklin).
Chromosome Structure
- Chromosomes consist of sequences of nucleotide bases forming genes.
- Genes have a constant position known as gene loci.
- DNA nucleotide base pairs are wrapped around histones to form chromatin.
- Chromosome has two sets of DNA instructions.
DNA Replication
- Occurs during interphase of mitosis & meiosis, creating an exact copy unless a mutation occurs.
- Process:
  - DNA is "unzipped" by helicase, exposing base pairs.
  - DNA polymerase inserts extra bases, forming two new DNA strands.
- Takes place in the nucleus.

RNA Structure
- Long chain of nucleotides.
- Ribose as sugar and generally single-stranded.
Types of RNA
- Messenger RNA (mRNA): Carries DNA’s message out of the nucleus.
- Ribosomal RNA (rRNA): The workbench for protein synthesis.
- Transfer RNA (tRNA): Transports amino acid to mRNA.
Genes to Proteins
- Genes express through protein formation.
- DNA is transcribed into RNA.
- RNA builds the polypeptide chain.
- Transcription
  - DNA strand is unzipped by helicase and read by RNA polymerase.
  - Proper base pairs join with one side of the DNA strand.
  - As mRNA is built, it allows the DNA strand to close.
  - Transcription can begin and end anywhere on the DNA strand.
  - Multiple mRNA strands can be built simultaneously.
Genetic Code
- mRNA has the "code" for protein within base pairs arrangement.
- Every three nucleotide bases form a codon, each providing specific protein formation instructions.
- 64 possible codons result in 20 different amino acids.
Translation
- Converts mRNA into a polypeptide chain.
- Occurs on the ribosomes or the rough endoplasmic reticulum (rER).
- Process controlled by ribosomes.