Comprehensive Notes on Genetics and Molecular Biology

Pairing of C, G, T, A in DNA: Cytosine (C) pairs with Guanine (G), and Thymine (T) pairs with Adenine (A) in DNA. This specific pairing is crucial for DNA replication and maintaining the integrity of the genetic code.
Genetic code: The set of rules by which information encoded in genetic material (DNA or RNA) is translated into proteins by living cells.
Prokaryotic chromosomes: Typically a single, circular chromosome located in the nucleoid region. They lack a membrane-bound nucleus.
DNA strands are antiparallel: DNA consists of two strands running in opposite directions. One strand runs 5' to 3', while the other runs 3' to 5'. This arrangement is essential for replication and transcription.
DNA pol III: The primary enzyme responsible for DNA replication in prokaryotes. It adds nucleotides to the 3' end of a growing DNA strand.
Telomerase: An enzyme that adds repetitive nucleotide sequences (telomeres) to the ends of eukaryotic chromosomes, preventing shortening during replication.

Genome: The entire set of genetic material (DNA) in an organism, including genes and non-coding sequences.
Haploid vs Diploid:
- Haploid (n): Having a single set of chromosomes (e.g., gametes - sperm and egg cells).
- Diploid (2n): Having two sets of chromosomes, one inherited from each parent (e.g., somatic cells).
Homologous chromosomes: Chromosome pairs (one from each parent) that are similar in length, gene position, and centromere location. They carry genes for the same traits but may have different alleles.
Karyotype: An organized visual representation of the chromosomes of a cell, arranged by size and structure. Used to identify chromosomal abnormalities.
Genetic content of human gametes: Human gametes (sperm and egg) are haploid, containing 23 chromosomes (n = 23).
Independent assortment of chromosomes: During meiosis, homologous chromosome pairs align randomly at the metaphase plate, leading to different combinations of chromosomes in each gamete. This is described by the formula $2^n$ , where n is the number of chromosome pairs. For humans, $2^{23}$ represents over 8 million possible combinations.

Monohybrid cross and dihybrid cross:
- Monohybrid cross: A cross between individuals that involves one pair of contrasting traits.
- Dihybrid cross: A cross between individuals that involves two pairs of contrasting traits.
Dominant and recessive traits:
- Dominant trait: A trait that is expressed in the phenotype when one or both alleles are present in the genotype.
- Recessive trait: A trait that is only expressed in the phenotype when two copies of the recessive allele are present in the genotype.
Punnett square: A diagram used to predict the genotypes and phenotypes of offspring in a genetic cross. It organizes all possible combinations of alleles from the parents.

Genetic code. Codon:
- Genetic Code: The set of rules by which information encoded in genetic material (DNA or RNA) is translated into proteins by living cells.
- Codon: A sequence of three nucleotides (triplet) in mRNA that specifies a particular amino acid or a stop signal during protein synthesis. There are 64 possible codons.
Transcription: The process by which RNA is synthesized from a DNA template. mRNA carries the genetic information from the DNA in the nucleus to the ribosomes in the cytoplasm.
Flow of information in a cell: DNA → RNA → Protein. This is the central dogma of molecular biology, describing how genetic information is transferred and used.
Prokaryotes vs eukaryotes. Transcription and translation compared:
- Prokaryotes: Transcription and translation occur simultaneously in the cytoplasm.
- Eukaryotes: Transcription occurs in the nucleus, and translation occurs in the cytoplasm. RNA processing (e.g., splicing, capping, tailing) occurs in eukaryotes but not prokaryotes.

Chromosome theory of inheritance: Genes are located on chromosomes, and the behavior of chromosomes during meiosis accounts for inheritance patterns.
X-linked traits (a few examples): Traits determined by genes located on the X chromosome. Examples include:
- Hemophilia
- Color blindness
Z-linked traits in birds: In birds, sex is determined by the Z and W chromosomes. Z-linked traits are determined by genes on the Z chromosome.
Translocation: A mutation in which a segment of a chromosome is transferred to another chromosome.
Gene linkage: Genes located close together on the same chromosome tend to be inherited together.
Testcross: A cross between an individual with an unknown genotype and a homozygous recessive individual. Used to determine the genotype of the unknown individual.
Recombination frequency and linkage maps:
- Recombination frequency: The percentage of offspring that show recombination (crossing over) between two linked genes. It is proportional to the distance between the genes on the chromosome.
- Linkage maps: A map of the relative locations of genes on a chromosome, based on recombination frequencies.
Genetic map unit: A unit of measurement for the distance between genes on a chromosome; one map unit (centimorgan, cM) is equal to a 1% recombination frequency.

CRISPR-Cas9: A gene editing technology that allows scientists to precisely edit DNA sequences in living organisms. Cas9 is an enzyme that cuts DNA at a specific location guided by a guide RNA.
Point mutations: A mutation affecting only one or very few nucleotides in a gene sequence, including:
- Substitutions (transition, transversion)
- Insertions
- Deletions
Frameshift mutation: A mutation caused by insertions or deletions of nucleotides in a DNA sequence that are not multiples of three. This shifts the reading frame, leading to a completely different amino acid sequence from the point of mutation onward and usually results in a nonfunctional protein.