Genetics: Chromosomal Inheritance, DNA Replication, and Gene Expression

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Chapter 12: Chromosomal Basis of Inheritance

Sex Chromosomes vs. Autosomes

Sex chromosomes determine the biological sex of an organism, while autosomes are all other chromosomes not involved in sex determination.

Mammalian Sex Chromosomes: Females have two X chromosomes (XX), males have one X and one Y chromosome (XY).
Fruit Fly (Drosophila) Sex Chromosomes: Similar to mammals; females are XX, males are XY.
Autosomes: Chromosomes that are not sex chromosomes; humans have 22 pairs of autosomes.

Example: In humans, color blindness is an X-linked recessive trait, more common in males due to their single X chromosome.

X-Linked and Y-Linked Traits

X-Linked Traits: Genes located on the X chromosome. Can be dominant or recessive.
Y-Linked Traits: Genes located on the Y chromosome; only present and expressed in males.
Inheritance Patterns:
- X-linked recessive traits are more common in males (e.g., hemophilia).
- Y-linked traits are passed directly from father to son.

Barr Bodies

A Barr body is an inactivated X chromosome in female mammals, ensuring dosage compensation between males and females.

Occurs via random inactivation of one X chromosome in each cell during early embryonic development.
Visible as a dense spot in the nucleus of female cells.

Gene Linkage

Linked genes are located on the same chromosome and tend to be inherited together.

Cause: Physical proximity of genes on the same chromosome.
Effect: Reduces independent assortment; parental combinations are more frequent than recombinant types.
Mechanism to Combat Linkage: Crossing over during meiosis exchanges segments between homologous chromosomes, increasing genetic diversity.

Chromosomal Maps

Chromosomal (genetic) maps show the relative positions of genes on a chromosome, based on recombination frequencies.

1 map unit (centimorgan, cM) = 1% recombination frequency.
Greater distance between genes increases the likelihood of crossing over.

Nondisjunction

Nondisjunction is the failure of homologous chromosomes or sister chromatids to separate properly during meiosis or mitosis.

Consequences: Results in gametes with abnormal chromosome numbers (e.g., trisomy 21 causes Down syndrome).

Inheritance of Non-Nuclear Traits

Some genes are located outside the nucleus, in organelles such as mitochondria and chloroplasts.

Mitochondrial Inheritance: Mitochondria are inherited maternally; mutations can cause diseases affecting energy production.
Chloroplast Inheritance: In plants, chloroplast genes are also usually inherited from the mother.
Prokaryotes: Inheritance is typically asexual, via binary fission; horizontal gene transfer can occur.

Chapter 13: The Molecular Basis of Inheritance

Viruses

Viruses are non-cellular infectious agents composed of genetic material (DNA or RNA) enclosed in a protein coat.

They infect host cells and use the host's machinery to replicate.
Can cause diseases in all forms of life.

DNA Replication Mechanism

Base Pairing: DNA nucleotides pair specifically: Adenine (A) with Thymine (T), Guanine (G) with Cytosine (C).
Chargaff’s Rules: In any DNA, %A = %T and %G = %C.
Antiparallel Strands: DNA strands run in opposite directions: one 5' to 3', the other 3' to 5'.
Replication Forks and Bubbles: Replication begins at origins, forming bubbles; forks are the active sites of DNA synthesis.

Key Proteins in DNA Replication

Helicase: Unwinds the DNA double helix.
Single-Strand Binding Proteins: Stabilize unwound DNA.
Primase: Synthesizes RNA primers.
DNA Polymerases:
- DNA Polymerase I: Removes RNA primers and fills in gaps.
- DNA Polymerase II: Involved in DNA repair.
- DNA Polymerase III: Main enzyme for DNA synthesis.
Ligase: Joins Okazaki fragments on the lagging strand.
Topoisomerase: Relieves supercoiling ahead of the fork.

DNA Replication Equation

Telomeres

Telomeres are repetitive DNA sequences at chromosome ends, protecting them from degradation.
Shorten with each cell division; telomerase enzyme can extend telomeres in some cells.
Telomere shortening is associated with aging and cell senescence.

Chapter 14: Gene Expression: From Gene to Protein

Transcription and Translation

Gene expression involves two main processes: transcription (DNA to mRNA) and translation (mRNA to protein).

Transcription: Occurs in the nucleus; RNA polymerase synthesizes mRNA from DNA template.
Translation: Occurs in the cytoplasm at ribosomes; tRNA brings amino acids to build proteins according to mRNA codons.
Genetic Code: Redundant (multiple codons for one amino acid) but specific (each codon codes for only one amino acid).

Key Enzymes and Locations

RNA Polymerase: Synthesizes RNA in the nucleus.
Ribosome: Site of protein synthesis in the cytoplasm.
Aminoacyl-tRNA Synthetase: Attaches amino acids to tRNA.

Practice: DNA, mRNA, tRNA, and Amino Acids

Transcription: DNA template is used to make mRNA (A pairs with U in RNA).
Translation: mRNA codons are read by tRNA anticodons, which bring the correct amino acids.
Use the genetic code table to translate mRNA codons into amino acids.

Types of Mutations

Nucleotide Substitution Mutations:
- Silent: No change in amino acid sequence.
- Missense: Changes one amino acid.
- Nonsense: Creates a premature stop codon.
Chromosomal Rearrangements:
- Deletions: Loss of a chromosome segment.
- Insertions: Addition of extra DNA segments.
- Inversions: Reversal of a chromosome segment.
- Translocations: Segment moves to a non-homologous chromosome.
- Polyploidy: Extra sets of chromosomes.

Table: Types of Point Mutations

Mutation Type	Effect on Protein	Example
Silent	No change in amino acid	GAA (Glu) to GAG (Glu)
Missense	One amino acid changed	GAA (Glu) to GUA (Val)
Nonsense	Premature stop codon	UAC (Tyr) to UAA (Stop)

Additional info: Understanding these concepts is essential for solving genetics problems and for appreciating the molecular mechanisms underlying inheritance and gene expression.