Chromosomal Basis of Inheritance, Molecular Basis of Inheritance, and Gene Expression: Study Notes

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Chapter 15: The Chromosomal Basis of Inheritance

Linked Genes

Linked genes are genes located near each other on the same chromosome and tend to be inherited together. This concept is fundamental to understanding genetic linkage and inheritance patterns.

Definition: Linked genes are physically close on a chromosome and do not assort independently.
Inheritance: Linked genes are inherited together unless crossing over occurs.
Example: Fruit fly genes for body color and wing type are often inherited together.

Crossing Over and Recombination

Crossing over during meiosis can break the physical link between genes on the same chromosome, resulting in genetic recombination and new allele combinations.

Recombination Frequency: The percentage of recombinant offspring is used to estimate the distance between genes.
Genetic Maps: Maps based on recombination frequencies show the relative positions of genes.
Example: If two genes have a recombination frequency of 50%, they are likely on different chromosomes.

Polyploidy and Extrachromosomal DNA

Polyploidy refers to organisms with more than two complete sets of chromosomes. Extrachromosomal DNA includes DNA found outside the nucleus, such as in mitochondria and chloroplasts.

Polyploidy: Common in plants, can result in triploidy (3n) or tetraploidy (4n).
Extrachromosomal DNA: Mitochondria and chloroplasts contain their own DNA, inherited maternally.
Example: Bread wheat is hexaploid (6n).

Chromosomal Rearrangements

Chromosomal rearrangements include changes in chromosome structure such as deletions, duplications, inversions, and translocations. These can have significant genetic consequences.

Deletion: Loss of a chromosome segment.
Duplication: Repetition of a chromosome segment.
Inversion: Reversal of a chromosome segment.
Translocation: Movement of a segment to a nonhomologous chromosome.
Example: Cri du chat syndrome is caused by a deletion on chromosome 5.

Chapter 16: The Molecular Basis of Inheritance

DNA Structure

DNA is composed of two antiparallel strands forming a double helix, with a sugar-phosphate backbone and nitrogenous bases paired by hydrogen bonds.

Base Pairing: Adenine (A) pairs with Thymine (T); Guanine (G) pairs with Cytosine (C).
Antiparallel Strands: The two strands run in opposite directions (5' to 3' and 3' to 5').
Example: The sequence 5'-ATGC-3' pairs with 3'-TACG-5'.

Key Enzymes in DNA Replication

DNA replication involves several enzymes that ensure accurate copying of genetic material.

DNA Polymerase: Synthesizes new DNA by adding nucleotides to the 3' end.
Helicase: Unwinds the DNA double helix.
Primase: Synthesizes RNA primers for DNA polymerase.
Ligase: Joins Okazaki fragments on the lagging strand.
Example: DNA polymerase III is the main enzyme for prokaryotic DNA replication.

DNA Replication Process

DNA replication is semi-conservative, meaning each new DNA molecule consists of one old strand and one new strand.

Origin of Replication: Replication begins at specific sites called origins.
Leading and Lagging Strands: The leading strand is synthesized continuously; the lagging strand is synthesized in fragments (Okazaki fragments).
Equation:

Chromatin Structure

DNA is packaged into chromatin, which consists of DNA wrapped around histone proteins to form nucleosomes.

Nucleosome: The basic unit of chromatin, consisting of DNA wrapped around histone proteins.
Heterochromatin: Densely packed chromatin, generally transcriptionally inactive.
Example: Barr bodies are examples of heterochromatin.

Chapter 17: Gene Expression: From Gene to Protein

The Central Dogma

The central dogma of molecular biology describes the flow of genetic information from DNA to RNA to protein.

Transcription: Synthesis of RNA from DNA template.
Translation: Synthesis of polypeptide (protein) from mRNA template.
Equation:

Transcription

Transcription is the process by which RNA is synthesized from a DNA template.

Initiation: RNA polymerase binds to the promoter region.
Elongation: RNA polymerase adds nucleotides to the growing RNA strand.
Termination: Transcription ends when RNA polymerase reaches a terminator sequence.
Example: The TATA box is a common promoter element in eukaryotes.

RNA Processing

In eukaryotes, the primary RNA transcript (pre-mRNA) undergoes processing before becoming mature mRNA.

5' Cap: Added to the 5' end for stability and ribosome recognition.
Poly-A Tail: Added to the 3' end for stability.
Splicing: Removal of introns and joining of exons.
Example: Alternative splicing allows for multiple proteins from a single gene.

Translation

Translation is the process by which ribosomes synthesize proteins using mRNA as a template.

mRNA: Carries genetic information from DNA to ribosome.
tRNA: Brings amino acids to the ribosome.
Ribosome: Site of protein synthesis.
Codon: Three-nucleotide sequence on mRNA that specifies an amino acid.
Example: AUG is the start codon for translation.

Mutations

Mutations are changes in the DNA sequence that can affect gene function and protein structure.

Point Mutation: Change in a single nucleotide.
Frameshift Mutation: Insertion or deletion of nucleotides not in multiples of three, altering the reading frame.
Silent Mutation: Does not change the amino acid sequence.
Missense Mutation: Changes one amino acid in the protein.
Nonsense Mutation: Changes a codon to a stop codon, truncating the protein.
Example: Sickle cell anemia is caused by a missense mutation in the hemoglobin gene.

Type of Mutation	Effect on Protein	Example
Silent	No change in amino acid sequence	GAA to GAG (both code for Glu)
Missense	One amino acid changed	Sickle cell anemia
Nonsense	Premature stop codon	Cystic fibrosis (some cases)
Frameshift	Altered reading frame	Tay-Sachs disease

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