Genetics Study Guide: DNA Structure, Chromosome Organization, Genome Transmission, and Mutations

DNA Structure and Organization

Structure of DNA

The structure of DNA is fundamental to its role in storing and transmitting genetic information. DNA is a double helix composed of two antiparallel strands of nucleotides.

• Nucleotide Components: Each nucleotide consists of a deoxyribose sugar, a phosphate group, and a nitrogenous base (adenine, thymine, cytosine, or guanine).

• Base Pairing: Adenine pairs with thymine, and cytosine pairs with guanine via hydrogen bonds.

• Double Helix: The two strands wind around each other, forming a right-handed helix.

• Antiparallel Orientation: One strand runs 5' to 3', the other 3' to 5'.

Example: The sequence 5'-ATCG-3' on one strand pairs with 3'-TAGC-5' on the complementary strand.

DNA Packaging: Histones, Nucleosomes, and Chromatin

DNA is highly compacted to fit within the nucleus, involving several levels of organization.

• Histones: Small, positively charged proteins that DNA wraps around.

• Nucleosomes: The basic unit of chromatin, consisting of ~146 base pairs of DNA wrapped around a histone octamer.

• Chromatin: The complex of DNA and proteins (mainly histones) that forms chromosomes. Chromatin can be euchromatin (less condensed, transcriptionally active) or heterochromatin (highly condensed, transcriptionally inactive).

Example: During interphase, DNA exists as chromatin, allowing access for transcription and replication.

Preservation and Duplication of the Genome

Faithful duplication of DNA is essential for inheritance. Several molecular components and mechanisms ensure accuracy:

• DNA Polymerases: Enzymes that synthesize new DNA strands using existing strands as templates.

• Origin of Replication: Specific sequences where DNA replication begins.

• Proofreading and Repair: DNA polymerases have proofreading activity; additional repair mechanisms correct errors.

Equation:

$\text{DNA}_{\text{parental}} \xrightarrow{\text{Replication}} \text{DNA}_{\text{daughter}}$

Genetic Markers: SNPs and STRs

Genetic markers are DNA sequences with known locations that can be used to identify individuals or species.

• Single-Nucleotide Polymorphism (SNP): A variation at a single base pair in the DNA sequence among individuals.

• Short Tandem Repeat (STR): Short sequences of DNA (2-6 base pairs) repeated in tandem.

• Use as Markers: SNPs and STRs are used in genetic mapping, forensics, and population genetics, even if they do not affect phenotype.

Example: STR analysis is commonly used in DNA fingerprinting.

Ploidy

Ploidy refers to the number of sets of chromosomes in a cell.

• Haploid (n): One set of chromosomes (e.g., gametes).

• Diploid (2n): Two sets of chromosomes (e.g., somatic cells in humans).

• Polyploid: More than two sets of chromosomes.

• Homologues: Chromosomes that are similar in structure and gene content; ploidy determines the number of homologues present.

Example: Humans are diploid (2n = 46 chromosomes).

Transmission of the Genome and Chromosomal Features

Key Chromosomal Features

Understanding chromosome structure is essential for studying inheritance and cell division.

Feature	Description
Centromere	Region where sister chromatids are joined; site of kinetochore formation.
Chromatid	One of two identical halves of a replicated chromosome.
Kinetochore	Protein complex assembled on the centromere; attaches to spindle fibers during cell division.
Telomere	Repetitive DNA sequences at chromosome ends; protect chromosomes from degradation.
Sister Chromatids	Identical copies of a chromosome, connected at the centromere.
Non-sister Chromatids	Chromatids from homologous chromosomes (not identical).

Nondisjunction and Abnormal Gametes

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

• Result: Gametes with abnormal chromosome numbers (e.g., trisomy, monosomy).

• Example: Down syndrome results from trisomy 21 due to nondisjunction.

Equation:

$\text{2n} \rightarrow \text{n} + 1,\ \text{n} - 1$

Epigenetic Changes and Gene Expression

Epigenetic modifications alter gene expression without changing the DNA sequence.

• DNA Methylation: Addition of methyl groups to cytosine bases, often silencing genes.

• Histone Modification: Chemical changes to histone proteins (e.g., acetylation, methylation) that affect chromatin structure and gene accessibility.

Example: X-chromosome inactivation in female mammals is an epigenetic process.

Protein Post-Translational Modifications

Proteins often undergo modifications after translation, affecting their function and localization.

• Phosphorylation: Addition of phosphate groups, often regulating enzyme activity.

• Glycosylation: Addition of carbohydrate groups, affecting protein folding and stability.

Additional info: Other modifications include ubiquitination and acetylation.

Mutations and Chromosomal Rearrangements

Types of Mutations and Their Effects

Mutations are changes in the DNA sequence that can affect gene function, mRNA, and protein products.

• Point Mutations: Single base changes (e.g., missense, nonsense, silent mutations).

• Insertions/Deletions: Addition or loss of nucleotides, potentially causing frameshifts.

• Effects: Can alter protein sequence, function, or expression; some are silent.

Example: Sickle cell anemia is caused by a missense mutation in the beta-globin gene.

Bacterial Regulation of Operons

Operons are clusters of genes under the control of a single promoter, common in prokaryotes.

• Lac Operon: Regulates lactose metabolism in Escherichia coli; induced in the presence of lactose.

• Mutations: Mutations in the operator, promoter, or regulatory genes can lead to constitutive or non-inducible expression.

Example: A mutation in the lac repressor gene can cause the operon to be expressed even without lactose.

Spontaneous Mutations

Spontaneous mutations occur naturally at low rates due to errors in DNA replication or repair.

• Causes: Tautomeric shifts, replication slippage, spontaneous base deamination.

• Low Rate: High-fidelity DNA polymerases and repair mechanisms minimize errors.

Chromosomal Rearrangements

Structural changes in chromosomes can affect gene function and genetic recombination.

Type	Description	Effect
Duplication	Segment of chromosome is copied	Gene dosage increase, possible novel functions
Deletion	Segment of chromosome is lost	Loss of gene function, possible lethality
Inversion	Segment is reversed end to end	May disrupt gene function or recombination
Translocation	Segment moves to a non-homologous chromosome	Gene disruption, altered regulation, possible disease

Mutation Induction by Environmental Factors

Environmental agents can increase mutation rates above spontaneous levels.

• Mutagens: Chemicals (e.g., base analogs, alkylating agents), radiation (e.g., UV, X-rays).

• Mechanisms: Induce base changes, strand breaks, or crosslinking.

Example: UV light causes thymine dimers, leading to replication errors.