Chromosomal Mutations and DNA Recombination
Chromosomal Mutations and DNA Recombination
Prokaryotic Chromosome
Prokaryotic chromosomes are typically found in nucleoid regions, which lack a membrane-bound nucleus.
Eukaryotic Chromosome
Eukaryotic cells contain chromosomes enclosed within a nuclear envelope.
Key components include:
Nucleolus: Site of ribosome production.
Chromatin: Complex of DNA and protein that makes up chromosomes.
Nuclear pores: Openings that allow the transport of molecules in and out of the nucleus.
Various organelles:
Lysosome: Contains enzymes for digestion.
Endoplasmic Reticulum:
Smooth: Lacks ribosomes, involved in lipid synthesis and detoxification.
Rough: Studded with ribosomes; synthesizes proteins.
Free and Bound Ribosomes: Free ribosomes float in cytoplasm, while bound ribosomes are attached to the endoplasmic reticulum.
Golgi Body: Modifies and packages proteins.
Centrioles: Involved in cell division.
Mitochondria: Powerhouse of the cell, site of ATP production.
Structural details:
Nucleosome: Core unit of chromatin, consisting of 147 base pairs of DNA wrapped around a histone octamer, dimensions are 6 nm x 11 nm.
Chromatin fiber: Appears as a 300 nm diameter structure when nucleosomes coil further.
Metaphase Chromosome: Resulting in highly condensed structures during cell division are 700 nm in diameter.
Looped Domains: Helps pack chromatin into 1400 nm structures.
Histone H1: Involved in the higher-order structure of chromatin.
Key measurements of chromatin structure:
DNA: 2 nm in diameter.
Solenoid structure: 30 nm in diameter.
Karyotype
A karyotype analyzes the lengths and centromere placements of chromosomes (excluding sex chromosomes).
Homologous chromosomes: Pairs of chromosomes sharing identical lengths and centromere positions.
Each chromosome type exhibits specific features, with identical homologous partner.
Structure and Conditions of Eukaryotic Chromosome
A chromosome consists of one chromatid or two sister chromatids.
Telomeres: Stable ends of chromosomes that protect genetic data.
Centromere: Constricted region of chromosomes where kinetochores form, anchoring spindle microtubules.
Chromosomes are categorized into four major types based on centromere position:
Metacentric: Centromere in the middle, resulting in p (short) and q (long) arms of equal length.
Submetacentric: Centromere located between the middle and end, p arm shorter than q arm.
Acrocentric: Centromere closer to one end, resulting in a notably shorter p arm.
Telocentric: Centromere located at the end of the chromosome.
Cell Cycle and Chromosome Definition
S Phase: DNA synthesis during which chromosomes are duplicated.
G1 Phase: Initial growth phase, the cell grows and prepares for DNA replication.
G2 Phase: Prepares again for mitosis, the G2/M checkpoint ensures readiness for division.
M Phase: Includes mitosis and cytokinesis, culminating in cell division.
The phases include:
Interphase
G1
S
G2
Mitosis
Prophase
Prometaphase
Metaphase
Anaphase
Telophase
Chromosome Changes and DNA
Changes in chromosome configuration across various cell phases.
Track number of chromosomes and DNA molecules per cell:
G1: 4 chromosomes, 4 DNA molecules
S: 4 chromosomes, 8 DNA molecules
G2: 4 chromosomes, 8 DNA molecules
Prophase: 4 chromosomes, 8 DNA molecules
Metaphase: 4 chromosomes, 8 DNA molecules
Anaphase: 4 chromosomes, 8 DNA molecules
Telophase and Cytokinesis: 4 chromosomes, 4 DNA molecules
Types of Chromosome Mutations
Duplication:
A segment of the chromosome is duplicated, e.g., A B C D E F G (original) -> A B C D D E F G (mutated).
Deletion:
A segment of the chromosome is removed, e.g., A B C D E F G -> A B C F G.
Inversion:
A segment of the chromosome is flipped 180 degrees, leading to altered gene order.
Translocation:
A segment of chromosome moves to a non-homologous chromosome or different region within the same chromosome.
The four types of mutations are duplication, deletion, inversion, and translocation.
Table of Mutation Types
Various types of chromosome mutations including:
Chromosome rearrangement: Structural change in chromosome.
Chromosome deletion: Segment removed.
Inversion: Segment inverted.
Paracentric: Does not include centromere.
Pericentric: Includes centromere.
Translocation: Movement of segments between non-homologous chromosomes or internal regions.
Reciprocal: Exchange between nonhomologous chromosomes.
Nonreciprocal: Movement without reciprocal exchange.
Aneuploidy: Change in the number of individual chromosomes.
Nullisomy: Loss of both homologous chromosomes.
Monosomy: Loss of one homologous chromosome.
Trisomy: Gain of one homologous chromosome.
Tetrasomy: Gain of two homologous chromosomes.
Polyploidy: Addition of entire chromosome sets
Autopolyploidy: Derived from the same species.
Allopolyploidy: Derived from two or more species.
Consequences of Inversion
Inversion results in an unusual structure:
Forms an inversion loop during prophase I of meiosis, allowing homologous sequences to align.
Gametes may produce nonviable recombinant forms lacking certain genes.
Result may prevent offspring viability due to genetic imbalance or loss.
Consequences of Translocation
Affect gametes produced during meiosis, leading to nonviable offspring due to the presence of duplicated or missing genes.
Robertsonian translocation: Involves the exchange of a short arm of one acrocentric chromosome with the long arm of another, producing a large metacentric chromosome and a fragment that often is lost.
Gene Dosage
The interaction of multiple genes may be required for proper development.
Changes in chromosomal structure can modify gene expression and lead to developmental issues.
Gene Duplications and Interactions:
Extra copies of genes change the relative amounts of gene products, potentially leading to abnormal development if one gene product increases disproportionately compared to others.
Final Notes
It is crucial to understand the implications of chromosomal mutations as they can lead to various genetic disorders or abnormal phenotypes depending on the nature of the mutation and the genes involved.