patho 4

Genetic Control of Cell Function and Inheritance

Function of DNA in Controlling Cell Function

DNA is the fundamental molecule that stores genetic information, governing cell structure and function. Although every cell in an organism contains the same DNA, only a subset of genes is expressed in each cell type, depending on its specialized role.

• Genetic Instructions: DNA contains the instructions for cellular activities and differentiation.

• Selective Expression: Each cell type expresses only the genes necessary for its function.

The DNA Double Helix

The structure of DNA is a double helix composed of nucleotide base pairs, which form the genetic code. The four bases are adenine (A), thymine (T), cytosine (C), and guanine (G).

• Base Pairing: Adenine pairs with Thymine; Cytosine pairs with Guanine.

• Transcription: The process by which a complementary messenger RNA (mRNA) copy is made from DNA, carrying genetic information to ribosomes for protein synthesis.

DNA Mutations

Mutations are permanent changes in the DNA sequence. They can occur spontaneously or be induced by environmental factors such as chemicals or radiation.

• Types of Mutations: Substitution, addition, loss, or rearrangement of base pairs.

• Somatic vs. Germ Cell Mutations: Somatic mutations affect non-reproductive cells; germ cell mutations affect reproductive cells and can be inherited.

• Inherited Disorders: Germ cell mutations may lead to inherited genetic disorders.

DNA Repair

Cells possess mechanisms to repair DNA damage and maintain genetic integrity. Key enzymes include endonucleases, DNA polymerase, and DNA ligase.

• Endonucleases: Recognize and remove distorted DNA regions.

• DNA Polymerase: Fills gaps using the complementary strand as a template.

• DNA Ligase: Joins DNA fragments during replication and repair.

• Loss of Repair: Accumulation of errors can lead to cancer via protooncogene activation or tumor suppressor gene inactivation.

Protein Synthesis

Protein synthesis is the process by which cells build proteins based on genetic instructions. It involves three main types of RNA:

• Messenger RNA (mRNA): Template for protein synthesis, produced by transcription.

• Ribosomal RNA (rRNA): Structural component of ribosomes, the site of protein synthesis.

• Transfer RNA (tRNA): Delivers amino acids to ribosomes during translation.

Genes Controlling Protein Synthesis

Genes are classified based on their roles in protein synthesis:

• Structural Genes: Code for the amino acid sequence of proteins with functional roles (e.g., enzymes, structural proteins).

• Regulator Genes: Produce proteins that control the expression of other genes, acting as switches or modulators.

Gene Expression

Gene expression refers to the degree to which a gene or group of genes is active. It is regulated by transcription factors and posttranslational modifications.

• Transcription Factors: Proteins that bind DNA and regulate transcription, either activating or repressing gene expression.

• Posttranslational Processing: Chemical modifications and folding of proteins after translation, essential for proper function.

Chromosome Composition

Human cells contain 23 pairs of chromosomes, classified as autosomes and sex chromosomes.

• Autosomes: 22 pairs, identical in males and females.

• Sex Chromosomes: 1 pair; males have XY, females have XX.

Cytogenetics for Karyotyping

Cytogenetics studies chromosome structure and number. Karyotyping is a technique to visualize chromosomes, aiding in the detection of chromosomal abnormalities.

• Karyotyping: Arranges chromosomes by size and shape for analysis.

• Colchicine Use: Arrests cells in metaphase for easier observation.

• Clinical Application: Detects genetic disorders such as Down syndrome.

Cell Division

Cell division occurs via mitosis or meiosis, ensuring genetic continuity and diversity.

• Mitosis: Occurs in somatic cells, producing two identical cells with 46 chromosomes.

• Meiosis: Occurs in germ cells, producing gametes with 23 chromosomes each.

Patterns of Inheritance

Inheritance patterns describe how traits are passed from parents to offspring. These include polygenic, multifactorial, and uniparental disomy.

• Polygenic Inheritance: Traits influenced by multiple genes, resulting in a range of phenotypes.

• Multifactorial Inheritance: Traits influenced by genes and environmental factors.

• Uniparental Disomy: Both chromosomes of a pair inherited from one parent, causing disorders such as Prader-Willi syndrome.

Discoveries of Gregor Mendel (1865)

Gregor Mendel established the foundational laws of inheritance through experiments with pea plants.

• Dominant and Recessive Factors: Traits are controlled by dominant and recessive alleles.

• Single-Gene Inheritance: Observable traits are inherited from parents.

• Punnett Square: Tool for predicting genetic outcomes.

Mendel Laws

Mendel's laws describe the behavior of alleles during gamete formation and fertilization.

• First Law (Law of Segregation): Each parent contributes one allele for each gene; alleles separate during meiosis.

• Second Law (Law of Independent Assortment): Alleles of different genes segregate independently and recombine randomly in offspring.

Allele: One of two or more versions of a gene; humans inherit two alleles for each gene, one from each parent.

Genetic Traits

Traits are classified based on allele combinations and expression.

• Recessive Trait: Expressed only in homozygous individuals (two identical alleles).

• Dominant Trait: Expressed in both homozygous and heterozygous individuals.

• Carrier: Heterozygous for a recessive trait; does not express the trait but can pass it on.

Arrangement of Alleles

Alleles can be arranged as homozygotes or heterozygotes at a gene locus.

• Homozygotes: Two identical alleles (AA or aa).

• Heterozygotes: Two different alleles (Aa).

Transmission of Genetic Information

Genetic information is transmitted from parents to offspring via alleles located at specific loci on chromosomes.

• Alleles: Alternate forms of a gene at a locus; one from each parent.

• Locus: The position a gene occupies on a chromosome.

• Example: Eye color is determined by different alleles at the eye color locus.

Genetic Information in the Cells

Genotype and phenotype are key concepts in genetics, describing genetic makeup and observable traits, respectively.

• Genotype: The complete set of genes in an individual, determining potential traits.

• Phenotype: The observable characteristics resulting from genotype and environmental influences (e.g., height, hair color).