Ongoing 3-1: Genes

Genes

  • Topic: Genetics

Genetic Vocabulary

  • Gene: A heritable factor of DNA that influences a specific characteristic.

    • Heritable means you get it from parents and give it to your offspring.

    • The same gene is found in a specific location on a chromosome for every human (all 46 chromosomes).

    • Humans differ by one or a few bases in their genes, which partially determine characteristics like height, hair color, and skin color.

  • Locus: Position of a gene on a chromosome.

    • Plural is loci.

Alleles

  • Allele: A specific/alternate form of a gene.

    • Two alleles are required to control a trait.

    • Differences arise from one or a few bases.

    • New alleles can arise by mutation, leading to genetic variation (e.g., attached or free earlobes).

Human Genome

  • Genome: All the genetic information of an organism (includes all genes + non-coding DNA).

    • Comprises 46 chromosomes, 21,000 genes, and 3.5 billion base pairs.

    • Approximately 1.5% of DNA codes directly for proteins.

  • Human Genome Project:

    • Mapped the number, location, sequence, and size of genes.

    • Completed from 1984 to 2003.

Gene Mutation

  • Gene mutation: A permanent change in the nucleotide sequence of a section of DNA.

    • New alleles are formed through mutations.

    • Mutations can be beneficial, detrimental, or neutral; they may be inherited or acquired.

Types of Gene Mutation

Substitution Mutations

  • Involves changing one base for another.

    • Affects only a single codon (point mutation).

    • Results in a Single Nucleotide Polymorphism (SNP): genomic variant at a single base position.

    • Effects can be neutral or negative (e.g., changing amino acids).

Effects of Substitution

  • Degeneracy of the genetic code: Redundancy in the code, with multiple codons coding for the same amino acid.

    • Substitution mutations may not change the resulting amino acid.

  • Silent mutation: No change in the resulting amino acid.

  • Missense mutations: Change a single amino acid in the polypeptide chain.

  • Nonsense mutations: Create a premature STOP codon, shortening the polypeptide.

Example of Negative Effect: Sickle Cell Mutation

  • Sickle cell anemia: Caused by a missense mutation in hemoglobin.

    • Alters hemoglobin structure, making it fibrous and insoluble.

    • Results in misshaped red blood cells that cannot carry O2 effectively, leading to blood clotting and anemia.

  • Normal hemoglobin's 6th amino acid is glutamic acid (GAG).

    • Mutation changes this to valine (GUG).

Mutation Types: Frameshift Mutations

  • Frameshift mutation: Involves insertion or deletion of bases, shifting the reading frame of codons in mRNA.

    • Result: A completely different chain of amino acids, often nonfunctional proteins.

  • Insertion mutation example:

    • Original: GGTCTCCTCACGCCA

    • Mutated: GGTGCTCCTCACGCCA

    • mRNA impact: CCACGAGGAGUGCGGU

  • Deletion mutation example:

    • Original: GGTCTCCTCACGCCA

    • Mutated: GGTC_CCTCACGCCA

    • mRNA impact: CCAG_GGAGUGCGGU

Causes of Mutation

  1. Errors in DNA replication or repair.

  2. Mutagen: Any agent that causes mutation.

  • Increases mutation chances significantly.

  • Types of mutagens:

    • Physical: Radiation (UV, X-rays).

    • Chemical: Heavy metals, nitrosamines (tobacco).

    • Biological: Viruses (e.g., HPV), bacteria.

Consequences of Mutation

Germline and Somatic Cells

  • Germline mutations: Occur in gametes (egg or sperm); can be inherited.

  • Somatic mutations: Occur in somatic cells; generally non-inheritable.

    • Many cancers arise from somatic mutations.

Germline vs. Somatic Variants

  • Germline variants: Present in gametes, passed to offspring.

  • Somatic variants: Present in body cells, not passed to offspring.

Random Nature of Mutations

  • Mutations occur randomly and can be beneficial, detrimental, or neutral.

  • Serve as the original source of genetic variation essential for evolution by natural selection.

Learning Objectives

  • Explain what a gene and allele are with examples.

  • Describe how new alleles arise.

  • Distinguish between substitution, insertion, and deletion mutations.

  • Explain the consequences of base substitutions, including SNPs and degeneracy of the genetic code.

  • Analyze the effects of insertion and deletion resulting in frameshift mutations.

  • Discuss causes of mutations (mutagens and replication errors).

  • Differentiate between somatic and germline variants.

  • Understand mutations as random occurrences essential for evolutionary processes.

Note on Genetic Variation: Genetic variation refers to the differences in DNA sequences among individuals within a population. These variations are crucial for a population's adaptation and evolution. Genetic variation can arise from mutations, gene flow between populations, and sexual reproduction leading to new allele combinations. Understanding genetic variation helps in studying inheritance patterns, population dynamics, and the genetic basis of diseases.