_Biology -Entrance Exam ( Universidad de Navarra) unit 5

Unit 5 - The Inheritance: Molecular Genetics

Mendelian Genetics

• Mendel's Contribution: Gregor Mendel, known as the father of genetics, proposed that discrete factors (now known as genes) are inherited from parents to offspring. These principles were derived from his experiments with pea plants, where he studied traits such as flower color and pea shape.

• Genes and DNA: In contemporary biology, genes are understood as segments of DNA residing on chromosomes. Each gene contains the information necessary to produce proteins that dictate hereditary traits and functions in living organisms.

• Diploid Cells: Most complex organisms, including humans, are made up of diploid cells, which possess two complete sets of chromosomes (23 pairs, totaling 46 chromosomes). These cells arise through mitosis and are responsible for growth and tissue repair.

• Haploid Cells: During sexual reproduction, diploid cells undergo meiosis, resulting in haploid cells (gametes), which contain only one set of chromosomes (23 unpaired chromosomes).

• Gametes: There are two types of gametes: egg cells produced by females and sperm cells produced by males. Each gamete contains half of the genetic information needed to form a new individual.

• Reestablishing Diploidy: Fertilization occurs when male and female gametes unite, resulting in a zygote that restores the diploid state, containing one set of chromosomes from each parent.

• Alleles: Alleles are different forms of a gene that can exist for a given trait. For example, for the trait of earlobe shape, there are two allelic forms: attached and free earlobes. The combination of alleles an individual possesses determines their genotype.

Genetics Terminology

• Genome: The genome is the complete set of DNA in an organism, encompassing all genetic information stored in the sequence of nucleotides. In humans, this includes approximately 20,000 genes.

• Genotype vs. Phenotype: The genotype refers to the specific genetic makeup of an organism concerning a trait, while the phenotype encompasses the physical appearance or observable traits resulting from the genotype, such as whether earlobes are attached or free.

• Homozygous vs. Heterozygous:

  • Homozygous: An individual is homozygous for a trait if they have two identical alleles (e.g., AA or aa).

  • Heterozygous: An individual is heterozygous if they possess two different alleles (e.g., Aa) for a trait; in this case, the dominant allele may mask the expression of the recessive allele.

Dominance Relationships

• Dominant and Recessive Alleles:

  • Dominant: An allele that expresses its phenotype even in the presence of a recessive allele (e.g., free earlobes represented by the dominant allele).

  • Recessive: An allele that only manifests its phenotype when homozygous (e.g., attached earlobes depicted by two recessive alleles).

• Karyotype: A karyotype is an organized profile of a person's chromosomes, which can reveal chromosomal abnormalities and is crucial in diagnosing genetic disorders.

• Law of Segregation: This principle states that during meiosis, the two alleles for a trait segregate from one another, ensuring that each gamete carries only one allele for each gene.

• Law of Independent Assortment: This law posits that alleles for different traits segregate independently during gamete formation, which is evident in Mendel’s dihybrid crosses leading to a phenotypic ratio of 9:3:3:1.

Protein Synthesis

• DNA and RNA: DNA serves as the hereditary molecule carrying genetic information necessary for the maintenance and reproduction of organisms.

• RNA: Messenger RNA (mRNA) plays a critical role in protein synthesis by transcribing the DNA code and transferring it to ribosomes for protein synthesis following the Central Dogma of molecular biology (DNA → mRNA → protein).

• Transcription: The process of synthesizing mRNA from a DNA template using RNA polymerase. This step modifies mRNA through splicing, where introns are removed, and exons are joined together.

• Translation: The mechanism through which ribosomes read the mRNA sequence and synthesize proteins by linking the appropriate amino acids in the specified sequence dictated by codons in the mRNA.

Types of RNA

1. Ribosomal RNA (rRNA): This type of RNA is a structural component of ribosomes, which are the sites of protein synthesis.

2. Transfer RNA (tRNA): tRNA molecules transport specific amino acids to ribosomes during protein synthesis, matching the amino acid with the corresponding mRNA codon.

3. Messenger RNA (mRNA): Carries genetic information from the DNA in the nucleus to the ribosomes in the cytoplasm for protein synthesis.

DNA Structure and Replication

• DNA Structure: Composed of two long chains of nucleotides twisted into a double helix. Each nucleotide consists of a nitrogenous base (adenine, guanine, cytosine, thymine), a phosphate group, and a deoxyribose sugar. The specific pairing of nitrogenous bases (A with T and C with G) ensures accurate genetic information transfer.

• DNA Replication: Occurs before cell division (mitosis or meiosis) to produce identical copies of DNA. This process involves the unzipping of the DNA double helix, followed by complementary base pairing and synthesis of new strands through a semi-conservative method, where each new strand retains one original strand.

Gene Regulation

• Gene Control: Gene expression is tightly regulated at multiple levels (during and after transcription), ensuring that genes are turned on or off as needed according to cellular requirements.

• Introns & Exons: Introns (non-coding regions) are removed from the pre-mRNA transcript, while exons (coding sequences) are spliced together to form mature mRNA, which exits the nucleus for translation.

Genetic Mutations

• Types of Mutations: Genetic mutations can be classified into several types, including point mutations (single base changes), insertions, deletions, and duplications. These alterations can affect protein structure and function, leading to diverse phenotypic outcomes.

• Effects of Mutations: The effects of mutations can vary widely, ranging from neutral (having no effect) to harmful (causing diseases). Some mutations may confer advantages, such as increased resistance to antibiotics or adaptability to environmental changes.

Genetic Engineering and Biotechnology

• Genetic Modifications: This process involves the deliberate alteration of an organism's genetic material to achieve desired traits. Techniques can include traditional breeding, genetic modification, and CRISPR-Cas9 gene editing.

• Genetically Modified Organisms (GMOs): Organisms whose genetic material has been altered through biotechnology, commonly used to enhance traits in agriculture (pest resistance, increased yield) or in medical applications (producing insulin or vaccines).

• Methods of Gene Transfer: Gene transfer methods include direct methods such as electroporation, vector-based methods using viral or plasmid vectors, and revolutionary gene editing technologies like CRISPR/Cas9, which allows precise changes to DNA sequences.

Ethical and Environmental Considerations

• Controversies Related to GMOs: The use of GMOs raises significant ecological and ethical discussions, including potential impacts on biodiversity, gene flow to non-GM crops, food safety, and concerns about the long-term effects of genetically altering organisms.

• Potential Risks of GMOs: The risks associated with GMOs encompass issues such as unintended consequences to non-GM organisms, ecological disruption, potential toxicity to humans or wildlife, and implications for sustainable agricultural practices and food sovereignty.