genetics 11

Genetics: Basic Concepts and Definitions

• Genetics is the branch of biology studying genetic materials and how traits pass from one generation to the next through genes. Genetic material carries hereditary information essential for perpetuating life, primarily DNA in most organisms, and RNA in some viruses like HIV and COVID-19.

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• DNA (Deoxyribonucleic Acid) is the hereditary material in humans and other organisms, existing as a double helix with base pairs attached to a sugar-phosphate backbone. It stores all genetic information needed for growth, development, reproduction, and cellular control.

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• RNA (Ribonucleic Acid) is single-stranded, contains ribose sugar, and uses uracil instead of thymine. It plays a critical role in protein synthesis and acts as genetic material in some viruses.

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Structure and Function of DNA

• DNA has a double helix structure composed of two polynucleotide chains twisted into a spiral. Each nucleotide consists of a sugar (deoxyribose), phosphate group, and one of four nitrogenous bases: adenine (A), thymine (T), guanine (G), and cytosine (C).

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• Nitrogenous bases pair specifically: A pairs with T and C pairs with G, forming base pairs held by hydrogen bonds. A and G are purines; C and T are pyrimidines.

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• The sugar-phosphate groups form the DNA backbone, and the two strands run antiparallel (5’ to 3’ and 3’ to 5’ ends).

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• Genes are DNA segments containing instructions for organism traits, located linearly on chromosomes. Each gene occupies a unique position called a locus.

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• Chromosomes are threadlike structures made of DNA and histone proteins, carrying hundreds to thousands of genes. Diploid organisms have chromosomes in pairs, one from each parent (e.g., humans have 46 chromosomes in 23 pairs).

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DNA Replication

• DNA replication copies the DNA during cell division, ensuring genetic information passes to daughter cells. It is a semiconservative process, where each original strand serves as a template for a new complementary strand, producing two DNA molecules each with one old and one new strand.

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• Key enzymes in replication:

• Replication stages:

  1. DNA helix unwinds and strands separate.

  2. Each strand acts as a template; nucleotides are added via complementary base pairing (A-T, C-G). Leading strand synthesizes continuously; lagging strand synthesizes discontinuously.

  3. Two new double helices form, identical to the original.

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Structure and Function of RNA

• RNA is single-stranded, containing ribose sugar and nitrogenous bases adenine (A), guanine (G), cytosine (C), and uracil (U) instead of thymine.

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• Three main types of RNA:

  • mRNA (messenger RNA): Carries genetic code from DNA to ribosomes.

  • tRNA (transfer RNA): Brings amino acids to ribosomes during protein synthesis.

  • rRNA (ribosomal RNA): Structural component of ribosomes aiding translation.

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• RNA functions in protein synthesis and acts as genetic material in some viruses. It translates DNA instructions into proteins.

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Cell Division and the Cell Cycle

• Cell division is vital for organism growth, repair, and reproduction, involving the transfer of genetic material via DNA replication.

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• The cell cycle comprises an ordered series of events: two major phases are interphase (cell growth, DNA replication) and the mitotic phase (cell division).

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• Interphase stages:

  • G1 phase: Cell grows, accumulates materials for DNA replication.

  • S phase: DNA synthesis occurs, producing sister chromatids.

  • G2 phase: Cell prepares for mitosis, replenishing energy and synthesizing proteins.

  • G0 phase: Resting phase where the cell is metabolically active but not dividing.

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Mitosis: Somatic Cell Division

• Mitosis results in two genetically identical daughter cells, essential for growth and tissue repair. It involves nuclear division (karyokinesis) and cytoplasmic division (cytokinesis).

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• Mitosis stages:

  1. Prophase: Chromosomes condense, nuclear envelope breaks down, spindle fibers form, centrosomes migrate to poles.

  2. Metaphase: Chromosomes align at the cell equator (metaphase plate).

  3. Anaphase: Sister chromatids separate and move to opposite poles.

  4. Telophase: Chromosomes decondense, nuclear envelopes re-form, followed by cytokinesis splitting the cytoplasm.

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Meiosis: Production of Gametes

• Meiosis produces haploid sex cells (gametes) with half the chromosome number, increasing genetic variation via crossing over and independent assortment. It consists of two divisions: Meiosis I and Meiosis II.

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• Meiosis I (Reduction Division):

  • Prophase I: Homologous chromosomes pair and exchange segments (crossing over), increasing variability.

  • Metaphase I: Homologous pairs align at the equator.

  • Anaphase I: Homologous chromosomes separate to opposite poles; sister chromatids remain attached.

  • Telophase I and Cytokinesis: Two haploid cells form, each with duplicated chromosomes.

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• Meiosis II (Similar to Mitosis):

  • Prophase II: Chromosomes condense; nuclear envelope dissolves.

  • Metaphase II: Sister chromatids align at equator.

  • Anaphase II: Sister chromatids separate to opposite poles.

  • Telophase II and Cytokinesis: Four genetically distinct haploid cells result.

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Protein Synthesis

• Proteins are polymers of amino acids essential for cell structure and function. Protein synthesis converts genetic information in DNA into functional proteins through transcription and translation.

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• Transcription: DNA is copied into messenger RNA (mRNA) in the nucleus by RNA polymerase. mRNA carries codons—triplets of nucleotides specifying amino acids.

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• Translation: mRNA is decoded by ribosomes with the help of transfer RNA (tRNA) and ribosomal RNA (rRNA) to assemble amino acids into polypeptide chains (proteins).

• The central dogma of molecular biology describes the flow of genetic information: DNA → RNA → Protein. Reverse transcription (RNA → DNA) occurs in some viruses.

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