DNA Structure and Replication

DNA Structure

• DNA (Deoxyribonucleic Acid) is a double helix composed of two antiparallel strands.

• Each strand has a 5' phosphate group and a 3' hydroxyl group.

• Nucleotide pairing: Guanine (G) pairs with Cytosine (C) (3 hydrogen bonds), and Adenine (A) pairs with Thymine (T) (2 hydrogen bonds).

• Strands are complementary and antiparallel (one runs 5' to 3', the other 3' to 5').

DNA Replication

• Replication is semiconservative: each new DNA molecule contains one parental and one new strand.

• Initiation begins at specific sites called origins of replication.

• Helicase unwinds the DNA double helix.

• Primase synthesizes a short RNA primer to provide a starting point for DNA synthesis.

• DNA polymerase extends the new DNA strand from the primer, always synthesizing in the 5' to 3' direction.

• After extension, the RNA primer is removed and the gap is filled with DNA.

• DNA ligase seals nicks in the sugar-phosphate backbone.

Key Enzymes and Functions

Example: If the template strand is 3'-AGCT-5', the new strand will be 5'-TCGA-3'.

Gene Expression: Transcription and Translation

Transcription

• Transcription is the process of synthesizing RNA from a DNA template.

• Promoter: DNA sequence where RNA polymerase binds to initiate transcription.

• Terminator: Sequence signaling the end of transcription.

• RNA polymerase synthesizes RNA in the 5' to 3' direction, using the DNA template strand (which is read 3' to 5').

• In eukaryotes, the initial RNA transcript (pre-mRNA) undergoes splicing to remove introns.

Translation

• Translation is the process of decoding mRNA into a polypeptide (protein).

• Occurs in the ribosome, involving mRNA, tRNA (transfer RNA), and rRNA (ribosomal RNA).

• Each codon (three nucleotides) codes for one amino acid.

• tRNA molecules bring amino acids to the ribosome, matching codons via their anticodons.

• Ribosome sites: E (Exit), P (Peptidyl), A (Aminoacyl).

Key Terms

• mRNA: Messenger RNA, carries genetic code from DNA to ribosome.

• tRNA: Transfer RNA, brings amino acids to ribosome.

• rRNA: Ribosomal RNA, structural and catalytic component of ribosomes.

Example: If the DNA template has an A, the RNA will have a U (uracil) opposite it.

Cell Cycle and Mitosis

Cell Theory

• All living organisms are made of one or more cells.

• The cell is the basic unit of life.

• All cells arise from pre-existing cells.

Cell Cycle

• The cell cycle is the series of events that cells go through as they grow and divide.

• Phases: G1 (growth), S (DNA synthesis), G2 (preparation for division), M (mitosis/cytokinesis).

• Checkpoints regulate progression and ensure errors are corrected.

• Cancer cells bypass checkpoints, leading to uncontrolled division.

Mitosis

• Mitosis is a type of cell division resulting in two genetically identical daughter cells (asexual reproduction, growth, repair).

• Stages:

  • Prophase: Chromosomes condense, centrosomes move to poles.

  • Prometaphase: Nuclear envelope breaks down, spindle fibers attach to kinetochores at centromeres.

  • Metaphase: Chromosomes align at the metaphase plate.

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

  • Telophase: Nuclear envelope reforms, chromosomes decondense.

  • Cytokinesis: Division of cytoplasm, forming two cells.

Key Terms

• Chromosome: Long DNA molecule with associated proteins.

• Genome: Entire set of genetic material in a cell.

• Diploid (2n): Two sets of chromosomes.

• Haploid (1n): One set of chromosomes.

• Homologous chromosomes: Chromosome pairs with same genes but possibly different alleles.

Example: Humans have 23 pairs of chromosomes (46 total).

Meiosis and Genetic Variation

Meiosis

• Meiosis is a type of cell division that produces four genetically unique haploid gametes (sperm or egg cells).

• Occurs in gonads (testes or ovaries).

• Consists of two divisions: Meiosis I and Meiosis II.

• Meiosis I: Homologous chromosomes pair (synapsis), recombine (crossing over), and separate, reducing chromosome number by half (2n → 1n).

• Meiosis II: Sister chromatids separate (like mitosis), resulting in four haploid cells.

Stages of Meiosis

• Genetic variation arises from crossing over (recombination) during Prophase I and independent assortment of chromosomes.

Example: After meiosis, each gamete contains a unique combination of alleles.

Mendelian Genetics

Basic Concepts

• Genotype: Genetic makeup of an organism (e.g., AA, Aa, aa).

• Phenotype: Observable traits (e.g., purple or white flowers).

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

• Heterozygous: Two different alleles (Aa).

• Dominant allele: Expressed trait (e.g., A).

• Recessive allele: Masked in presence of dominant (e.g., a).

• Alleles: Alternative forms of a gene.

• Autosome: Non-sex chromosome.

Inheritance Patterns

• Gametes from a heterozygote (Aa) will carry either A or a, not both.

• Dominant traits mask recessive traits in heterozygotes.

• Some traits may show incomplete dominance (blending of traits).

Example Table: Genotype and Phenotype

Example: In pea plants, purple flower color (A) is dominant to white (a).

Additional Key Terms and Concepts

• Sister chromatids: Identical copies of a chromosome, connected at the centromere.

• Daughter chromatids: Chromatids that have separated during cell division and are now individual chromosomes.

• Homologous chromosomes: Chromosomes of the same size and shape, carrying the same genes but possibly different alleles.

• Nonhomologous chromosomes: Chromosomes that differ in size, shape, and gene content.

Example: Blue and red chromosomes in diagrams often indicate chromosomes from different parents.

Summary Table: DNA, RNA, and Protein Synthesis

Additional info: Some explanations and definitions have been expanded for clarity and completeness, including the distinction between sister and daughter chromatids, and the summary tables for enzyme functions and inheritance patterns.