Genetics and Molecular Biology Review - DNA, Replication, Inheritance
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Key concepts and definitions
Chargaff's rules for base pairing:
The amount of adenine equals thymine, and the amount of guanine equals cytosine. In symbols: [A] = [T], \quad [G] = [C].
DNA nucleotide composition can be calculated from these rules. If you know the percentage of one base, you can deduce the rest:
Example given (from the transcript): If A% = 15%, then T% = 15% and G% + C% = 70%. Since G and C are equal, each is 35%:
[A] = 15\%, [T] = 15\%, [G] = [C] = \frac{100\% - (15\% + 15\%)}{2} = \frac{70\%}{2} = 35\%.
Watson and Crick developed the model of the DNA double helix.
Rosalind Franklin’s X-ray diffraction crystallography work contributed critical data to understanding DNA structure; a mnemonic: Crystal/ro salind are females (memory aid).
Hershey–Chase experiment demonstrated that DNA is the genetic material in bacteriophages (bacteriophage infection).
Structural components:
Sister chromatids: copies of the same chromosome.
Centromere: center region of the chromosome where sister chromatids are held together and where spindle fibers attach during mitosis.
Histones: proteins around which DNA wraps to form nucleosomes (spools of protein that help package DNA).
Nucleotide differentiation and base pairing:
The component that differentiates each nucleotide is the base (A, T, C, G).
Base pairs are held together by hydrogen bonds: A–T pairs with 2 hydrogen bonds; G–C pairs with 3 hydrogen bonds.
Genetic diversity across species:
The sequence of bases differs among species, contributing to diversity.
Chromosome types and sex determination:
Autosomes are the same in males and females.
Sex chromosomes determine sex of the individual.
Chromosome number and ploidy:
Humans have 46 total chromosomes.
A diploid cell contains 2 sets of chromosomes: 2n = 46.
Karyotype:
A karyotype shows the chromosomes of an individual and reveals sex; information inferred includes sex and chromosomal abnormalities.
DNA replication and enzymes
Functions of DNA polymerase:
Adds nucleotides to a growing DNA strand during replication.
Primer and attachment point:
A primer provides the attachment point for DNA polymerase to begin synthesis.
Semiconservative model of replication:
Each daughter DNA molecule consists of one original (old) strand and one newly synthesized strand: ext{DNA replication is semiconservative: } 1 ext{ old strand} + 1 ext{ new strand}.
Steps of DNA replication
The major players and order (as listed in the transcript):
Helicase – unwinds/unzips the double helix.
Primase – lays down RNA primers.
DNA polymerase – adds nucleotides to synthesize the new strand.
Ligase – seals gaps in the sugar-phosphate backbone.
Note: In canonical textbooks, primase is typically involved in laying down RNA primers for DNA polymerase to extend; helicase unwinds the helix; ligase seals nicks. The transcript lists these components in a similar order.
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Mutations and gene expression
Mutation: Changes in a DNA sequence.
Nucleotide dimers: Often caused by UV light producing dimers (e.g., thymine dimers) that disrupt DNA replication and transcription.
Gene and genetic information:
A gene is the information that codes for a product (RNA or protein).
Gene expression and its steps:
Involves transcription (DNA to RNA) and translation (RNA to protein).
DNA vs RNA – form and function:
DNA: double-stranded, bases A, T, C, G.
RNA: single-stranded, base U replaces T; bases A, U, C, G.
Transcription and its components (as listed):
Helicase, primase, RNA polymerase, ligase (note: classic transcription uses RNA polymerase; some items may reflect replication components).
Introns and exons:
Introns are removed during RNA processing; exons remain and code for amino acids.
Exon mutations can alter the amino acid sequence and thus the protein.
RNA transcription of a DNA strand (example from transcript):
DNA: ACGTA → RNA: UGCAU (Note: the transcript’s example contains a typographical error; the standard relationship is that the RNA sequence is the same as the DNA coding strand with T replaced by U. If DNA coding strand is 5'-ACGTA-3', then mRNA is 5'-ACGUA-3'.)
Codons and translation:
Codons are three-nucleotide bases on mRNA that encode amino acids.
Translation is the process by which ribosomes read mRNA and synthesize proteins using tRNA to deliver amino acids.
tRNA and anticodons:
tRNA delivers/transfers amino acids to the ribosome and contains a 3-nucleotide anticodon that pairs with mRNA codons.
Substitution, insertion, deletion mutations:
Substitution: one base replaces another within a codon.
Insertion: an extra nucleotide is inserted.
Deletion: a nucleotide is lost.
Frameshift mutations arise from insertions or deletions, altering the reading frame of the codons downstream.
Example note (incomplete from transcript): AUGCAC illustrates codons and reading frame shifts in substitution/insertion contexts; the key concept is that such changes can alter the resulting amino acid sequence.
The cell cycle, mitosis, and cancer cell lines
HeLa cells and cell lines:
HeLa cells are an immortal human cell line widely used to study human diseases and to develop vaccines (e.g., polio vaccine).
Benefits of cell lines:
Enable controlled study of cellular processes, disease mechanisms, and drug testing.
The cell cycle and mitosis
Overview: cell cycle consists of interphase (G1, S, G2) and mitosis/cytokinesis.
Interphase:
G1: the cell grows and carries out normal functions.
S: DNA replication occurs.
G2: the cell grows further in preparation for division.
Most of the time is spent in interphase.
Mitosis and cytokinesis:
Prophase: nuclear envelope breaks down; chromosomes condense and attach to microtubules.
Metaphase: sister chromatids align at the equator of the cell.
Anaphase: sister chromatids separate and move to opposite poles.
Telophase: nuclear envelope reforms around two sets of chromosomes.
Cytokinesis: cytoplasmic division resulting in two diploid daughter cells.
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Cytoplasmic division in plants vs animals
Animal cells:
Cytokinesis occurs via a contractile actin ring forming a cleavage furrow that pinches the membrane inward to split the cell.
Plant cells:
Instead of a cleavage furrow, vesicles derived from the Golgi coalesce at the center to form a cell plate, which develops into a separating cell wall between daughter cells.
Summary:Animals use cleavage furrow and contraction; plants produce a cell plate that forms a separating cell wall.
Meiosis vs Mitosis (sexual vs asexual reproduction)
Meiosis (sexual reproduction): results in four haploid cells; includes two rounds of division.
Prophase I: homologous chromosomes pair and crossing over occurs.
Metaphase I: homologous chromosome pairs align at the equator.
Anaphase I: homologous chromosomes separate to opposite poles.
Telophase I: cells divide (cytokinesis) to form two haploid cells.
Prophase II, Metaphase II, Anaphase II, Telophase II: separate sister chromatids into four haploid products.
Mitosis (asexual reproduction): produces two identical diploid daughter cells.
Crossing over:
Occurs during Prophase I of meiosis and increases genetic diversity.
Homologous chromosomes:
Pairs of chromosomes that are the same length, shape, and carry the same genes in the same order (though not necessarily identical alleles).
Cancer biology terms from the transcript
Tumors (neoplasms): abnormal growths of cells.
Benign: nonmalignant (e.g., warts).
Malignant: cancerous, can invade surrounding tissue.
Oncogenes:
Genes that promote an increase in cell division and can turn a normal cell into a cancer cell when mutated or overexpressed.
Genotypes vs phenotypes:
Genotype: genetic makeup (the alleles present).
Phenotype: observable traits.
Genetic concept highlights:
Homozygous: same alleles for a gene.
Heterozygous: different alleles for a gene.
Dominant vs recessive alleles (capital letters for dominant, lowercase for recessive).
Monohybrid cross:
A cross examining the inheritance of a single trait.
If two heterozygous parents (Aa x Aa), the probability of recessive offspring (aa) is 25\%.
ABO blood groups (polygenic-like in observation but controlled by a few alleles with codominance):
Alleles: IA, IB, i (O). IA and IB are codominant; i is recessive.
Genotypes and corresponding phenotypes:
IAIA or IAi -> blood type A
IBIB or IBi -> blood type B
IAIB -> blood type AB
ii -> blood type O
Polygenic inheritance:
Traits influenced by multiple genes; examples include skin color, height, and weight.
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Pedigrees and human inheritance patterns
Pedigrees are used to study how traits or disorders run in families.
Modes of inheritance:
Autosomal dominant: at least one dominant allele is present; affected individuals appear in every generation; an affected parent has a 50% chance of passing the trait to offspring if heterozygous.
Autosomal recessive: two recessive alleles are required; carriers (heterozygotes) are typically unaffected; affected individuals may appear in siblings without affected parents.
X-linked disorders: more common in males because males have only one X chromosome; any affected X is expressed in males, whereas females may be carriers if they have one affected X.
Nondisjunction and aneuploidy
Nondisjunction: failure of chromosomes to separate correctly during meiosis, leading to an abnormal number of chromosomes in gametes.
Effects of nondisjunction: too many or too few chromosomes in offspring (aneuploidy).
Common aneuploidies mentioned:
Down syndrome (trisomy 21): three copies of chromosome 21.
Turner syndrome: females with only one X chromosome (monosomy X).
Real-world relevance and examples for study
Understanding these inheritance patterns helps in genetic counseling and predicting risks for offspring.
Examples of conditions by these patterns appear in pedigrees and clinical genetics contexts.
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