Introduction to Getting Up to Speed in Biology | MIT OCW

DNA replication is a semi-conservative process requiring approximately 20 minutes, involving unwinding of the double helix and synthesis of complementary strands. Key enzymes include DNA-dependent DNA polymerase, single strand binding proteins (SSBL), and helicase. Replication occurs at the replication fork with leading strand synthesis continuous in 5' to 3' direction and lagging strand synthesis discontinuous. Transcription copies genetic information from DNA to RNA, proceeding in 5' to 3' direction through initiation, elongation, and termination stages. RNA polymerase synthesizes RNA complementary to the DNA template strand. Eukaryotic RNA undergoes processing: 5' capping, polyadenylation, and splicing to remove introns. Translation occurs at ribosomes, where mRNA codons are read in triplets by tRNA anticodons. The genetic code is degenerate and nearly universal. Translation proceeds through initiation, elongation, and termination, with stop codons (UAA, UAG, UGA) signaling completion. Ribosomes consist of large and small subunits that facilitate peptide bond formation between amino acids.

This extensive section covers the mechanisms of DNA replication, transcription, and translation. DNA replication is semi-conservative, meaning each new DNA molecule consists of one parental strand and one newly synthesized strand. Meselson and Stahl proved this in 1958 using heavy nitrogen (N15) and light nitrogen (N14). E. coli divides every 20 minutes, and DNA replication takes approximately 18 minutes. DNA polymerase synthesizes new strands in the 5' to 3' direction, requiring a template strand. The leading strand is synthesized continuously, while the lagging strand is synthesized discontinuously as Okazaki fragments. The origin of replication is where DNA replication begins, typically at regions with high A-T content. Transcription copies DNA into RNA, beginning at a promoter region and ending at a terminator region. RNA polymerase reads the template strand in the 3' to 5' direction and synthesizes RNA in the 5' to 3' direction. In prokaryotes, transcription and translation occur simultaneously in the cytoplasm. In eukaryotes, transcription occurs in the nucleus while translation occurs in the cytoplasm. Eukaryotic genes contain exons (expressed sequences) and introns (intervening sequences removed during splicing). The genetic code is a triplet code where three nucleotides specify one amino acid. With four nucleotides, there are 64 possible codons, more than enough for 20 amino acids. The code is degenerate (multiple codons specify the same amino acid) and nearly universal. AUG is the start codon (methionine), while UAA, UAG, and UGA are stop codons. tRNA is an adapter molecule that brings amino acids to the ribosome. Gene expression can be regulated at multiple levels: transcriptional, RNA processing, RNA stability, translational, and post-translational. The lac operon demonstrates negative regulation in prokaryotes.

The central dogma describes the flow of genetic information in living organisms: DNA replication (duplication), transcription (DNA to mRNA), and translation (mRNA to protein). DNA replication involves three main steps: helicase separates DNA strands by breaking hydrogen bonds between complementary base pairs (A-T with 2 bonds, C-G with 3 bonds), creating a Y-shaped replication fork; DNA polymerase adds complementary nucleotides to each original strand; DNA ligase seals gaps between newly synthesized fragments. Replication is semi-conservative, with each new DNA molecule containing one original and one new strand, occurring with high accuracy (one error per billion base pairs). Transcription synthesizes mRNA using one DNA strand as template, with RNA polymerase binding to promoter sequences near genes. The DNA double helix unwinds, exposing the template strand. RNA polymerase adds complementary RNA nucleotides: adenine in DNA pairs with uracil in RNA, cytosine with guanine, guanine with cytosine, and thymine with adenine. After reaching termination signals, the enzyme releases mRNA, and DNA strands rejoin. Translation occurs in ribosomes within the cytoplasm, reading mRNA codons to assemble amino acids into polypeptide chains. The process involves three stages: initiation (ribosome binds mRNA at start codon AUG with methionine-carrying tRNA), elongation (tRNAs deliver amino acids as anticodons match codons, forming peptide bonds), and termination (stop codons UAA, UAG, or UGA trigger release factors to detach the polypeptide). In prokaryotes, transcription and translation can occur simultaneously due to the absence of a nuclear membrane.

DNA replication produces two different strands due to the 5' to 3' directionality constraint. The leading strand is synthesized continuously toward the replication fork, requiring only one RNA primer. The lagging strand is synthesized discontinuously away from the fork in short segments called Okazaki fragments (100-200 nucleotides), each requiring its own RNA primer. DNA polymerase I removes RNA primers and fills gaps, while DNA ligase seals the fragments. Transcription involves RNA polymerase synthesizing RNA from a DNA template. The process begins at the promoter region, where RNA polymerase binds and unwinds the DNA. RNA polymerase adds complementary nucleotides to the template strand, synthesizing RNA in the 5' to 3' direction. The key difference from DNA is that RNA contains uracil (U) instead of thymine (T). Pre-mRNA undergoes post-transcriptional modifications: 5' capping (addition of modified guanine), polyadenylation (addition of poly-A tail), and splicing (removal of introns, joining of exons). These modifications produce mature mRNA ready for translation. Translation is the process by which ribosomes synthesize proteins from mRNA. A codon is a sequence of three nucleotides specifying an amino acid. The genetic code is nearly universal and degenerate. Translation occurs in three phases: initiation (ribosome assembles around mRNA and recognizes start codon), elongation (amino acids are added one by one), and termination (stop codons signal completion). Ribosomes consist of small (40S) and large (60S) subunits with three binding sites: A site (accepts new aminoacyl-tRNA), P site (holds growing polypeptide chain), and E site (releases empty tRNA). tRNA carries specific amino acids with anticodons complementary to mRNA codons. The start codon (AUG) codes for methionine and initiates translation. Stop codons (UAA, UAG, UGA) signal termination. Post-translational modifications (phosphorylation, glycosylation, etc.) further modify proteins for specific functions.

DNA replication is semi-conservative, proven by Meselson-Stahl (1958) using N15/N14 isotope labeling and cesium chloride centrifugation. Replication begins at origins, forming Y-shaped forks. Leading strand synthesizes continuously toward the fork; lagging strand synthesizes discontinuously in Okazaki fragments. DNA polymerase adds nucleotides only in 5' to 3' direction, requiring RNA primers. Transcription differs between prokaryotes (cytoplasm, coupled with translation) and eukaryotes (nucleus, hnRNA processing with 5' capping, splicing, polyadenylation). Eukaryotes have three RNA polymerases: I (rRNA), II (mRNA), III (tRNA, 5S rRNA). Translation occurs on ribosomes with A, P, E sites. tRNA charging attaches amino acids using ATP. Peptide bonds form catalyzed by peptidyl transferase (rRNA enzyme). The genetic code features: triplet, non-overlapping, degenerate, unambiguous, nearly universal. Stop codons (UAA, UAG, UGA) signal termination; AUG is the start codon.

Cellular metabolism (الأيض الخلوي) represents the sum of chemical transformations in cells with enzyme assistance, including catabolism (هدم) where materials are broken down and energy is released, and anabolism (بناء) where new products are built and energy is consumed. Photosynthesis exemplifies anabolism, while cellular respiration exemplifies catabolism. Glycolysis (التحلل السكري) is the first stage of cellular respiration where glucose is converted into two molecules of pyruvic acid in the cytoplasm. The process involves sequential molecular transformations: glucose activation by adding a phosphate group from ATP, isomerization to fructose-6-phosphate, second phosphorylation to fructose-1,6-bisphosphate, splitting into two glyceraldehyde-3-phosphate molecules, and conversion to pyruvic acid. The net gain of two ATP represents the energy profit from breaking down one glucose molecule. The fate of pyruvic acid depends on oxygen availability: in aerobic respiration, it is oxidized to acetyl-CoA in the mitochondria for the Krebs cycle; in anaerobic respiration, it undergoes fermentation to produce ethanol (alcoholic fermentation) or lactic acid (lactic acid fermentation).

This comprehensive section covers the fundamental energy-producing pathways in cellular metabolism. Glycolysis breaks down glucose to produce 2 ATP and 2 NADH2 molecules per glucose molecule. The citric acid cycle (Krebs cycle) produces 2 ATP per glucose molecule. Each NADH2 carries approximately 2.5 ATP equivalents. The citric acid cycle is also known as the tricarboxylic acid cycle or Krebs cycle, named after Hans Krebs who discovered it. These pathways work together to generate the majority of cellular ATP from glucose breakdown, forming the foundation of cellular energy metabolism.

Cellular respiration involves two main pathways: glycolysis (anaerobic) and the citric acid cycle (aerobic). Glycolysis breaks down glucose into two pyruvate molecules, producing 2 ATP. The citric acid cycle further processes pyruvate to generate additional energy. Understanding these pathways is essential for biology exams, especially when professors include blank sections for drawing molecular processes.

Cellular metabolism consists of two complementary processes: anabolism (building) and catabolism (breaking down). Anabolism converts small, simple molecules into large, complex molecules like in photosynthesis, storing energy in chemical bonds. Catabolism breaks down large, complex molecules into smaller ones to release energy, like in cellular respiration. Photosynthesis requires light, chlorophyll, and enzymes, converting CO2 and water into glucose and oxygen. Cellular respiration uses glucose and oxygen to produce CO2, water, and energy. Cellular respiration occurs in three forms: aerobic (with oxygen), anaerobic (without oxygen), and fermentation (insufficient oxygen). All begin with glycolysis. The mitochondrion, where aerobic respiration occurs, has an outer membrane, inner membrane with cristae (folds increasing surface area), intermembrane space, and matrix containing enzymes, DNA, ribosomes, and proteins. Glycolysis is the first stage, occurring in the cytoplasm without requiring oxygen. One glucose molecule (six carbons) is split into two pyruvate molecules (each with three carbons), producing two ATP (direct energy) and two NADH (indirect energy).

Cellular metabolism encompasses photosynthesis and cellular respiration. Photosynthesis captures light energy to produce glucose: 6CO2 + 6H2O + light → C6H12O6 + 6O2. Light-dependent reactions in thylakoid membranes split water, producing ATP and NADPH. The Calvin cycle in stroma fixes CO2 into glucose. Cellular respiration releases energy from glucose. Glycolysis breaks glucose into pyruvate, producing 2 ATP and 2 NADH. Aerobic respiration in mitochondria produces much more ATP through Krebs cycle and electron transport chain. Oxygen serves as final electron acceptor.

This section covers Mendelian genetics and inheritance patterns. The F2 generation of a dihybrid cross shows 9:3:3:1 ratio. Chromosomes are made of DNA and histones. Harmful mutations persist because they are dominant. Mendelian disorders follow Mendel's laws. The term 'gene' was coined by Johannsen.

This section covers Mendel's laws of inheritance. Gregor Johann Mendel is the father of genetics, working with garden peas (Pisum sativum). His three laws are: Law of Dominance, Law of Segregation, and Law of Independent Assortment. In a monohybrid cross (Tt × Tt), four gamete types are produced with 25% each. The F2 phenotypic ratio is 3:1. Sex determination follows Mendelian principles: father's X or Y chromosome determines child's sex (50% male, 50% female probability).

This extensive section covers the complete foundation of Mendelian genetics: heredity (trait transfer between generations), genetics (study of heredity), characteristics (transferable traits), traits (specific variations), genes (DNA segments controlling traits), alleles (different gene versions at same locus), and locus (gene position). It explains genotype notation: homozygous (two identical alleles), heterozygous (two different alleles), dominant genes (capital letters, expressed in heterozygotes), and recessive genes (lowercase letters, expressed only in homozygotes). The section distinguishes genotype (complete gene set) from phenotype (observable characteristics), and explains how environment can modify phenotype without changing genotype (modification). It covers independent genes (on different chromosomes, inherited independently, number equals chromosome pairs) and linked genes (on same chromosome, inherited together unless crossing over occurs). The section introduces Mendel's foundational contributions and probability principles: probabilistic events are independent, and probability of independent events equals product of individual probabilities. For independent genes, gamete variety is calculated as 2^n where n is heterozygous gene pairs. For linked genes without crossing over, all heterozygous pairs on same chromosome are treated as one unit. For linked genes with crossing over, all heterozygous pairs are treated as independent genes.

This section covers fundamental genetics. Mendel selected seven pairs of contrasting traits in garden peas (Pisum sativum). A dihybrid cross produces a 9:3:3:1 phenotypic ratio. Down syndrome results from trisomy of chromosome 21. The ABO blood group demonstrates codominance and multiple alleles. Klinefelter syndrome (47,XXY) involves an extra X chromosome. Hemophilia is a sex-linked recessive disorder. The chromosome theory of inheritance was proposed by Sutton and Boveri in 1902.

This section covers Mendelian genetics and inheritance patterns. In incomplete dominance, heterozygotes show intermediate phenotypes with 1:2:1 genotypic and phenotypic ratios. Codominance shows both alleles expressed equally. Back cross involves crossing F1 with either parent, while test cross uses homozygous recessive to determine genotype. Monohybrid crosses show 1:2:1 genotypic and 3:1 phenotypic ratios.