Cellular Metabolism, Cell Cycle, DNA Replication, and Protein Synthesis: Study Guide

Cellular Metabolism

Metabolism Overview

Metabolism refers to all chemical reactions occurring within living cells, enabling them to grow, reproduce, and maintain their structures. It is divided into two main categories:

• Anabolism: The synthesis of complex molecules from simpler ones, requiring energy.

• Catabolism: The breakdown of complex molecules into simpler ones, releasing energy.

ATP (Adenosine Triphosphate) is the primary energy carrier in cells.

Cellular Respiration

Cellular respiration is the process by which cells extract energy from glucose. It consists of several stages:

• Glycolysis: Occurs in the cytoplasm; glucose is split into two pyruvate molecules. Net gain: 2 ATP, 2 NADH.

• Preparatory Step: Pyruvate is converted to acetyl-CoA, producing NADH and CO2.

• Citric Acid Cycle (Krebs Cycle): Acetyl-CoA enters the cycle, generating NADH, FADH2, ATP, and CO2.

• Electron Transport System: NADH and FADH2 donate electrons, leading to ATP synthesis via ATP synthase.

Aerobic respiration requires oxygen; anaerobic processes (e.g., lactic acid fermentation) occur without oxygen.

Key molecules: NAD+, FAD, Coenzyme A, α-ketoglutarate, succinate, fumarate, oxaloacetate, citric acid, G3P, pyruvate.

Inner and outer mitochondrial membrane spaces and cristae are important for electron transport and ATP production.

ATP Production Steps and Molecular Counts

• Glycolysis: 2 ATP (net), 2 NADH

• Preparatory Step: 2 NADH

• Citric Acid Cycle: 2 ATP, 6 NADH, 2 FADH2

• Electron Transport Chain: ~34 ATP (from NADH and FADH2)

Total ATP per glucose: ~36-38 ATP

Cell Cycle

Phases of the Cell Cycle

The cell cycle is the sequence of events that cells undergo to grow and divide. It consists of:

• Interphase: Cell grows and DNA replicates. Subdivided into:

  • G1 phase: Cell growth

  • S phase: DNA synthesis

  • G2 phase: Preparation for mitosis

  • G0 phase: Resting phase; cells may exit the cycle

• Mitotic phase (M phase): Cell divides (mitosis and cytokinesis)

Chromatin condenses into chromosomes during mitosis. Each chromosome consists of two chromatids joined at the centromere.

DNA Replication and Repair

Steps of DNA Replication

DNA replication is the process by which a cell copies its DNA before division. It is semiconservative, meaning each new DNA molecule contains one old and one new strand.

• Initiation: Begins at the origin of replication, forming a replication bubble.

• Elongation: Helicase unwinds DNA; single-stranded binding proteins stabilize strands; primase synthesizes RNA primers; DNA polymerase adds nucleotides.

• Leading strand: Synthesized continuously (3’ direction).

• Lagging strand: Synthesized in fragments (Okazaki fragments), later joined by ligase.

• Termination: Replication ends; repair enzymes correct errors.

Restriction enzymes cut DNA at specific sequences; mutations are changes in DNA sequence.

DNA Structure

• Double helix: Two strands held together by complementary base pairing.

• Histones: Proteins that package DNA into chromatin.

• Genes: Segments of DNA coding for proteins.

Protein Synthesis and Structure

Transcription and Translation

Protein synthesis involves two main steps:

• Transcription: DNA is copied into mRNA by RNA polymerase at the promoter region.

• 1° transcript: Initial mRNA; contains introns (non-coding) and exons (coding).

• Translation: mRNA is decoded by ribosomes to synthesize a polypeptide.

• tRNA: Transfers amino acids; contains anticodon matching mRNA codon.

• rRNA: Ribosomal RNA, part of ribosome structure.

Steps of translation:

• Initiation: Ribosome assembles at start codon.

• Elongation: Amino acids added via peptide bonds.

• Termination: Ribosome reaches stop codon; polypeptide released.

Protein Structure

• Primary structure: Sequence of amino acids.

• Secondary structure: Alpha helix and beta sheet formed by hydrogen bonding.

• Peptide bonds: Link amino acids.

• Ribozymes: RNA molecules with enzymatic activity.

Enzymes in DNA Replication and Protein Synthesis

• DNA Polymerase: Synthesizes new DNA strands.

• Helicase: Unwinds DNA helix.

• Ligase: Joins Okazaki fragments.

• Primase: Synthesizes RNA primers.

• RNA Polymerase: Synthesizes mRNA during transcription.

• Repair enzymes: Correct DNA errors.

Key Terms Table

The following table summarizes key terms and their definitions:

Term	Definition
Metabolism	All chemical reactions in a cell
Anabolism	Synthesis of complex molecules
Catabolism	Breakdown of molecules
ATP synthase	Enzyme producing ATP in mitochondria
Glycolysis	Glucose breakdown in cytoplasm
Citric Acid Cycle	Cycle generating NADH, FADH2, ATP
Electron Transport System	Final stage of cellular respiration
Chromatin	DNA-protein complex in nucleus
Chromatid	One of two identical DNA strands
Chromosome	Condensed DNA structure
Gene	DNA segment coding for protein
mRNA	Messenger RNA; carries genetic code
tRNA	Transfer RNA; brings amino acids
rRNA	Ribosomal RNA; part of ribosome
Codon	Three-base sequence in mRNA
Anticodon	Three-base sequence in tRNA
Peptide bond	Covalent bond between amino acids
Mutation	Change in DNA sequence
Semiconservative	Each new DNA has one old strand

Example: ATP Production Calculation

For one glucose molecule:

• Glycolysis: 2 ATP

• Citric Acid Cycle: 2 ATP

• Electron Transport Chain: ~34 ATP

Total: ~36-38 ATP

Example: DNA Replication

During S phase, DNA is replicated. Helicase unwinds the DNA, primase adds primers, DNA polymerase synthesizes new strands, and ligase joins fragments.

Example: Protein Synthesis

Transcription produces mRNA from DNA. Translation uses mRNA, tRNA, and ribosomes to assemble a polypeptide.

Additional info: Some terms (e.g., G3P, α-ketoglutarate, succinate, fumarate, oxaloacetate) are intermediates in glycolysis and the citric acid cycle. The cell cycle phases (G1, S, G2, M, G0) are essential for understanding cell division and growth. Enzymes such as DNA polymerase, helicase, ligase, and primase are crucial for DNA replication, while RNA polymerase is key for transcription.