Level 2 Bio: Gene Expression

Metabolic Pathway

A series of enzyme-controlled biochemical reactions that convert a starting molecule into an end product. Each step is catalyzed by a specific enzyme, and pathways can be linear, branched, or cyclic.

Functions of Metabolic Pathways

Metabolic pathways are essential because they provide:

1. Efficiency: Smaller steps release energy gradually.
2. Control: Each step can be regulated to meet cellular needs.
3. Intermediates: Products from one step can be used in other pathways.
4. Flexibility: Pathways can be switched on/off depending on environmental conditions.

Role of Enzymes in Metabolic Pathways

Enzymes are proteins that speed up biochemical reactions by lowering activation energy, without being used up. They have an active site that fits the substrate, and their shape (determined by genes) is crucial for pathway function; mutations can alter pathways.

Photosynthesis as a Metabolic Pathway

Photosynthesis is a complex anabolic pathway with multiple enzyme-controlled steps. It includes:

1. Light-dependent reactions: Convert light energy to chemical energy (ATP, NADPH).
2. Light-independent reactions (Calvin cycle): Use ATP and NADPH to build glucose from CO_{2}.

Gene Expression

The process by which a cell makes an RNA copy of a piece of DNA. It involves transcription and translation to produce proteins from the gene's encoded information.

Gene

A length of DNA (100–2,000,000 base pairs) that codes for one protein; humans have ~25,000.

Chromosome

A package of DNA containing thousands of genes which a human cells carry two copies of

Genome

The complete set of genes or genetic material present in a cell or organism.

Nucleus

Membrane-bound organelle in eukaryotes where genetic information (DNA) is stored.

DNA (Deoxyribonucleic Acid)

Double-stranded, helical nucleic acid that stores and transmits genetic information.

RNA (Ribonucleic Acid)

Single-stranded nucleic acid that helps make proteins; contains ribose and bases A, U, C, G.

Nucleotide

The monomer of nucleic acids, consisting of a nitrogenous base, a pentose sugar, and a phosphate group.

Nitrogenous Base

Component of a nucleotide; in DNA: A, T, C, G; in RNA: A, U, C, G.

Pentose Sugar

Five-carbon sugar in nucleotides; deoxyribose in DNA, ribose in RNA.

Phosphate Group

Part of a nucleotide that links sugars of adjacent nucleotides, forming the nucleic-acid backbone.

Complementary Base Pairing

Specific hydrogen bonding of A with T (or U) and C with G in nucleic acids.

Purine

Double-ring nitrogenous base (adenine or guanine).

Pyrimidine

Single-ring nitrogenous base (cytosine, thymine, or uracil).

Monomer

A small molecule that can join others to form a polymer.

Polymer

A large molecule made of repeating monomers; DNA and RNA are nucleic-acid polymers.

Polynucleotide

A chain of many nucleotides linked together, forming DNA or RNA strands.

Double Helix

The twisted-ladder shape of double-stranded DNA.

Amino Acid

Building block of proteins; contains an alpha carbon, amino group, carboxyl group, and variable R-group.

Peptide Bond

Covalent bond joining amino acids in a protein backbone.

Protein

Large molecules made of chains of amino acids

Protein Primary Structure

Linear sequence of amino acids in a polypeptide held by peptide bonds.

Protein Secondary Structure

Local folding of a polypeptide into α-helices or β-pleated sheets via hydrogen bonds.

α-Helix

Spiral secondary-structure motif stabilized by hydrogen bonds (e.g., in keratin).

β-Pleated Sheet

Folded secondary-structure motif forming sheets (e.g., in silk).

Protein Tertiary Structure

Overall 3-D shape of a polypeptide stabilized by hydrogen bonds, ionic bonds, disulfide bridges, and hydrophobic interactions.

Disulfide Bridge

Covalent S–S bond between two cysteine residues, stabilizing tertiary protein structure.

Hydrophobic Interaction in Proteins

Non-polar side chains clustering away from water, aiding protein folding.

Protein Quaternary Structure

Assembly of two or more polypeptide chains into a functional protein.

Hemoglobin

Quaternary protein with four subunits that transports oxygen in blood.

DNA Polymerase

A type of enzyme that is responsible for forming new copies of DNA, in the form of nucleic acid molecules

Transcription

Process in which DNA is transcribed into messenger RNA (mRNA).

Translation

Process in which mRNA is decoded to synthesize a protein at the ribosome.