Concept 2 Unit 4 Summary & Study Notes

🧬 Overview

Protein synthesis is the process of reading instructions in DNA to build a polypeptide (which folds into a functional protein). It follows the Central Dogma: DNA → RNA → protein, carried out in two main stages: transcription and translation.

🧩 Protein Structure: Four Levels

• Primary structure: the linear amino acid sequence (a polypeptide is a chain of amino acids).
• Secondary structure: local folding into alpha helices or beta sheets due to hydrogen bonds.
• Tertiary structure: 3D folding of a single polypeptide chain into its functional shape.
• Quaternary structure: association of two or more polypeptide chains to form a functional protein.

Proteins are the most diverse macromolecules because of these hierarchical structural levels and varied amino acid sequences.

🧾 Types of RNA

• mRNA (messenger RNA): copies instructions from DNA and carries them out of the nucleus to ribosomes.
• tRNA (transfer RNA): carries specific amino acids and has an anticodon that pairs with mRNA codons.
• rRNA (ribosomal RNA): together with proteins makes up the ribosome and helps catalyze peptide bond formation.

🏛️ Transcription (DNA → mRNA)

Purpose: To carry the genetic code from the nucleus (where DNA is confined) to the cytoplasm as mRNA. Location: Nucleus. Starts with DNA; ends with mRNA. Process:

• The gene (a DNA segment) unwinds/unzips.
• RNA nucleotides pair with the exposed DNA template strand using complementary base-pairing rules (in RNA, uracil (U) pairs with adenine (A), and cytosine (C) pairs with guanine (G)).
• The completed mRNA is released, DNA zips back up, and mRNA exits the nucleus into the cytoplasm.

Example (template DNA → mRNA): DNA (template): TACGCTAGTACGATT mRNA: AUGCGAUCAUGCUAA

🔤 Translation (mRNA → Polypeptide)

Purpose: To interpret the mRNA message and assemble a polypeptide (protein) at the ribosome. Location: Ribosomes in the cytoplasm. Starts with mRNA; ends with a polypeptide. Process:

• mRNA binds to a ribosome. Translation begins at the start codon AUG.
• The ribosome reads mRNA in sets of three nucleotides called codons (1 codon = 3 nucleotides).
• Each tRNA with a complementary anticodon brings the matching amino acid to the ribosome.
• The ribosome forms peptide bonds between amino acids, elongating the polypeptide chain.
• When a stop codon is reached, translation ends and the completed polypeptide is released.

Example breakdown: mRNA sequence: AUG CGA UCA UGC UAA Amino acids: Met – Arg – Ser – Cys – stop tRNA anticodons: UAC GCU AGU ACG AUU

🧩 Key Vocabulary

• Codon: three nucleotides on mRNA that specify an amino acid or stop signal.
• Anticodon: three complementary nucleotides on tRNA.
• Start codon: AUG (codes for methionine; translation initiation).
• Stop codons: codons that terminate translation (no matching amino acid).
• Peptide bond: covalent bond linking amino acids in a polypeptide.

✅ Summary of the Processes

• DNA contains the instructions (A, T, C, G) and stays in the nucleus.
• Transcription produces mRNA (A, U, C, G) from DNA in the nucleus.
• Translation uses mRNA in the ribosome to assemble amino acids into a protein.

🧠 Gene Expression and Regulation

Although every body cell contains the full DNA sequence, cells differ because of gene expression: only certain genes are turned on or off in each cell type.

• Gene expression is tightly regulated and can be controlled before, during, or after transcription and translation.
• Transcription factors are regulatory proteins that influence gene activity.
• Repressors decrease transcription; activators increase transcription.
• Disruption of gene regulation can lead to disease (e.g., some viruses interfere with regulatory mechanisms).

🔬 Epigenetics vs Mutation

• Mutation: a change in the DNA sequence itself.
• Epigenetics: heritable changes in gene expression that do not alter the DNA sequence. Instead, epigenetic modifications (for example, histone modification) change how accessible DNA is for transcription, affecting which genes are expressed.

✳️ Why Cells Differ (Cell Differentiation)

Cell specialization results from selective gene expression regulated by transcription factors and epigenetic modifications. Different combinations of active and inactive genes produce different sets of proteins, giving each cell type its unique structure and function.