Key Processes in DNA and Protein Synthesis PDF

Key Processes 1. DNA Replication DNA replication ensures that each new cell receives an exact copy of the DNA. It occurs during the S phase of the cell cycle, entirely within the nucleus. Steps of DNA Replication 1. Unwinding and Unzipping DNA Helicase breaks the hydrogen bonds between ni...

Key Processes 1. DNA Replication DNA replication ensures that each new cell receives an exact copy of the DNA. It occurs during the S phase of the cell cycle, entirely within the nucleus. Steps of DNA Replication 1. Unwinding and Unzipping DNA Helicase breaks the hydrogen bonds between nitrogenous bases, creating a replication fork. The two DNA strands are now exposed for complementary base pairing. 2. Complementary Base Pairing Free nucleotides in the nucleus pair with the exposed bases on each DNA strand. Base Pairing Rules: Adenine (A) pairs with Thymine (T). Cytosine (C) pairs with Guanine (G). 3. Joining of the New Strand DNA polymerase synthesizes the new DNA strand by forming covalent bonds between the sugar-phosphate backbones. DNA ligase seals gaps in the sugar-phosphate backbone. Result: Two identical DNA molecules, each with one original (parent) strand and one new strand (semi-conservative replication). Key Enzymes in Replication Helicase: Unwinds and unzips DNA. DNA Polymerase: Adds nucleotides to form the new strand. DNA Ligase: Joins fragments of the DNA backbone. 2. Protein Synthesis Protein synthesis converts genetic information from DNA into functional proteins. It consists of transcription and translation. A. Transcription (DNA → mRNA) Occurs in the nucleus and produces a single-stranded RNA molecule (mRNA) based on the DNA template. Steps of Transcription 1. Unzipping the DNA Helicase unwinds the DNA double helix. 2. RNA Complementary Base Pairing Free RNA nucleotides pair with the exposed DNA strand. Base Pairing Rules: Adenine (A) pairs with Uracil (U) in RNA (instead of Thymine). Cytosine (C) pairs with Guanine (G). 3. mRNA Synthesis RNA polymerase links the RNA nucleotides, forming the mRNA strand. The mRNA strand detaches and exits the nucleus to the cytoplasm. B. Translation (mRNA → Protein) Occurs in the cytoplasm on ribosomes. Translation converts mRNA into a chain of amino acids (protein). Steps of Translation 1. Initiation Ribosome attaches to the start codon (AUG) on mRNA. A tRNA with the complementary anticodon (UAC) brings the amino acid methionine. 2. Elongation Ribosome reads mRNA codons, and tRNA brings specific amino acids based on complementary anticodons. Amino acids are joined by peptide bonds to form a growing polypeptide chain. 3. Termination Ribosome reaches a stop codon (UAA, UAG, or UGA). The polypeptide chain is released, folding into a functional protein. Key Molecules in Protein Synthesis 1. DNA: The template for genetic information. 2. mRNA: Carries genetic instructions from DNA to ribosomes. 3. tRNA: Transfers amino acids to ribosomes. Each tRNA has an anticodon that pairs with the mRNA codon. 4. rRNA: A component of ribosomes that catalyzes protein synthesis. Key Codon-Anticodon Example DNA: GAC mRNA: CUG tRNA (anticodon): GAC Key Concepts and Terms Nitrogenous Bases DNA: Adenine (A), Thymine (T), Cytosine (C), Guanine (G). RNA: Adenine (A), Uracil (U), Cytosine (C), Guanine (G). Genetic Code Codons: Groups of three mRNA bases (e.g., AUG). Each codon specifies an amino acid. Mutations Point Mutation: A single base change. Silent Mutation: No effect on protein. Missense Mutation: Changes one amino acid. Nonsense Mutation: Introduces a stop codon, terminating the protein early. Frameshift Mutation: Insertion or deletion of bases, altering the reading frame. Key Enzymes Helicase: Unzips DNA. RNA Polymerase: Synthesizes mRNA. DNA Ligase: Joins DNA fragments. Comparison: Replication vs. Transcription vs. Translation DNA Replication and Protein Synthesis Practice Questions and Answers Section 1: DNA Replication 1. Q: What is the purpose of DNA replication? A: To create an identical copy of DNA for cell division. 2. Q: During which phase of the cell cycle does DNA replication occur? A: The S phase (synthesis phase). 3. Q: What enzyme unwinds and unzips the DNA during replication? A: Helicase. 4. Q: Which enzyme adds nucleotides during DNA replication? A: DNA polymerase. 5. Q: What is semi-conservative replication? A: Each new DNA molecule consists of one original strand and one newly synthesized strand. 6. Q: Which enzyme joins fragments of the sugar-phosphate backbone? A: DNA ligase. 7. Q: What are the complementary base pairing rules in DNA? A: Adenine pairs with Thymine, and Cytosine pairs with Guanine. 8. Q: What structure is formed when helicase unzips DNA? A: A replication fork. 9. Q: What are the two strands of DNA referred to during replication? A: The leading strand and the lagging strand. 10. Q: Why is the lagging strand synthesized in fragments? A: Because DNA polymerase can only add nucleotides in the 5’ to 3’ direction. Section 2: Transcription 11. Q: What is transcription? A: The process of creating mRNA from a DNA template. 12. Q: Where does transcription occur? A: In the nucleus. 13. Q: What enzyme is responsible for synthesizing mRNA during transcription? A: RNA polymerase. 14. Q: What is the complementary RNA base pair for adenine in DNA? A: Uracil (U).  15. Q: What is the primary product of transcription? A: mRNA (messenger RNA). 16. Q: What sequence signals the start of transcription? A: The promoter sequence. 17. Q: What sequence signals the end of transcription? A: The terminator sequence. 18. Q: What happens to the mRNA after transcription? A: It exits the nucleus and travels to the ribosome. 19. Q: What is the role of the template strand in transcription? A: It provides the pattern for mRNA synthesis. 20. Q: What is the difference between DNA and RNA? A: DNA is double-stranded and contains thymine, while RNA is single- stranded and contains uracil. Section 3: Translation 21. Q: What is translation? A: The process of synthesizing a protein from mRNA. 22. Q: Where does translation occur? A: In the cytoplasm, at the ribosome. 23. Q: What molecule carries amino acids to the ribosome? A: tRNA (transfer RNA). 24. Q: What is a codon? A: A sequence of three nucleotides on mRNA that codes for a specific amino acid. 25. Q: What is an anticodon? A: A sequence of three nucleotides on tRNA that pairs with a codon on mRNA.  26. Q: What is the start codon? A: AUG (methionine). 27. Q: What signals the end of translation? A: A stop codon (UAA, UAG, or UGA). 28. Q: What type of bond joins amino acids together? A: Peptide bonds. 29. Q: What is the function of rRNA in translation? A: It forms part of the ribosome and catalyzes peptide bond formation. 30. Q: How does tRNA know which amino acid to bring? A: The anticodon on tRNA matches the codon on mRNA. Section 4: General Questions 31. Q: What is the central dogma of molecular biology? A: DNA → RNA → Protein. 32. Q: What would happen if a mutation changed a stop codon into a normal codon? A: Translation would not stop, and the protein could be too long or nonfunctional. 33. Q: What is a silent mutation? A: A mutation that does not change the amino acid sequence of the protein. 34. Q: What is a missense mutation? A: A mutation that changes one amino acid in the protein. 35. Q: What is a nonsense mutation? A: A mutation that introduces a premature stop codon. 36. Q: How can a frameshift mutation occur? A: By the insertion or deletion of nucleotides that are not in multiples of three. 37. Q: What is the difference between mRNA and tRNA? A: mRNA carries genetic information, while tRNA transfers amino acids to the ribosome. 38. Q: Why is protein synthesis important? A: Proteins are essential for cell structure, function, and regulation. 39. Q: What would happen if helicase were not functional? A: DNA could not be unzipped, and replication would not occur. 40. Q: What role does the ribosome play in protein synthesis? A: It reads the mRNA and facilitates the assembly of amino acids into a protein.