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CHAPTER 16 & 17: REPLICATION, TRANSCRIPTION & TRANSLATION Summary: An essential characteristic of all living organisms is their ability to store and express genetic information. The genetic information in a human cell is contained within several DNA molecules. DNA contains both unique coding sequences (EXONS) that are expressed and noncoding sequences (INTRONS) that are not expressed. In the human genome, only about 1.5% of the DNA is functional. During cell division, DNA molecules are duplicated via a process called replication and then passed on to each daughter cell. Within a cell, the information in DNA is expressed through the formation of RNAs and proteins. Transcription involves making RNA molecules that are complementary to one of the two strands of DNA. There are 3 main types of RNA: messenger RNA (mRNA), transfer RNA (tRNA) and ribosomal RNA (rRNA). Transcription involves enzymes called RNA polymerases that bind to the DNA and separate the 2 strands to create a transcription bubble. As transcription occurs, ribonucleotides are joined together as follows: A pairs with T, U pairs with A, G pairs with C, and C pairs with G. As the RNA polymerase moves along the DNA, a new RNA strand is formed in the 5’3’ direction. Transcription begins at specific sites within the DNA called promoters and ends at sites within the DNA called terminators. Translation involves using messenger RNAs to direct the formation of specific proteins by the ribosomes (protein factories). The mRNA is read in 3-letter blocks or codons. The genetic code is unambiguous in that each codon has one specific meaning. The code is degenerate in that some amino acids are encoded for by more than 1 codon. The codon AUG, which specifies methionine, is used to START translation. The codons UAA, UAG, and UGA do not encode for amino acids and are used to STOP translation. Transfer RNAs are RNA molecules that transfer amino acids to the growing polypeptide chain. tRNAs have a characteristic clover-leaf shape and an anticodon region that is complementary to the codon region of the mRNA. Amino acids are attached to tRNA by enzymes called aminoacyl-tRNA synthetases. Ribosomes are RNA/protein complexes composed of large and small subunits. The complete ribosome binds to mRNA and has 3 sites at which tRNAs can bind: the aminoacyl site (A) site, the peptidyl (P) site, and the exit (E) site. During the initiation phase of translation, a mRNA combines with the small and large ribosomal subunits and a transfer RNA carrying methionine binds to the start codon (AUG). During the elongation phase of translation, several tRNAs carrying amino acids bind to the ribosome as it moves along the mRNA and the amino acids are linked together by peptide bonds. During the termination phase of translation, the ribosomal subunits release both the mRNA and the newly formed protein. These proteins then carry out specific functions to give each cell its characteristic properties. While replication and transcription of DNA occur in the nucleus, translation occurs in the cytoplasm.

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CHAPTER 17: TRANSCRIPTION & TRANSLATION Steps in Protein Synthesis 1. Promoter binds to DNA & opens it up at a particular site 2. RNA polymerase "unzips" the molecule 3. The 3'5' DNA strand serves as the TEMPLATE for addition of new nucleotides 4. The Genetic Code of DNA is transcribed into messenger-RNA (mRNA) (A, U, C, G) 5. The mRNA detaches from DNA & is spliced by the spliceosome before leaving the nucleus 6. The newly spliced mRNA goes to ribosomes (sites of protein synthesis) in cytoplasm 7. MRNA attaches to the ribosome and the ribosome moves down the mRNA reading the

sequence of nitrogenous bases 8. Triplets (3) of nucleotides on the mRNA constitute a CODON 9. Transfer RNA (tRNA) is involved in the next stage of protein synthesis = TRANSLATION 10. AUG codon on mRNA = the INITIATOR CODON. AUG binds to the P site of the ribosome.

AUG helps align the tRNA, the mRNA, and the ribosome and then signals the beginning of the gene to be expressed.

11. tRNAs carry amino acids. Each tRNA has an ANTICODON that binds to a complementary

CODON on the mRNA 12. A 2nd tRNA molecule binds to the 2nd codon of the mRNA and a di-peptide is formed 13. The 1st tRNA is released from the “E site” of the ribosome 14. The process continues until a nonsense "STOP" codon is reached: UAA, UGA, UAG 15. The codons from mRNA have now been translated into a polypeptide 16. The mRNA, polypeptide, tRNA, and ribosome disassemble

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CHAPTER 17 KEY TERMS A site Aminoacyl-tRNA synthetase Anticodon Base-pair substitution Codon Deletion E site Exon Insertion Intron Messenger RNA (mRNA) Mutation P site Poly-A tail Primary transcript Promoter Reading frame Ribosomal RNA (rRNA) Ribosome RNA polymerase Signal peptide Signal recognition particle (SRP) TATA box Template strand Transcription Transfer RNA (tRNA) Translation Triplet code

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ABBREVIATIONS (CODONS) FOR ALL 20 AMINO ACIDS ALA - ALANINE *LEU - LEUCINE ARG -ARGININE *LYS -LYSINE ASN -ASPARAGINE *MET -METHIONINE (START) ASP -ASPARTIC ACID *PHE -PHENYLALANINE CYS - CYSTEINE PRO -PROLINE GLN -GLUTAMINE SER - SERINE GLU -GLUTAMIC ACID *THR -THREONINE GLY -GLYCINE *TRP -TRYPTOPHAN *HIS - HISTIDINE TYR - TYROSINE *ILE - ISOLEUCINE *VAL -VALINE A codon is a 3-letter mRNA sequence used to determine which amino acid should be added to the growing polypeptide. For example, AUG is the codon for Methionine &UGA is the code for “STOP”

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The following is a list of steps and components involved in DNA replication, transcription or translation. Put an X in the space for each component that is a part of replication, transcription or translation.

Components Involved Replication Transcription Translation

DNA mRNA

DNA DNA

mRNA proteins

Promoter

Origin of replication

AUG start codon & UAA, UAG, UGA stop codons

Helicase, ligase, primase,

Leading & lagging strands & Okazaki fragments

A, P, and E sites of ribosomes

DNA polymerase & ATCG

RNA polymerase & AUCG

tRNA & amino acids

5’ Guanine cap, 3’ poly-adenine tail, introns & exons