I. Gene Expression Figure 1: Central Dogma of Molecular Biology

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1 I. Gene Expression Figure 1: Central Dogma of Molecular Biology Central Dogma: Gene Expression: RNA Structure RNA nucleotides contain the pentose sugar Ribose instead of deoxyribose. Contain the bases adenine, guanine, cytosine, & Uracil, not thymine. RNA is single stranded. Types of RNA differ in abundance, structure, function & size: a) rrna: most abundant (80%) & largest of all varieties of RNA ( bp). b) trna: comprise 15% of all cellular RNA; size ranges between 75-90bp. c) mrna: least abundant (5%); size is variable.

2 Figure 2: RNA vs DNA RNA Nucleotide RNA vs DNA Structure II. Gene Structure Figure 3: Gene Structure The Promoter, located upstream from the start end of a gene, contains the base sequence TATAAT called a TATA Box. The promoter marks the beginning of the gene & the site of attachment for the proteins involved in transcription. The region of a gene containing the stretch of DNA that codes for a polypeptide is called a Transcriptional Unit. At the end of the transcriptional unit is a Terminator that signals the end of a gene sequence. Introns increase the probability for crossing over between genes of homologous chromosomes leading to the formation of new genes & polypeptide products.

3 Exon: Intron: III. Mechanism of Gene Expression Stages of Transcription Figure 4: Transcription (DNA mrna) Initiation: Elongation: Termination:

4 Messenger RNA Figure 4.1: Primary mrna Transcript & Codons The end product of transcription is an unrefined Primary mrna Transcript that is complementary to the original DNA (gene) template. The Coding Segment contains the actual message for the polypeptide. Within the coding sequence, 3 consecutive bases constitute a Codon, which specifies a single amino acid. A Start Codon signals the beginning of the coding sequence whereas a Stop Codon signals the end. Figure 4.2: The Genetic Code (mrna Codons)

5 Messenger RNA Processing Figure 4.3: 5 CAP & Poly-A-Tail Before leaving the nucleus, the mrna must be modified so that it will survive in the cytoplasm long enough to be translated into a polypeptide. First, its 5 end is capped with a form of the guanine nucleotide. This 5 CAP protects the mrna from being broken down by hydrolytic enzymes. To the 3 end is attached a Poly-A Tail consisting of adenine nucleotides. The poly A tail also protects the mrna from being broken down in the cytoplasm. RNA polymerase transcribes both introns & exons. Particles called Spliceosomes remove introns from the primary transcript & splices the exons together to form a final transcript that is much shorter than the original gene sequence. Figure 8: mrna Splicing

6 Translation Transfer RNA: Figure 5: Transfer RNA Complementary base-pairing within each trna molecule causes it to fold. Three or more loops of unpaired nucleotides form, one of which contains a sequence called an Anticodon, which is complementary to certain codons on mrna. The anticodon enables the genetic code to be deciphered, whereby appropriate amino acids are transferred to the ribosome in a specific sequence to synthesize a polypeptide. Ribosome Structure Ribosome: Figure 6: Ribosome Structure

7 During translation, the mrna fits in a groove between the contact surfaces of the two subunits. In addition to the binding site for mrna, ribosomes have 3 binding sites for amino-acyl trnas: a) P Site: holds the trna carrying the growing polypeptide chain. b) A Site: site where trnas first enter the ribosome. c) E Site: site from which discharged trnas (those that have released their amino acids) leave the ribosome. Stages of Translation (mrna Polypeptides) Figure 7: Initiation Initiation: mrna attaches to the small subunit of the ribosome. The initiator trna met attaches to the start codon AUG on the mrna molecule. This is followed by the attachment of the large subunit.

8 Figure 7.1: Elongation & Translocation Elongation: amino acids are added one by one to the initial amino acid. An anticodon on trna reads the codon of mrna exposed in the ribosome s A site. If complementary, the amino acid carried by the trna will form a peptide bond with the amino acid of the trna occupying the P site. Translocation: the trna in the P site dissociates from the ribosome, & the trna in the A site, carrying the growing polypeptide, is shifted to the P site. This movement brings into the A site the next codon to be translated on the mrna. Figure 7.2: Termination

9 Termination: elongation continues until a stop codon (UAA, UAG, & UGA) reaches the A site of the ribosome. A protein called a release factor binds to the termination codon in the A site, causing the polypeptide chain to release from the trna. This hydrolysis initiates the dissociation of the ribosomal subunits as well as the release of the mrna transcript. The liberated polypeptide chains will undergo a modification process as amino acids may be modified by the addition of sugars, lipids, phosphate groups, etc. The chain will assume a specific conformation with the help of molecular chaperones & join with other polypeptide chains to form a functional protein or enzyme.