Site hosted by Build your free website today!

Protein Synthesis

Reading Assignment - Mader: Chapter 14

  1. Introduction
    DNA is like a book of instructions written in the alphabet: A,T,C,G. Merely knowing the letters does not tell how the genes work

  2. One Gene, One Polypeptide
    In the early 1900’s, Sir Archibald Garrod studied a metabolic disorder and concluded that genes function through the synthesis of a specific enzyme; intermediate molecules collected when an enzyme was defective and can lead to disease; Garrod used the terms inborn errors of metabolism to describe these genetic defects

    In 1940, Beadle and Tatum studied the bread mold, Neurospora crassa and came to the same conclusion

    1. the mold would normally grow on a minimal medium of sucrose, salt, and biotin (a vitamin)
    2. a nutritional mutant was found that also required B6; another was found that required B1
    3. each inherited mutant corresponded to a defective gene hence, one gene codes for one enzyme

    Studies of sickle cell hemoglobin by Linus Pauling and Harvey Itano used electrophoresis to show chemical differences between normal and sickle cell hemoglobin. Vernon Ingram showed that abnormal hemoglobin has a nonploar valine (VAL) instead of a negatively charged glutamic acid (GLU)

    1. because two genes code for hemoglobin, the theory was modified by Pauling and Itano to: one gene:one polypeptide
    2. thus, one gene codes for the amino acid sequence in a polypeptide

  3. The Path from Genes to Proteins
    A. RNA
    1. In the previous lesson you found that Watson and Crick discovered that DNA was a double helix and that during replication one strand serves as a template for the new, complementary strand
    2. because protein synthesis occurs in the cytoplasm of eukaryotes, another molecule must carry the instructions of the DNA to the site of protein synthesis - RNA or ribonucleic acid
    3. RNA consists of a single strand of nucleotides
    4. each nucleotide consists of a ribose sugar, a phosphate group, and one of four bases, A, C, G, and Uracil (U) instead of thymine
    5. thus, RNA could form using a portion of DNA as a template

  4. Overview of Protein Synthesis Go to DNA Workshop Activity and do the Protein Synthesis animation.
    1. The retrovirus responsible for AIDS is an exception to direction of information flow
    2. During transcription, a portion of DNA unwinds and serves as a template to produce an RNA transcript
    3. Each region of DNA can be transcribed thousands of times during the life of a cell
    4. During translation, three types of RNA convert the message of DNA into the sequence of amino acids in a polypeptide

    1. Ribosomal RNA (rRNA) combines with proteins to form the ribosomes
    2. Messenger RNA (mRNA) carries the “blueprint” to the ribosome
    3. Transfer RNA (tRNA) brings the correct amino acid to the ribosomal position of messenger RNA required for protein synthesis

  5. Transcription
    A. Synthesis of RNA
    An RNA transcript is complementary to its DNA template
    B. RNA Transcripts
    1. Exons are parts of a gene that are translated; it is a portion of the DNA code in the primary mRNA transcript
    2. Introns, or intervening DNA, are parts of a gene that do not get translated into a polypeptide; they are removed by sliceosomes before the mRNA leaves the nucleus
    3. Because exons and introns are both transcribed into RNA, the primary RNA transcript contains more than the code for the primary structure of the polypeptide
    4. After the messenger RNA leaves the DNA gene section, a“cap” is added to the front (5’ end) and a “tail” is added to the (3’)end of the molecule
    5. The cap (a modified guanine) may function as a start signal for translation; the function of the tail, which is made of a 150-200 adenine nucleotide chain, is to ease the transport of mRNA from the nucleus and to prevent its breakdown by catabolic enzymes
    6. The introns are removed by enzymes and the exons are spliced together by other enzymes, forming the mature mRNA transcript

  6. Translation
    A. The Genetic Code
    1. Nucleic acids must construct “words” from four kinds of nucleotides to designate each of the twenty amino acids found in a polypeptide chain
    2. A sequence of three nucleotides (a triplet) provides 64 choices (43), more than enough to specify twenty amino acids
    3. Crick, Brenner, and others deduced that the nucleotide bases are read three at a time and that a “start” signal establishes the correct “reading frame”
    4. The genetic code consists of 61 triplets that specify amino acids and 3 that serve to stop protein synthesis
    5. Each triplet that codes for an amino acid is called a CODON
    6. The code is universal for all life forms with few exceptions

    B. Codon-Anticodon Interaction
    1. Each kind of tRNA has an ANTICODON that is complementary to a mRNA CODON
    2. After the mRNA arrives in the cytoplasm, an anticodon on a tRNA bonds to the codon on the mRNA, thus a correct amino acid is brought into position
    3. Often, only the first two bases of an anticodon must be precisely complementary; the third base may vary - the “wobble” effect

    C. Ribosome Structure
    1. A eukaryotic cell may have thousands of ribosomes, and each has two parts
    2. The large and small subunit are both made of rRNA plus protein
    3. A cluster of ribosomes on the same mRNA is called a polyribosome or polysome

    D. Stages of Translation
    1. A complex of ribosomal units, mRNA, and tRNA forms
    2. Transfer RNAs bring appropriate amino acids to the site
    3. An enzyme forms a peptide bond between the newly arrived amino acid and the growing polypeptide chain
    4. The bond between the old amino acid and its tRNA is broken, and the old tRNA leaves the complex
    5. Once a stop codon is reached the polypeptide chain detaches
    6. Enzymes, called release factors, are involved in the detachment process
    7. The polypeptide can join the cytoplasmic pool of proteins or be further processed by the cytomembrane system

    Overview of the process of Protein Synthesis

    Some descriptions of difference between TRANSCRIPTION and TRANSLATION:
    Transcription involves the transfer of information from one form to another in the same language, for example, an office memo in shorthand is transcribed into typed copy but both in English; likewise a section of genetic code in DNA is copied to RNA (both nucleic acids).

    Translation is the transfer of information in one language to another language, for example, a story in French to English; likewise, genetic code in RNA is transferred to amino acids (nucleic acid to protein).

  7. Mutation and Protein Synthesis
    A. Mutation at the Molecular Level
    1. A gene mutation is a change in one to several bases in the nucleotide sequence of DNA
    2. Bases can be added, deleted, or replaced
    3. Mutagens include viruses, chemicals, and ultraviolet radiation
    4. Spontaneous mutations can arise from replication errors, or can be “frameshift mutations” due to insertions or deletions
    5. Genetic instructions read incorrectly due to frameshift result in abnormal proteins

    B. Mutation Rates
    1. The average rate is one mutation/gene/million replications
    2. Because it is unlikely for two genes to mutate at the same time, two antibiotics may be used together

    C. Mutation and Evolution
    1. While mutations are rare and most are harmful, beneficial mutations have been selected for by evolution
    2. Some mutations have resulted in genetic regions with no known function

    D. Perspective
    1. All life on earth shares the same chemical heritage - DNA is the source of the unity of life
    2. Mutations, genetic recombinations, and other mechanisms are the source of life’s diversity
    3. The changing environment is the testing ground for the novel combinations of DNA that appear

  8. The Importance of Genetic Control
    A. Because all cells in your body have the same genetic instructions, only a relatively small number of genes are active at any given time in any given tissue (e.g. only red blood cells activate hemoglobin genes)
    B. Gene control occurs through molecules that interact with DNA, RNA, or polypeptide chains. These molecules include enzymes and hormones

  9. Mechanisms of Regulation
    A. Most controls affect the role of transcription
    B. Most regulators are protein
    C. In vertebrates, hormones and other signaling molecules alter gene expression
    D. Genes control the rate of cell division
    1. the rate of cell division for different tissues varies with conditions
    2. When controls are lost, cancer can occur