Biology 100/101
Lecture 14
DNA to RNA to Protein
(Print Version)

Announcements

Objectives

Web Resources

Overview

Transcription

RNA Processing

3 Types of RNA

Genetic Code

Translation

Examples

Post-Translation

Summary

Lecture Syllabus

IB 100/101 Home Page


Announcements

Text readings in Hoefnagels

Chapter 12: Gene Function, Gene Regulation, and Biotechnology

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Objectives:

The content of today's lecture will help you answer the questions on these assignments:

After studying this material you should be able to:

  1. Draw a diagram, create a concept map, or write a paragraph that explains the relationships among these terms:

    2. chromosome allele gene expression
    trait DNA RNA polymerase
    messenger RNA transfer RNA ribosomal RNA
    codons anticodons ribosomes
    transcription translation RNA processing
    amino acids polypeptides protein
    protein folding primary structure quaternary structure

  2. Explain how the sequence of DNA nucleotides for a specific allele, such as the allele for sickle cell disease or the allele for cystic fibrosis, is related to the production of a specific protein.

  3. Explain transcription and translation, and the roles of RNA polymerase, messenger RNA, transfer RNA, ribosomal RNA, and ribosomes in carrying out these two processes.

  4. Explain, in general terms, how the order and kinds of amino acids that make up a protein determine its final conformation and, ultimately, its function.

Web Resources:

Protein Synthesis: An Overview

Example of the production of breast milk (Hoefnagels page 62, Figure 3.15)

DNA
(Gene
Allele)		 -- Transcription
(in nucleus)		 --> RNA -- Translation
(in cytoplasm)		 --> Protein

Transcription: DNA to RNA

  • Transcription occurs in the nucleus.

  • Transcription is the process by which RNA is assembled from a DNA template by the enzyme RNA Polymerase.

    • Roger Kornberg Wins Nobel Prize in Chemistry for his work on RNA Polymerase

  • Transcription DNA --> RNA From DNA Interactive - a MUST SEE!!!!

    • Chose "Copying the Code" toward the bottom of the screen

    • then select "puting it together" from the top of the next screen.

    • Then choose the "Transcription animation"

  • Transcription is the synthesis of a molecule of RNA that is complementary in nucleotide sequence to one side (the transcribed or template side) of a section of the DNA double helix (that would be an allele for a specific trait). The information is copied, but in a complementary form:

    • C in the RNA is complementary with G in the DNA

    • G in the RNA is complementary with C in the DNA

    • A in the RNA is complementary with T in the DNA

    • U not T in the RNA is complementary with A in the DNA

  • DNA vs. RNA (Figure 12.2, in Hoefnagels, page 237). RNA is a single-stranded molecule, its nucleotides have the sugar ribose instead of deoxyribose and the nucleotide base uracil instead of thymine.

  • Transcription Factors (Figure 12.11, in Hoefnagels, page 246) are protein molecules that determine which genes are expressed in which tissues at which stages of development. The promotor, a control sequence near the start of the gene, attracts a binding protein and then other transcription factors. It tells the enzyme RNA polymerase where to bind and begin making RNA.

  • Enzymes unwind the DNA strand, and RNA polymerase builds the RNA chain using the transcribed strand of the DNA double helix as a template.

  • DNA, RNA, and the Transcribed (Template) Strand of DNA (Figure 12.3, in Hoefnagels, page 238).

  • The Three Stages of Transcription(Figure 12.3, in Hoefnagels, page 238). Many identical copies of RNA are simultaneously transcribed, with one RNA polymerase starting after another. RNA is relatively short-lived, so a cell must constantly transcribe certain genes to maintain supplies of essential proteins.

    • RNA Processing

  • RNA undergoes processing in the nucleus after transcription.

    • Messenger RNA Processing(See Figure 12.4 in your Hoefnagels text.)

    • A "cap" is added to the 5' end of the molecule, and a "poly-A tail" is added to the 3' end. (Think of this as a "hall pass," permitting the molecule to leave the nucleus.)

    • Noncoding sequences called introns are removed. Introns (intervening or noncoding sections of DNA) produce sections of RNA that are removed by enzymes, leaving only the sections of RNA produced by exons in the DNA to be put back together.

    • The messenger RNA is now "mature" and can exit the nucleus. RNA molecules move into the cytoplasm via nuclear membane pores.

    Three Types of RNA are Produced by Transcription of Specific Genes

    • Messenger RNA (mRNA) is a complimentary copy of one strand (the template, or transcribed strand) of a section of a DNA molecule making up an allele. It acts as a messenger to carry information stored in the DNA in the nucleus to the cytoplasm where the ribosomes on the Endoplasmic Reticulum can translate it to synthesize protein molecules. Each three mRNA bases in a row forms a Codon (from accessexcellence.org) that specifies a particular amino acid.

    • Transfer RNA (tRNA) (see Hoefnagels text fig. 12.6, pg. 241) is small and has a very specific secondary and tertiary structure such that it can bind an amino acid at one end and mRNA at the other. It carries each amino acid to the ribosome. tRNA contains a sequence of 3 nucleotide bases at one end of the molecule called an anticodon. This Anticodon (from accessexcellence.org) is complementary to a particular codon of an mRNA molecule.

    • Ribosomal RNA (rRNA) is one of the structural components of a Ribosome (see Hoefnagels  text fig. 12.7, pg. 241). Ribosomes structurally support and catalyze protein synthesis. In eukaryotes, a ribosome has two subunits (large and small), containing 82 proteins and four rRNA molecules all together.

    • Three Scientists Awarded Nobel prize in Chemistry for Research on the Structure of Ribosomes.

    The Genetic Code

    (Hoefnagels Text table 12.2, pg. 240)

    The Genetic Code (from accessexcellence.org), for the translation of codons to amino acids

    • Three consecutive bases in a mRNA molecule form a Codon (from accessexcellence.org) that is a code for one amino acid.

    • The code is redundant, with some amino acids having more than one codon. For example, the codons GCU, GCC, GCA, and GCG all code for alanine (Ala).

    • A change in the first or second bases of a codon are more likely to affect the "meaning" of a codon than a change in the third base.

    • The codon AUG starts translation, and the codons UGA, UAA, and UAG stop translation.

    Translation: RNA to Protein

    • Translation occurs in the cytoplasm at the ribosomes on the E.R.

    • Translation is the process by which the information carried in messenger RNA is used to direct the synthesis of a polypeptide. See Fig. 12.8, pg. 242 in Hoefnagels  text.

    • Translation mRNA --> Protein From DNA Interactive - a MUST SEE!!!!

      • Chose "Reading the Code" toward the bottom of the screen

      • then select "puting it together" from the top of the next screen.

      • Then choose the "Translation animation"

    • The Three Stages of Translation

      • Initiation: the first mRNA codon AUG forms a complex with an initiator tRNA (carrying the amino acid methionine) and the small ribosomal subunit. See Fig. 12.8, pg. 242 in Hoefnagels  text.  The large ribosomal subunit then joins this complex to begin the next stage.

      • Elongation: the stepwise addition of amino acids to a growing polypeptide chain. The amino acids are carried to the ribosome by the tRNAs. The ribosome moves along the mRNA one codon at a time, transferring new amino acids to the growing polypeptide chain via peptide bonds. See Fig. 12.8, pg. 242 in Hoefnagels  text.

      • Termination: elongation stops at an mRNA stop codon (UGA, UAA, UAG), and the new polypeptide is released. The ribosome breaks into its large and small subunits, releasing the new protein and the mRNA. See Fig. 12.8, pg. 242 in Hoefnagels  text.

    • Several ribosomes (polyribosomes) can translate the same mRNA molecule simultaneously. See fig. 12.9, pg. 244 in Hoefnagels  text.
      Chaparone proteins help guide the folding of the new protein (polypeptide).

Examples of Transcription and Translation

  • Relationship among DNA, mRNA, and amino acid sequences from from Dr. Robert J. Huskey.

    • In this illustration the transcribed strand of the DNA is the upper line of letters (TAC CAC, etc).

    • Note that the mRNA sequence looks very much like the non-transcribed side of the DNA except, of course, that there are U's in the RNA and T's in the DNA.

  • RNA transcript of the beta-globin gene and corresponding amino acid sequence, from from Dr. Robert J. Huskey.

    • Here you can see the entire transcript (new mRNA molecule) just as it is produced by the RNA polymerase from the transcribed strand of DNA.

    • The introns, magenta colored sections, are cut out by enzymes in the nucleus.

    • The exons, the black sections, are spliced back together by other enzymes and sent out to the ribosomes for translation.

    • The abreviations of the amino acids are lined up with the codons in the exons so you can see the primary structure of the protein beta-globin.

Post-translation

Summary

  • Transcription and Translation - Graphic Representation from Dr. Robert J. Huskey.

    • Genes (DNA) are transcribed into RNA by the enzyme RNA polymerase. This process is controlled by proteins called transcription factors.

    • Prior to leaving the nucleus, the RNA is processed. To mRNA, a cap and tail are added and noncoding sequences (introns) are removed.

    • In the cytoplasm, mRNA molecules are translated by ribosomes (rRNA + ribosomal proteins) which match the 3-base codons of the mRNA to the 3-base anticodons of the appropriate tRNA molecules. The first AUG codon initiates translation, the message is read three consecutive bases at a time, and translation ceases when a stop codon is reached.

    • Newly synthesized proteins are often modified after translation, so that they can do their job properly.

  • Protein Synthesis from Access Excellence: