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

Text Readings in Lewis et al.	Testing Your Knowledge	Thinking Scientifically
Chapter 13, Gene Function	Pages 264-5, Questions 1-9	Page 266, Questions1-2

Answers to many of these questions can be found at the Text On-Line Learning Center

You may also ask questions and see answers to your classmates' questions in Web Crossing in the "Talk to Ross and Ed" discussion.

Objectives:

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:

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:

• DNA Interactive
• DNA Replication (Making sister chromatids during the S phase of the cell cycle)
• 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 "Replication animation"
• Transcription DNA --> RNA
• 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"
• Translation mRNA --> Protein
• 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"

• DNA From The Beginning
  Everything you would ever want to know about DNA with understandable animations and tutorials.
• Human Biology (BIOL121) from the University of Virginia, with lots of nice graphics.
• Transcription and Translation, from the University of Virginia. CAUTION! Music plays!
• What are genes? from Access Excellence
• Essential Study Partner, detailing Protein Synthesis, accessed from the Text On-Line Learning Center. Choose Genetics, then Protein Synthesis.

Protein Synthesis: An Overview

Transcription: DNA to RNA

• Transcription occurs in the nucleus.
• Transcription in the process by which RNA is assembled from a DNA template.
• 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"
• Essential Study Partner detailing the process of transcription.
• 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 13.2, in Lewis et al., page 245). RNA is another nucleic acid found in cells. It is a single-stranded molecule, its nucleotides have the sugar ribose instead of deoxyribose and the nucleotide base uracil instead of thymine.
• Transcription Factors 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 13.6, in Lewis et al., page 247).
• The Three Stages of Transcription (Figure 13.7, in Lewis et al., page 247). 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.
• An animated graphic from the University of Texas, Austin:
  
  • The orange and blue strand is the double helical DNA molecule
  • The blue portion is the section of DNA being transcribed (again and again and again...)
  • The light blue moving ball is a molecule of RNA polymerase
  • The yellow strands are single stranded molecules of RNA

RNA Processing

• Messenger RNA Processing (See Figure 13.10 in your 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.

The Three Types of RNA

• Messenger RNA (mRNA) is a complimentary copy of one strand (the 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 E. R. can translate it to synthesize protein molecules. Each three mRNA bases in a row forms a Codon that specifies a particular amino acid.
• Transfer RNA (tRNA) 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 is complementary to a particular codon of an mRNA molecule.
• Ribosomal RNA (rRNA) is one of the structural components of a Ribosome. 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 (See Fig. 13.8, text).

The Genetic Code

• Three consecutive bases (a Codon) in a mRNA molecule 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).
• The first and second bases of each codon are more important than the last.
• 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. 13.11 in your 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"

• Essential Study Partner detailing the process of translation.
• Movie: The Three Stages of Translation, from the University of Virginia.
• 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. 13.14, in your 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. 13.15, in your 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. 13.15, in your text.

Examples of Transcription and Translation

• 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.

• 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

• Newly synthesized proteins are often modified after translation (post-translation) before they can carry out their function.
• Proteins fold into a specific 3-D structure (conformation) as they emerge from the ribosome. Other proteins (e.g., chaperone proteins) oversee the process of proper folding.
• They may join other polypeptide units to form a larger, functional protein.
• Errors in protein folding can cause illness, such as sickle cell disease or cystic fibrosis.
• For more information, see our last lecture.

Summary

• 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.