Lesson 14: How DNA and RNA Code for Proteins (3.1.2)
All living things have in common several distinctive characteristics including the production of protein. Cells consist of a variety of proteins whose function is determined by its shape. In this lesson you will learn how DNA and RNA code for proteins that determine our traits. DNA (deoxyribonucleic acid) directs the activities of the cell and contains the sugar deoxyribose. RNA (ribonucleic acid) is involved in protein synthesis and contains the sugar ribose.
THE STRUCTURE OF RNA
Ribonucleic acid (RNA) is a molecule used to translate the code from the DNA molecule into protein. It is similar to DNA, except it is single stranded. Its sugar is ribose, and both RNA and DNA contain adenine, guanine, and cytosine. In RNA thymine is replaced with uracil (U).
There are three types of RNA and each has its own function. Messenger RNA (mRNA) is a linear molecule responsible for carrying the DNA code from the nucleus to the ribosomes. A group of three bases found on the mRNA molecule is called a codon. The second type of RNA is transfer RNA (tRNA) responsible for transporting amino acids and a group of three bases called an anticodon to the ribosomes. The third type of RNA is called ribosomal RNA (rRNA) and is responsible for bringing mRNA and tRNA together to assemble protein in the ribosomes. Table 14.1 contrasts DNA and RNA.
Table 14.1 DNA vs RNA
DNA / RNASugar / Deoxyribose / Ribose
Bases / G, C, A, T / G, C, A, U
Number of Strands / Two / One
Shape / Double Helix / mRNA - linear
tRNA- clover (“t”) shaped
rRNA – globular
PROTEIN SYNTHESIS
The molecule below represents a single stranded molecule of mRNA and sample codons that might make up the molecule. Pairing of the codons on mRNA with anticodons on tRNA follows the complimentary base pairing rule for RNA: A always pair with U and ‘G’ always pair with ‘C.’
TRANSCRIPTION
The first step of protein synthesis is the manufacture of mRNA during the process called transcription. Transcription occurs in the nucleus and is very similar to DNA replication. The process of transcription begins when weak hydrogen bonds in a region of the DNA are broken and the double helix unwinds and separates as shown in Figure 14.3. The separated segment of DNA represents a gene and it serves as a template for the soon-to-be-formed mRNA strand. Free floating nucleotides of RNA complimentary base pair with the exposed bases of DNA with the help of the enzymes. This continues until the entire gene segment has been paired, and a complete RNA strand has been formed in the nucleus of the cell. The cell then alters this RNA into one of three forms, mRNA, tRNA, or rRNA, according to the cell’s needs. The newly formed mRNA has a sequence complementary to the original DNA segment. mRNA separates and leaves the nucleus, moving out into the cytoplasm to settle on the ribosome, an organelle composed of rRNA.
TRANSLATION
Translation is the step in protein synthesis where mRNA is decoded (translated) and a sequence of amino acids are linked together to form a polypeptide which is folded into a protein. Let’s look at the “language” of mRNA.
One way to think of a strand of mRNA is as a chain of nucleotides, as in:
AUGACAGAUUAG
While this is correct, another way of thinking of the chain is that is divided into a segments consisting of three nucleotides called a codon, as stated earlier in the description of mRNA.
AUG ACA GAU UAG
Writing the mRNA code in this way emphasizes the importance of codons in the sequence of amino acids.
The codons in the chart in Figure 7.6 above are written in the RNA code and not the DNA code. Each of the abbreviations in the chart represents one of the twenty amino acids. To read the chart you must look at a single codon such as AUG. A is the first base, U is the second base and G is the third base. To find the amino acid, locate the first base in the row to the left of the chart. Locate the second base at the top of the chart to find the column. Locate the third base to the right of the chart. Where the first, second and third base meet in the grid, the corresponding amino acid will be found.
The codon AUG codes for the start codon methionine (MET). When the start codon bonds to the surface of the ribosome, it initiates the beginning of protein synthesis. It is possible for other codons to line up ahead of the start codon; however, protein synthesis does not begin until the start codon is read by the ribosomes. Transfer RNA is located in the cytoplasm. Each tRNA molecule has a three-part nucleotide segment called an anticodon, which will pair up with one mRNA codon. An amino acid is found on the opposite end of tRNA. When mRNA’s start codon is read by the ribosomes, it signals tRNA molecules to transfer amino acids that are coded in the complimentary mRNA codon as each is read by the ribosomes. The ribosomes facilitates this process by moving along the mRNA chain until it reaches a stop codon, a three-nucleotide segment that tells the ribosome that the translation process is complete. As amino acids are deposited, they are linked together by peptide bonds to form a polypeptide. The ribosome then releases the newly-formed polypeptide chain which is folded into a protein and moves out into the cell as a fully functioning polypeptide (protein). The function of this protein is determined by the structure of the polypeptide (amino acid sequence) and the way it folds on itself. The big idea is “form fits function” or the shape of the protein determines its function.
PROTEIN SYNTHESIS
The manufacture of proteins involves the three basic steps that are carried out by the three types of RNA, as shown above in Figure 14.5. The basic steps are:
· The DNA code of the gene segment must be copied in the nucleus of the cell. (Transcription)
· The code must then be carried from the nucleus into the cytoplasm and finally to the ribosome.
· The protein is then assembled from the code and released from the ribosome (Translation)
Lesson 14 Review: How DNA and RNA Code for Proteins
A. Define the following terms.
DNA nucleotide ribose amino acid
gene deoxyribose transcription translation
RNA base messenger RNA (mRNA) thymine
anticodon ribosomes ribosomal RNA ( rRNA) start codon
protein synthesis Adenine Transfer RNA (tRNA) codon
polymerase cytosine complimentary pairs stop codon
guanine uracil
B. Choose the best answer
1. Protein synthesis begins with the manufacture of a molecule of
A. mRNA C. tRNA
B. rRNA D. nucleotide
2. What are ribosomes made of?
A. mRNA C. tRNA
B. ATP D. protein
3. Proteins are made up of polypeptide chains. Polypeptide chains are composed of
A. mRNA C. tRNA
B. rRNA D. amino acids
4. What does transfer RNA (tRNA) carry?
A. the mRNA to the ribosome C. an amino acid to the ribosome
B. the nucleotide bases to the mRNA D. an amino acid to the cytoplasm
5. Which of the following is the last step in protein synthesis?
A. tRNA bonds to an amino acid in the cytoplasm
B. The stop codon binds to the ribosome and the polypeptide is released
C. DNA unravels to expose a gene segment
D. mRNA bonds to tRNA
C. Answer the following questions.
1. Which sugars are found in DNA and RNA?
2. What are proteins made of?
3. What role does DNA play in protein synthesis?
4. Discuss the role of transcription and translation in the process of making proteins.