Types of RNA

Essential Question

How is RNA different from DNA, and why does that matter?

DNA stays safe in the nucleus, but proteins are built at ribosomes in the cytoplasm. RNA is the molecule that bridges the two. In this lesson you compare RNA with DNA, learn the three main types of RNA, and see how information flows from DNA to RNA to protein.

RNA Compared With DNA

• RNA (ribonucleic acid) is a nucleic acid made of nucleotides, like DNA.
• RNA has the sugar ribose, and DNA has the sugar deoxyribose.
• RNA has the base uracil (U) in place of thymine (T). In RNA, A pairs with U.
• RNA is usually a single strand, and DNA is a double helix.
• RNA molecules are much shorter than a DNA chromosome, because each one is a copy of only a gene or a small part of it.

The differences in sugar, base, and strand number make RNA a temporary working copy, while DNA is the long-term storage.

Three Main Types of RNA

• Messenger RNA (mRNA) carries the instructions from a gene. Its bases are read in groups of three called codons.
• Transfer RNA (tRNA) carries one amino acid. It has an anticodon, a set of three bases that pairs with a codon on the mRNA.
• Ribosomal RNA (rRNA) combines with proteins to make ribosomes, the structures that build proteins.
• Cells also make other kinds of RNA, but these three are the ones needed to build a protein.

mRNA carries the message, tRNA brings the amino acids, and rRNA builds the ribosome where the protein is made.

From DNA to Protein

• Information flows from DNA to RNA to protein.
• Transcription makes an mRNA copy of a gene. Translation uses the mRNA to build a protein. You will study each in the next lessons.
• In eukaryotes, transcription takes place in the nucleus and translation takes place at ribosomes in the cytoplasm.
• Prokaryotes have no nucleus, so both steps happen in the cytoplasm.
• Proteins carry out most of the work of cells, so the sequence of a gene ends up affecting the traits of an organism.

A gene in DNA is copied into mRNA, and the mRNA is used to build a protein.

Why a Temporary Copy Helps

• DNA is the master copy and stays in the nucleus. Using RNA copies protects it from damage.
• One gene can be copied many times, so a cell can make a lot of one protein quickly.
• mRNA does not last forever, so a cell can change which proteins it makes.
• All cells in an organism have the same DNA, but different cell types use different genes. This is how a liver cell and a muscle cell can differ.

Different cells use different genes, so they make different mRNAs and different proteins from the same DNA.

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Worked Examples

Try each problem on your own first, then open the solution to check your work.

Example 1: DNA or RNA?

Three nucleic acid samples were analyzed (percent of each base). These are sample values for practice. For each sample, decide whether it is DNA or RNA and whether it is likely single-stranded or double-stranded. Write your answer as Claim, Evidence, Reasoning.

Sample	% A	% T	% U	% G	% C
Sample 1	30	30	0	20	20
Sample 2	26	0	31	24	19
Sample 3	22	29	0	21	28

Show the solution

1. Claim: Sample 1 is double-stranded DNA, Sample 2 is RNA, and Sample 3 is DNA that is probably single-stranded.
2. Evidence: Sample 1 has T and no U, with A = 30 and T = 30, and G = 20 and C = 20. Sample 2 has U and no T. Sample 3 has T and no U, but A = 22 does not match T = 29 and G = 21 does not match C = 28.
3. Reasoning: DNA contains thymine and RNA contains uracil, which identifies the type. In a double strand, A pairs with T and G pairs with C, so their amounts match. Sample 1 matches. In Sample 3 the amounts do not match, so it is probably single-stranded. Sample 2 has U but its A (26) and U (31) do not match, and G (24) and C (19) do not match, which fits a single-stranded RNA.
4. Note: the pairing pattern only tells us about the strands when a molecule is double-stranded.
5. Answer: Sample 1: double-stranded DNA. Sample 2: RNA. Sample 3: DNA, probably single-stranded.

Example 2: Match the RNA to the Job

Use the table. Write mRNA, tRNA, or rRNA for each description.

Description	Which RNA?
Has an anticodon that pairs with a codon
Is a part of the ribosome
Is read three bases at a time as codons
Carries one amino acid

Show the solution

1. An anticodon is found on tRNA, where it pairs with a codon on mRNA.
2. Ribosomes are made of rRNA and proteins.
3. Codons are groups of three bases on mRNA.
4. Each tRNA carries one amino acid.
5. Answer: tRNA, rRNA, mRNA, tRNA.

Example 3: Base Percentages in RNA

A 100-nucleotide RNA molecule has 22 adenine, 31 uracil, and 26 guanine nucleotides. How many cytosine nucleotides are there, and what percent of each base is present? Does this RNA follow the rule A = U and G = C?

Show the solution

1. Add the known bases: 22 + 31 + 26 = 79.
2. Cytosine = 100 – 79 = 21 nucleotides.
3. With 100 nucleotides, each count is also its percent: A 22%, U 31%, G 26%, C 21%.
4. A (22) does not equal U (31) and G (26) does not equal C (21), so the rule does not apply. That fits a single-stranded molecule, where bases are not paired across two strands.
5. Answer: C = 21 nucleotides (21%). The RNA does not follow A = U and G = C, as expected for a single strand.

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Practice

Use the practice worksheet to compare DNA and RNA, match RNA types to their jobs, and use base percentages to tell DNA from RNA.

• Types of RNA Practice Worksheet (PDF)
• Types of RNA Practice Answer Key (PDF)

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Lesson Slides

• 9.4 Types of RNA Slides (PDF)

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Big Ideas

• RNA has ribose, uracil instead of thymine, and is usually single-stranded.
• mRNA carries a gene’s message, tRNA brings amino acids, and rRNA helps make ribosomes.
• Information flows from DNA to RNA to protein.
• Different cell types use different genes, so they make different RNAs and proteins from the same DNA.

Back to the essential question: RNA differs from DNA in its sugar, one base, and strand number. These differences make RNA a short-lived working copy, so DNA can stay protected while cells make the proteins they need.

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