RNA

RNA

High School

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Learning Objectives

• Define the structure of RNA and compare it with the structure of DNA.

• Describe the main types of RNA and their roles in making proteins.

• Explain the process of transcription, including its stages and specific base-pairing rules.

• Summarize the changes mRNA undergoes after transcription before it is ready for translation.

Key Vocabulary

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Ribonucleic Acid (RNA)

A single-stranded nucleic acid that is essential for making proteins inside the cell.

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Transcription

The process of copying genetic info from DNA into a new molecule of messenger RNA.

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Messenger RNA (mRNA)

Carries the genetic code from DNA in the nucleus to the ribosomes for protein synthesis.

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Transfer RNA (tRNA)

Brings the correct amino acids to the ribosome during the process of building new proteins.

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Ribosomal RNA (rRNA)

A key molecular component of the ribosome, which is the actual site of protein assembly.

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What is RNA?

• RNA, or Ribonucleic Acid, is a single-stranded molecule vital for protein synthesis.

• An RNA strand has a phosphate group, a ribose sugar, and nitrogenous bases.

• Its four bases are Adenine (A), Guanine (G), Cytosine (C), and Uracil (U).

• Uracil replaces Thymine, and RNA is found anywhere inside a cell.

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Solved Example 1
During transcription, a DNA template strand has the sequence 3'-ATG-GCC-TAG-5'. What is the corresponding mRNA sequence that will be produced?

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Step 1: Analyze and Sketch the Problem

• Goal: Determine the mRNA sequence transcribed from a given DNA strand.

• Knowns: The DNA template strand sequence is 3'-ATG-GCC-TAG-5'.

• Unknown: The corresponding mRNA sequence.

• Transcription Pairing Rules: A (DNA) pairs with U (RNA), T (DNA) pairs with A (RNA), G (DNA) pairs with C (RNA), and C (DNA) pairs with G (RNA).

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Solved Example 1
During transcription, a DNA template strand has the sequence 3'-ATG-GCC-TAG-5'. What is the corresponding mRNA sequence that will be produced?

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Step 2: Solve for the Unknown

• Apply the base-pairing rules to each DNA base to find the complementary mRNA base.

• For DNA 3'-A-T-G-5', the mRNA will be 5'-U-A-C-3'.

• For DNA 3'-G-C-C-5', the mRNA will be 5'-C-G-G-3'.

• For DNA 3'-T-A-G-5', the mRNA will be 5'-A-U-C-3'.

• Combine the transcribed codons to get the full mRNA sequence.

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Solved Example 1
During transcription, a DNA template strand has the sequence 3'-ATG-GCC-TAG-5'. What is the corresponding mRNA sequence that will be produced?

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Step 3: Evaluate the Answer

• The final mRNA sequence is 5'-UAC-CGG-AUC-3'.

• To verify the answer, check the base pairings (A with U, T with A, G with C) and the direction. The 3' to 5' DNA template correctly produces a 5' to 3' mRNA strand. The sequence and direction are correct.

Multiple Choice

Which of the following is a key structural difference between RNA and DNA?

• RNA is only found in the nucleus, while DNA can be found anywhere in the cell.

• RNA contains the base Thymine, while DNA contains the base Uracil.

• RNA is a double-stranded helix, while DNA is single-stranded.

• RNA contains the sugar ribose and the base Uracil, while DNA has deoxyribose and Thymine.

Types of RNA

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Messenger RNA

• ​Copies genetic instructions from a DNA gene in the nucleus.

• ​​It then carries these instructions out of the nucleus.

• ​The instructions are delivered to the ribosomes in the cytoplasm.

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Transfer RNA

• ​This RNA molecule transports specific amino acids to the ribosomes.

• ​​It reads the coded message on the messenger RNA sequence.

• ​It brings the correct amino acid to the growing protein chain.

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Ribosomal RNA

• ​This is a structural component of the ribosomes themselves.

• ​​It helps to form the overall structure of the ribosome.

• ​It helps assemble amino acids into a functioning protein chain.

Multiple Choice

Which type of RNA is responsible for copying the genetic code from DNA and carrying it to the cytoplasm?

• dRNA

• rRNA

• mRNA

• tRNA

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What Is the Central Dogma?

• The Central Dogma describes the flow of genetic information: DNA → RNA → Protein.

• This two-step process of transcription and translation determines an organism's traits.

• Transcription copies a DNA segment into a complementary messenger RNA (mRNA) strand.

• In transcription, Adenine (A) pairs with Uracil (U), and Cytosine (C) pairs with Guanine (G).

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Multiple Choice

According to the Central Dogma, what is the primary role of transcription?

• To directly form a protein from a DNA template.

• To create a permanent, exact copy of the entire DNA molecule.

• To translate the RNA sequence into a chain of amino acids.

• To copy the genetic information from DNA into a strand of mRNA.

The 3 Steps of Transcription

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1. Initiation

• RNA polymerase binds to a specific region on the DNA called a promoter.

• This binding signals the DNA double helix to unwind and separate.

• This allows the enzyme to access and read one of the DNA strands.

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2. Elongation

• RNA polymerase moves along the unwound DNA, creating a complementary mRNA molecule.

• It adds nucleotides one by one, following the base-pairing rules.

• For a short time, the newly created mRNA is bound to the DNA.

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3. Termination

• Transcription ends when RNA polymerase reaches a specific stop sequence on the gene.

• The newly made mRNA strand then detaches from the DNA template.

• Finally, the process of transcription is completed, and the enzyme is released.

Multiple Choice

In which stage of transcription does the enzyme RNA polymerase bind to the promoter region of a gene?

• Elongation

• Splicing

• Initiation

• Termination

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Post-Transcriptional Modifications

• New mRNA must be processed before it can be used for translation.

• Splicing removes non-coding introns and joins the coding exons together.

• Editing alters nucleotides, allowing a single gene to make multiple proteins.

• A poly-A tail is added to protect the mRNA and signal its end.

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Solved Example 5
A pre-mRNA transcript is 2500 nucleotides long. Splicing removes two introns of 300 and 450 nucleotides, and polyadenylation adds a 150-nucleotide poly-A tail. What is the final length of the mature mRNA molecule?

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Step 1: Analyze and Sketch the Problem

• Goal: Determine the final length of the mature mRNA.

• Knowns: Initial length = 2500 nucleotides, Intron 1 length = 300 nucleotides, Intron 2 length = 450 nucleotides, Poly-A tail length = 150 nucleotides.

• Unknown: Final mRNA length.

• Formula: Final Length = Initial Length − Total Intron Length + Poly-A Tail Length

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Solved Example 5
A pre-mRNA transcript is 2500 nucleotides long. Splicing removes two introns of 300 and 450 nucleotides, and polyadenylation adds a 150-nucleotide poly-A tail. What is the final length of the mature mRNA molecule?

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Step 2: Solve for the Unknown

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Solved Example 5
A pre-mRNA transcript is 2500 nucleotides long. Splicing removes two introns of 300 and 450 nucleotides, and polyadenylation adds a 150-nucleotide poly-A tail. What is the final length of the mature mRNA molecule?

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Step 3: Evaluate the Answer

• The final mature mRNA is 1900 nucleotides long.

• This makes sense, as splicing shortens the transcript by removing introns, while polyadenylation adds a tail, resulting in a net change to the original length.

Multiple Choice

What is the purpose of 'splicing' the mRNA molecule after transcription?

• To remove non-coding introns and join the coding exons together.

• To attach the mRNA molecule to the ribosome for translation.

• To add a protective tail of adenine units to the end of the mRNA.

• To change some of the nucleotides to create different protein forms.

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Common Misconceptions About Transcription

Misconception	Correction
RNA is just a simple copy of DNA.	RNA has diverse roles, like messenger (mRNA) and transporter (tRNA).
Transcription and DNA replication are the same process.	Replication copies all DNA; transcription copies specific genes into RNA.
All parts of a gene are translated into protein.	Only coding regions (exons) are translated; non-coding introns are removed.
mRNA is immediately ready for translation after transcription.	In eukaryotes, mRNA is processed (e.g., splicing) before translation.

Multiple Choice

How does the function of mRNA explain why it is single-stranded and able to leave the nucleus, unlike DNA?

• Its single strand is less stable, which is why it can be translated into proteins.

• It must be small and flexible to pass through nuclear pores and carry the genetic code to the ribosomes in the cytoplasm.

• Being single-stranded allows it to replicate itself quickly in the cytoplasm.

• It remains in the nucleus to give instructions directly to the DNA.

Multiple Choice

What is the fundamental difference in the outcome of transcription versus DNA replication?

• Transcription produces a single-stranded RNA molecule from a gene, while replication creates a complete double-stranded DNA copy.

• Replication results in a protein, while transcription results in an amino acid.

• Transcription happens in the cytoplasm, while replication happens in the nucleus.

• Transcription uses Uracil to pair with Guanine, while replication uses Thymine to pair with Adenine.

Multiple Choice

A genetic mutation prevents the termination sequence of a gene from being recognized by RNA polymerase. What is the most likely outcome of this mutation?

• The cell will produce a shortened protein from the resulting mRNA.

• The process of transcription will not be able to start for this gene.

• The RNA polymerase will continue to transcribe DNA beyond the end of the gene, creating an abnormally long mRNA strand.

• The mRNA strand will be unable to leave the nucleus.

Multiple Choice

If the splicing process were to fail and an intron was not removed from an mRNA molecule, what would be the effect on the resulting protein?

• The mRNA would not be able to attach to the ribosome for translation.

• The protein would likely be non-functional because it would contain extra, incorrect amino acids.

• There would be no effect, as introns do not contain genetic information.

• The protein would be shorter than usual due to a premature stop codon.

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Summary

• RNA is a single-stranded nucleic acid with three main types: mRNA, tRNA, and rRNA.

• The Central Dogma describes the flow of information from DNA to RNA to Protein.

• Transcription is the process where an mRNA copy is created from a DNA template.

• Transcription involves initiation, elongation, and termination, followed by mRNA processing.

Poll

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