Learning Objectives

• Describe the structures of DNA and RNA and distinguish between them.

• Explain the central dogma of biology, including replication, transcription, and translation.

• Detail the specific roles of mRNA, tRNA, and rRNA in protein synthesis.

• Analyze the relationship between genes, proteins, and traits.

Key Vocabulary

Gene

A section of DNA that contains the information to make a specific protein.

Replication

The process of producing two identical replicas of DNA from one original DNA molecule.

Transcription

The process of converting the genetic code from DNA into a messenger RNA molecule.

Translation

The process of converting information from an mRNA molecule into a sequence of amino acids.

Codon

A sequence of three nucleotides on an mRNA strand that codes for one amino acid.

DNA vs. RNA

• DNA and RNA are nucleic acids built from units called nucleotides.

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

• DNA uses deoxyribose sugar and Thymine (T); RNA uses ribose sugar and Uracil (U).

• Base pairing in DNA is A-T and G-C; for RNA, A pairs with U.

Solved Example 1
A DNA template strand has the base sequence TACGGTCCA. What is the complementary mRNA sequence that will be produced during transcription?

Step 1: Analyze and Sketch the Problem

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

• Knowns: The DNA template strand sequence is TACGGTCCA.

• Unknown: The complementary mRNA sequence.

• Formula: Apply the base-pairing rules for transcription: DNA Adenine (A) pairs with RNA Uracil (U), DNA Thymine (T) pairs with RNA Adenine (A), DNA Cytosine (C) pairs with RNA Guanine (G), and DNA Guanine (G) pairs with RNA Cytosine (C).

Solved Example 1
A DNA template strand has the base sequence TACGGTCCA. What is the complementary mRNA sequence that will be produced during transcription?

Step 2: Solve for the Unknown

• Write down the DNA template strand: TAC GGT CCA.

• Transcribe each DNA base to its complementary RNA base.

• T (Thymine) in DNA pairs with A (Adenine) in RNA.

• A (Adenine) in DNA pairs with U (Uracil) in RNA.

• C (Cytosine) in DNA pairs with G (Guanine) in RNA.

• G (Guanine) in DNA pairs with C (Cytosine) in RNA.

• Applying these rules: TAC becomes AUG, GGT becomes CCA, and CCA becomes GGU.

• Combine the new bases to form the mRNA sequence: AUG CCA GGU.

Solved Example 1
A DNA template strand has the base sequence TACGGTCCA. What is the complementary mRNA sequence that will be produced during transcription?

Step 3: Evaluate the Answer

• To verify, check each base pair against the transcription rules: T in DNA pairs with A in mRNA, A with U, C with G, and G with C.

• The final mRNA sequence AUG CCA GGU is the correct complement to the DNA template strand TAC GGT CCA.

The Three Types of RNA

Messenger RNA (mRNA)

• ​Carries genetic instructions from the DNA in the nucleus to the ribosomes.

• ​​Its sequence is complementary to the DNA strand it was copied from.

• ​It acts as the template for building a specific protein chain.

Transfer RNA (tRNA)

• ​It delivers the correct amino acids to the ribosome for protein assembly.

• ​​It reads the message on the mRNA in three-letter sequences called codons.

• ​Its anticodon is complementary to the codon on the mRNA strand.

Ribosomal RNA (rRNA)

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

• ​​Ribosomes are the cellular machines responsible for synthesizing all proteins.

• ​It helps bond amino acids together to form a long polypeptide chain.

Solved Example 2
Given a DNA template strand of TAC GGT CCA, determine the complementary mRNA codons and the corresponding tRNA anticodons.

Step 1: Analyze and Sketch the Problem

• Goal: Find the mRNA codons and tRNA anticodons for the given DNA sequence.

• Knowns: DNA template strand is TAC GGT CCA.

• Unknowns: mRNA sequence, tRNA sequence.

• Rules: DNA to mRNA (A→U, T→A, C→G, G→C), mRNA to tRNA (A→U, U→A, C→G, G→C).

Solved Example 2
Given a DNA template strand of TAC GGT CCA, determine the complementary mRNA codons and the corresponding tRNA anticodons.

Step 2: Solve for the Unknown

• First, transcribe the DNA template strand to find the mRNA codons.
  DNA: T A C | G G T | C C A
  mRNA: A U G | C C A | G G U

• Next, translate the mRNA codons to find the complementary tRNA anticodons.
  mRNA: A U G | C C A | G G U
  tRNA: U A C | G G U | C C A

Solved Example 2
Given a DNA template strand of TAC GGT CCA, determine the complementary mRNA codons and the corresponding tRNA anticodons.

Step 3: Evaluate the Answer

• Check the transcription: TAC on DNA correctly pairs with AUG on mRNA, GGT with CCA, and CCA with GGU.

• Check the translation: AUG on mRNA correctly pairs with UAC on tRNA, CCA with GGU, and GGU with CCA. The sequences follow the rules of complementary base pairing.

Genes, Proteins, and Traits

• A gene is a section of DNA with instructions to build a protein.

• ​Proteins are molecules made from smaller units called amino acids.

• These proteins are essential for cell structures and reactions in your body.

• This gene-to-protein flow determines your body’s specific traits.

Central Dogma & DNA Replication

Central Dogma

• ​Describes the one-way flow of genetic information within a biological system.

• ​​Information flows in one direction: from DNA to RNA to a protein.

• ​This involves two main processes which are called transcription and translation.

DNA Replication

• ​The process where a cell makes an identical copy of its DNA.

• ​​This must occur before a cell can divide and happens in the nucleus.

• ​DNA Polymerase is the enzyme that builds and proofreads new DNA strands.

Solved Example 3
If a segment of DNA is 900 nucleotides long, how many amino acids will be in the resulting protein? (Assume no introns and the segment starts with a start codon and ends with a stop codon).

Step 1: Analyze and Sketch the Problem

• Goal: Determine the number of amino acids coded by a DNA segment.

• Knowns: The DNA segment is 900 nucleotides long. It starts with a start codon and ends with a stop codon.

• Unknown: The number of amino acids in the protein.

• Formula: Each codon consists of 3 nucleotides and codes for one amino acid. The stop codon does not code for an amino acid.

Solved Example 3
If a segment of DNA is 900 nucleotides long, how many amino acids will be in the resulting protein? (Assume no introns and the segment starts with a start codon and ends with a stop codon).

Step 2: Solve for the Unknown

Solved Example 3
If a segment of DNA is 900 nucleotides long, how many amino acids will be in the resulting protein? (Assume no introns and the segment starts with a start codon and ends with a stop codon).

Step 3: Evaluate the Answer

What is Transcription?

• Transcription copies a DNA gene's code into a messenger RNA (mRNA) molecule.

• The enzyme RNA polymerase reads the DNA template strand in the cell nucleus.

• It adds complementary RNA bases, pairing Uracil (U) with DNA’s Adenine (A).

• The finished mRNA molecule then leaves the nucleus to find a ribosome.

The Three Types of RNA in Translation

Messenger RNA (mRNA)

• Carries genetic code from DNA in the nucleus to the ribosome.

• The message is read in three-letter groups called codons.

• Each codon specifies a particular amino acid for the protein chain.

Ribosomal RNA (rRNA)

• This is a key structural component of the cell's ribosomes.

• It helps position mRNA and tRNA molecules during protein synthesis.

• It helps form peptide bonds between the individual amino acids.

Transfer RNA (tRNA)

• This acts as a molecular bridge connecting codons to amino acids.

• It has a special sequence called an anticodon to match mRNA.

• It carries specific amino acids to the ribosome to build proteins.

Translating the Genetic Code

• The genetic code is the set of rules for building proteins from mRNA instructions.

• ​Information is read in three-nucleotide “words” called codons, which specify an amino acid.

• Translation starts at the AUG codon and halts at one of three stop codons.

• Transfer RNA (tRNA) delivers the correct amino acid by matching its anticodon to the codon.

Solved Example 6
Given the mRNA codon sequence AUG-GGC-UCA, determine the complementary tRNA anticodon sequence that will bind to it during translation.

Step 1: Analyze and Sketch the Problem

• Goal: Find the tRNA anticodon sequence that is complementary to the given mRNA codon sequence.

• Knowns: The mRNA codon sequence is AUG-GGC-UCA.

• Key Concept: The base pairing rules for RNA are Adenine (A) pairs with Uracil (U), and Guanine (G) pairs with Cytosine (C).

Solved Example 6
Given the mRNA codon sequence AUG-GGC-UCA, determine the complementary tRNA anticodon sequence that will bind to it during translation.

Step 2: Solve for the Unknown

• Break down the mRNA sequence into individual codons: AUG, GGC, and UCA.

• Apply the complementary base pairing rules to each nucleotide in the sequence.

• For the first codon (AUG): A pairs with U, U pairs with A, G pairs with C. The tRNA anticodon is UAC.

• For the second codon (GGC): G pairs with C, G pairs with C, C pairs with G. The tRNA anticodon is CCG.

• For the third codon (UCA): U pairs with A, C pairs with G, A pairs with U. The tRNA anticodon is AGU.

Solved Example 6
Given the mRNA codon sequence AUG-GGC-UCA, determine the complementary tRNA anticodon sequence that will bind to it during translation.

Step 3: Evaluate the Answer

• Verify the pairings: A pairs with U, and G pairs with C. The pairings for each codon (AUG → UAC, GGC → CCG, UCA → AGU) are correct.

• The final combined tRNA anticodon sequence is UAC-CCG-AGU.

Common Misconceptions About DNA

Summary

• The Central Dogma describes genetic flow: DNA is transcribed to RNA, which is translated to protein.

• DNA stores genetic instructions, while RNA, in forms like mRNA, tRNA, and rRNA, builds proteins.

• DNA replication is the process of creating identical copies of DNA for cell division.

• The expression of genes as proteins is what ultimately determines an organism's traits.