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Biology: Genetics, Reproduction, and Biotechnology - DNA Replication, Transcription, Translation, and Mutation

Grade 8IB

Review the key concepts, formulae, and examples before starting your quiz.

🔑Concepts

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DNA Structure: DNA is a double helix made of nucleotides. Each nucleotide consists of a phosphate group, a deoxyribose sugar, and a nitrogenous base (A,T,C,GA, T, C, G).

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Complementary Base Pairing: In DNA, Adenine (AA) always pairs with Thymine (TT) via two hydrogen bonds, and Cytosine (CC) always pairs with Guanine (GG) via three hydrogen bonds (A=TA=T and C≡GC \equiv G).

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DNA Replication: A semi-conservative process where the DNA molecule unwinds (via Helicase) and new strands are synthesized (via DNA Polymerase) using the original strands as templates.

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Transcription: The process in the nucleus where a segment of DNA is copied into mRNA. In RNA, Thymine is replaced by Uracil (UU), so AA pairs with UU.

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Translation: The process occurring at the ribosome where mRNA codons (sequences of 3 bases) are read to assemble amino acids into a polypeptide chain (protein) using tRNA molecules.

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Mutations: Changes in the DNA sequence. Point mutations (substitution) affect one base, while frameshift mutations (insertion or deletion) can change every subsequent codon in the sequence.

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Biotechnology: The use of biological systems or organisms to develop products, such as using CRISPR for gene editing or PCR (Polymerase Chain Reaction) to amplify DNA segments.

📐Formulae

Chargaff’s Rule: [A]=[T] and [G]=[C]\text{Chargaff's Rule: } [A] = [T] \text{ and } [G] = [C]

Total Nucleotides=[A]+[T]+[G]+[C]\text{Total Nucleotides} = [A] + [T] + [G] + [C]

Central Dogma: DNA→TranscriptionmRNA→TranslationProtein\text{Central Dogma: } \text{DNA} \xrightarrow{\text{Transcription}} \text{mRNA} \xrightarrow{\text{Translation}} \text{Protein}

Number of Codons=Number of Nucleotides in mRNA3\text{Number of Codons} = \frac{\text{Number of Nucleotides in mRNA}}{3}

💡Examples

Problem 1:

If a double-stranded DNA molecule contains 30%30\% Adenine (AA), calculate the percentage of Cytosine (CC) present.

Solution:

20%20\%

Explanation:

According to Chargaff's rule, [A]=[T][A] = [T]. If [A]=30%[A] = 30\%, then [T]=30%[T] = 30\%. Together, A+T=60%A + T = 60\%. The remaining 40%40\% must be split equally between GG and CC (100%−60%=40%100\% - 60\% = 40\%). Therefore, [C]=40%2=20%[C] = \frac{40\%}{2} = 20\%.

Problem 2:

Transcribe the following DNA template strand into mRNA: 3′−TAC−GGG−ATC−5′3'-TAC-GGG-ATC-5'.

Solution:

5′−AUG−CCC−UAG−3′5'-AUG-CCC-UAG-3'

Explanation:

During transcription, DNA bases are paired with their RNA complements. T→AT \rightarrow A, A→UA \rightarrow U (since RNA has no TT), C→GC \rightarrow G, and G→CG \rightarrow C. The orientation also flips from 3′→5′3' \rightarrow 5' to 5′→3′5' \rightarrow 3'.

Problem 3:

A DNA sequence changes from ...ATG−CAG−GTA......ATG-CAG-GTA... to ...ATG−TAG−GTA......ATG-TAG-GTA.... Identify the type of mutation and its potential effect.

Solution:

Point Mutation (Substitution)

Explanation:

The second codon changed from CAGCAG to TAGTAG. This is a substitution mutation. Because TAGTAG is often a 'Stop' codon, this specific mutation is called a 'nonsense mutation,' which would prematurely terminate the protein synthesis.