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Nucleic Acids (AHL) - Transcription and Gene Expression

Grade 11IBBiology

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

🔑Concepts

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Transcription occurs in a 5′→3′5' \rightarrow 3' direction, where RNARNA polymerase adds ribonucleoside triphosphates to the 3′3' end of the growing mRNAmRNA strand.

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The promoter is a non-coding DNADNA sequence located upstream of the gene that serves as the binding site for RNARNA polymerase. In eukaryotes, this often includes the TATATATA box.

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Eukaryotic cells modify mRNAmRNA after transcription through three main processes: 5′5' capping (addition of a methylguanosine cap), polyadenylation (addition of a poly-A tail to the 3′3' end), and splicing.

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Splicing involves the removal of non-coding sequences called introns and the joining of coding sequences called exons. Alternative splicing allows a single gene to code for multiple proteins.

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Gene expression is regulated by proteins that bind to specific DNADNA sequences: enhancers (increase transcription) and silencers (decrease transcription).

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Nucleosomes help regulate transcription through histone tail modification. Acetylation (adding −COCH3-COCH_3) neutralizes the positive charge on histones, loosening the DNADNA binding and increasing transcription. Methylation (adding −CH3-CH_3) can either increase or decrease transcription depending on the site.

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The environment of a cell or an organism can influence gene expression through epigenetic tags, which are chemical modifications to DNADNA or histones that do not change the DNADNA sequence itself.

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DNADNA methylation typically involves adding a methyl group to cytosine. Higher levels of DNADNA methylation are generally associated with reduced gene expression (gene silencing).

📐Formulae

Direction of synthesis: 5′→3′\text{Direction of synthesis: } 5' \rightarrow 3'

Template strand (DNA): 3′…5′  ⟹  mRNA: 5′…3′\text{Template strand (DNA): } 3' \dots 5' \implies \text{mRNA: } 5' \dots 3'

Base Pairing Rules: A→U,T→A,C↔G\text{Base Pairing Rules: } A \rightarrow U, T \rightarrow A, C \leftrightarrow G

Reaction: (NMP)n+NTP→(NMP)n+1+PPi\text{Reaction: } (NMP)_n + NTP \rightarrow (NMP)_{n+1} + PP_i

💡Examples

Problem 1:

A specific DNADNA template strand has the sequence 3′−TAC GGC ATA ATT−5′3'-TAC\,GGC\,ATA\,ATT-5'. Determine the sequence of the mRNAmRNA produced during transcription and identify the direction of synthesis.

Solution:

5′−AUG CCG UAU UAA−3′5'-AUG\,CCG\,UAU\,UAA-3'

Explanation:

The mRNAmRNA is synthesized antiparallel to the template strand. Using complementary base pairing rules (AA pairs with UU in RNARNA, TT with AA, CC with GG, and GG with CC), the 3′3' end of the DNADNA corresponds to the 5′5' end of the mRNAmRNA. The synthesis always occurs in the 5′→3′5' \rightarrow 3' direction.

Problem 2:

How does the acetylation of histone tails affect the structure of chromatin and the expression of genes?

Solution:

Acetylation leads to a more relaxed chromatin structure (euchromatin), increasing gene expression.

Explanation:

Histones have a positive charge due to lysine residues, which attract the negatively charged PO43−PO_4^{3-} groups of DNADNA. The addition of acetyl groups (−COCH3-COCH_3) neutralizes these positive charges, weakening the interaction between histones and DNADNA. This results in 'open' chromatin that is more accessible to RNARNA polymerase and transcription factors.

Problem 3:

Explain how alternative splicing increases the proteome size relative to the genome.

Solution:

By selectively including or excluding different exons from the primary mRNAmRNA transcript.

Explanation:

A single gene (part of the genome) can be transcribed into a pre-mRNAmRNA. During splicing, different combinations of exons can be joined together (e.g., Exons 1−2−31-2-3 vs. Exons 1−3−41-3-4). This results in multiple distinct mRNAmRNA molecules, which are translated into different proteins (part of the proteome), thereby allowing one gene to code for several functional products.

Transcription and Gene Expression - Revision Notes & Key Diagrams | IB Grade 11 Biology