DNA, Transcription, and Translation: Key Concepts in Cell Biology

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DNA

B DNA vs Z DNA

The structure of DNA can exist in several forms, with B DNA and Z DNA being the most common. B DNA is the standard right-handed helix found in cells, while Z DNA is a left-handed helix that occurs under certain conditions.

B DNA: Right-handed helix, 10 base pairs per turn, most common in vivo.
Z DNA: Left-handed helix, 12 base pairs per turn, forms in regions with alternating purines and pyrimidines.
Example: Z DNA may play a role in gene regulation.

Supercoiling

Supercoiling refers to the overwinding or underwinding of DNA, which helps compact the molecule and affects its accessibility for replication and transcription.

Positive supercoiling: DNA is overwound.
Negative supercoiling: DNA is underwound, making strand separation easier.
Enzymes called topoisomerases regulate supercoiling.

Chromosome Structure/DNA Packing

DNA is packed into chromosomes through multiple levels of organization, affecting how condensed the DNA is.

Nucleosome: DNA wrapped around histone proteins.
Chromatin: Nucleosomes further folded into higher-order structures.
Condensation: Heterochromatin is more condensed and less accessible; euchromatin is less condensed and more accessible.

DNA vs RNA Structure

DNA and RNA differ in their sugar components and nitrogenous bases.

DNA: Deoxyribose sugar, bases A, T, C, G; no uracil.
RNA: Ribose sugar, bases A, U, C, G; uracil replaces thymine.
Example: DNA does not contain uracil.

Nucleus

Structure of Nucleus

The nucleus is a membrane-bound organelle that contains the cell's genetic material.

Surrounded by a double membrane called the nuclear envelope.
Contains nuclear pores for transport of molecules.

Structure and Function of Nuclear Pore Complex

The nuclear pore complex regulates the movement of molecules between the nucleus and cytoplasm.

Allows selective transport of proteins, RNA, and other molecules.
Composed of multiple proteins called nucleoporins.

Permeability and Import/Export Processes

The nuclear envelope is selectively permeable, allowing regulated exchange of materials.

Small molecules can diffuse freely; larger molecules require active transport.
Import/export is mediated by importins and exportins.

Transcription

Location in Prokaryotes vs Eukaryotes

Transcription occurs in different cellular locations depending on the organism.

Prokaryotes: Cytoplasm.
Eukaryotes: Nucleus.

Prokaryotic Transcription

Transcription in prokaryotes involves initiation, elongation, and termination.

Initiation: RNA polymerase binds to promoter regions.
Elongation: RNA strand is synthesized.
Termination: RNA polymerase releases the newly made RNA.
Promoters are specific DNA sequences where RNA polymerase binds.
Some require a sigma factor for initiation.

Eukaryotic Transcription

Eukaryotic transcription is more complex, involving multiple RNA polymerases and regulatory elements.

Types of RNA polymerases: I, II, III (each transcribes different types of RNA).
Initiation: Requires transcription factors and promoter sequences.
Promoters differ for each RNA polymerase.

Processing of mRNA, rRNA, and tRNA

After transcription, RNA molecules undergo processing before becoming functional.

mRNA: 5' capping, splicing, 3' polyadenylation.
rRNA and tRNA: Cleavage and chemical modification.
Spliceosome: Complex responsible for removing introns from pre-mRNA.
Alternative splicing: Allows a single gene to code for multiple proteins.

Translation

One Gene, One Polypeptide

The one gene, one polypeptide hypothesis states that each gene encodes a single polypeptide chain.

Exceptions exist, such as alternative splicing.

Types of Mutations

Mutations can alter the polypeptide sequence and function.

Missense mutation: Changes one amino acid.
Nonsense mutation: Introduces a stop codon.
Silent mutation: No change in amino acid sequence.
Example: Identify mutation type by examining changes in primary structure.

Wobble

The wobble hypothesis explains how tRNA can recognize multiple codons due to flexible base pairing at the third codon position.

Allows fewer tRNAs to cover all codons.

Structure of Ribosome

Ribosomes are the site of protein synthesis and differ between prokaryotes and eukaryotes.

Prokaryotic ribosome: 70S (30S + 50S subunits).
Eukaryotic ribosome: 80S (40S + 60S subunits).
mRNA binds to the small subunit; tRNAs interact at the A, P, and E sites.

tRNA

tRNA molecules carry amino acids to the ribosome and match them to the mRNA codon.

Structure includes an anticodon loop and amino acid attachment site.
Charging tRNA involves forming an ester bond between the amino acid and tRNA.

Prokaryotic Translation

Translation in prokaryotes involves initiation factors and assembly of the ribosome on mRNA.

Initiation factors help the ribosome recognize the start codon.
Assembly begins with the small subunit binding to mRNA, followed by the large subunit.

Eukaryotic Translation

Eukaryotic translation is more complex, involving additional factors and steps.

Pre-initiation complex includes GTP, mRNA, and ribosomal subunits.
Poly-A binding protein stabilizes mRNA.
GTP hydrolysis is required for assembly and elongation.

Elongation

Elongation is the process of adding amino acids to the growing polypeptide chain.

Occurs at the A (aminoacyl), P (peptidyl), and E (exit) sites of the ribosome.
Requires elongation factors and GTP.

Molecular Chaperones

Molecular chaperones assist in the proper folding of newly synthesized proteins.

Two main types: Hsp70 and chaperonins.
Prevent misfolding and aggregation.

Post-Translational Processing

Proteins often undergo modifications after translation to become fully functional.

Includes cleavage, phosphorylation, glycosylation, and more.
Differences exist between prokaryotes and eukaryotes.

Key Equations

Central Dogma:
Charged tRNA formation: