What happens during transcription vs. translation?
Where do each of these processes occur (nucleus or cytoplasm)?
What are the main molecules involved in each step (like mRNA, tRNA, etc.)?
Sample Answer
In molecular biology, the flow of genetic information within a cell is often described by the “Central Dogma,” which states that DNA is transcribed into RNA, and RNA is then translated into protein. These two processes, transcription and translation, are fundamental to life.
What happens during Transcription vs. Translation?
Transcription: Transcription is the process by which the genetic information encoded in DNA is copied into a molecule of RNA. Think of it like taking a specific recipe from a large cookbook (DNA) and writing it down on a smaller, temporary note (RNA).
- Process:
- Initiation: An enzyme called RNA polymerase binds to a specific region on the DNA called a promoter. The DNA double helix unwinds and separates at this point, exposing the nucleotide bases.
- Elongation: RNA polymerase moves along one strand of the DNA (the template strand) and synthesizes a complementary RNA molecule. RNA nucleotides are added according to base-pairing rules: Adenine (A) pairs with Uracil (U) in RNA (instead of Thymine (T) as in DNA), and Guanine (G) pairs with Cytosine (C).
Full Answer Section
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- Termination: RNA polymerase reaches a specific sequence on the DNA called a terminator, signaling the end of transcription. The newly formed RNA molecule detaches from the DNA, and the DNA double helix re-forms.
- Product: The primary product is a messenger RNA (mRNA) molecule. In eukaryotes, this initial RNA transcript (pre-mRNA) often undergoes further processing (splicing to remove introns, adding a 5′ cap and a poly-A tail) to become mature mRNA before it leaves the nucleus.
Translation: Translation is the process by which the genetic information carried by mRNA is decoded to synthesize a protein (a chain of amino acids). This is like taking that temporary recipe note (mRNA) and using it to actually assemble the dish (protein) by combining specific ingredients (amino acids).
- Process:
- Initiation: The mRNA molecule binds to a ribosome (the cellular machinery for protein synthesis). The ribosome scans the mRNA until it finds the start codon (AUG), which signals the beginning of the protein sequence. A transfer RNA (tRNA) molecule carrying the first amino acid (methionine) binds to this start codon.
- Elongation: The ribosome moves along the mRNA, reading its sequence in groups of three nucleotides called codons. For each codon, a complementary tRNA molecule carrying a specific amino acid arrives at the ribosome. The ribosome then catalyzes the formation of a peptide bond between the amino acid carried by the incoming tRNA and the growing polypeptide chain. The empty tRNA then detaches.
- Termination: The ribosome encounters a stop codon on the mRNA (UAA, UAG, or UGA), which does not code for any amino acid. Instead, release factors bind to the stop codon, causing the polypeptide chain to be released from the ribosome. The ribosome then disassembles from the mRNA.
- Product: A polypeptide chain, which folds into a functional protein.
Where do each of these processes occur?
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Transcription:
- In eukaryotic cells (cells with a nucleus, like human cells): Occurs in the nucleus.
- In prokaryotic cells (cells without a nucleus, like bacteria): Occurs in the cytoplasm.
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Translation:
- In eukaryotic cells: Occurs in the cytoplasm (specifically on ribosomes, which can be free in the cytoplasm or attached to the endoplasmic reticulum).
- In prokaryotic cells: Occurs in the cytoplasm. In prokaryotes, transcription and translation can even occur simultaneously because there’s no nuclear membrane separating the processes.
What are the main molecules involved in each step?
Transcription:
- DNA: Serves as the template strand from which the RNA is synthesized.
- RNA polymerase: The enzyme that synthesizes the RNA molecule by adding complementary RNA nucleotides.
- RNA nucleotides (A, U, G, C): The building blocks for the RNA molecule.
- Transcription factors: Proteins that help RNA polymerase bind to the promoter and regulate gene expression (primarily in eukaryotes).
- Promoter: A specific DNA sequence that signals the start of a gene and where RNA polymerase binds.
- Terminator: A specific DNA sequence that signals the end of transcription.
Translation:
- mRNA (messenger RNA): Carries the genetic code from DNA in the form of codons, which specify the amino acid sequence.
- Ribosomes: Cellular organelles composed of ribosomal RNA (rRNA) and proteins. They serve as the site of protein synthesis, facilitating the binding of mRNA and tRNA and catalyzing peptide bond formation. Ribosomes have a large and small subunit.
- tRNA (transfer RNA): Small RNA molecules that act as adapters. Each tRNA molecule has an anticodon (a three-nucleotide sequence that is complementary to an mRNA codon) at one end and carries a specific amino acid at the other end.
- Amino acids: The building blocks of proteins, linked together in a specific sequence determined by the mRNA codons.
- Aminoacyl-tRNA synthetases: Enzymes that attach the correct amino acid to its corresponding tRNA molecule.
- Initiation factors, elongation factors, release factors: Proteins that assist in various stages of translation (starting, continuing, and stopping the process).
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