Chapter 13: Problem 20
Write a paragraph describing the abbreviated chemical reactions that summarize RNA polymerase-directed transcription.
Short Answer
Expert verified
Answer: The main steps of RNA polymerase-directed transcription are: 1) Template DNA recognition, where RNA polymerase binds to the promoter; 2) DNA unwinding and separation, forming the transcription bubble; 3) RNA synthesis, with the addition of complementary ribonucleotide triphosphates; 4) Elongation, as RNA polymerase moves along the template DNA strand; and 5) Termination, where the enzyme encounters a termination sequence and releases the synthesized RNA molecule.
Step by step solution
01
Template DNA Recognition
In the first step of transcription, RNA polymerase recognizes and binds to the DNA at a specific sequence called the promoter. The promoter is located upstream of the coding region of the gene, and tells RNA polymerase the correct start site and direction of transcription.
02
DNA Unwinding and Separation
Once RNA polymerase binds to the promoter, it unwinds and separates the two DNA strands, creating an open DNA region known as the transcription bubble. In this region, one of the DNA strands will serve as a template for RNA synthesis (called the template strand), while the other strand is called the non-template strand.
03
RNA Synthesis
RNA polymerase begins synthesizing the RNA by adding ribonucleotide triphosphates that are complementary to the template DNA strand. The process starts at a specific site called the transcription start site, and the nucleotides A, C, G, and U are paired to their complementary bases on the template DNA strand (A-T, T-A, G-C, and C-G). The phosphate groups from the ribonucleotide triphosphates provide the energy needed for the formation of the phosphodiester bond between the nucleotides in the RNA chain.
04
Elongation
The RNA polymerase moves along the template DNA strand in the 3' to 5' direction, synthesizing the RNA in the 5' to 3' direction. During elongation, the DNA double helix is rewound and the RNA molecule separates from the template DNA strand as it is synthesized, ensuring that the synthesized RNA is only complementary to the template DNA.
05
Termination
Once the RNA polymerase encounters a specific termination sequence, it releases the newly synthesized RNA molecule and dissociates from the template DNA. The RNA molecule undergoes further modifications before it becomes functional in the cell.
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Key Concepts
These are the key concepts you need to understand to accurately answer the question.
Template DNA Recognition
Understanding the first step in transcribing DNA into RNA involves recognizing the starting point. RNA polymerase, the key enzyme in this process, specifically identifies a region known as the promoter. This region contains specific DNA sequences that signal the start of a gene. Much like finding the right address before sending a letter, RNA polymerase must locate the correct promoter sequence to initiate transcription.
It's crucial to note that the promoter not only marks where transcription starts but also determines the direction of RNA synthesis. In other words, it sets the stage for the entire transcription process. For students, imagine RNA polymerase as a conductor, with the promoter sequence being the starting note of a music score – without it, the melody cannot be played correctly.
It's crucial to note that the promoter not only marks where transcription starts but also determines the direction of RNA synthesis. In other words, it sets the stage for the entire transcription process. For students, imagine RNA polymerase as a conductor, with the promoter sequence being the starting note of a music score – without it, the melody cannot be played correctly.
DNA Unwinding
Once RNA polymerase has anchored itself to the promoter, it's time to get to work on the DNA. This step is reminiscent of unzipping a jacket; the enzyme unwinds the double-stranded DNA, creating a 'transcription bubble'. This bubble exposes part of the DNA, revealing the template strand that will guide RNA synthesis.
In this critical moment, enzymes must maintain the unwound state. For learners, this can be visualized like holding a book open; the pages need to remain separated to read the words. Similarly, the separated DNA strands ensure the enzyme has a clear guide for building the RNA molecule.
In this critical moment, enzymes must maintain the unwound state. For learners, this can be visualized like holding a book open; the pages need to remain separated to read the words. Similarly, the separated DNA strands ensure the enzyme has a clear guide for building the RNA molecule.
RNA Synthesis
With the template strand exposed, RNA polymerase begins constructing the RNA molecule, a process akin to stringing beads on a necklace. It adds nucleotides one by one, following the base-pairing rules where adenine (A) pairs with uracil (U) in RNA, and cytosine (C) pairs with guanine (G).
The synthesis kicks off at the transcription start site, and the building blocks used are ribonucleotide triphosphates (rNTPs). Like linking LEGO blocks, the phosphodiester bonds form between the nucleotides to extend the RNA strand. The energy for this comes from the rNTPs themselves, as they lose two phosphate groups during the binding process. This step-by-step assembly of the RNA strand is an essential concept as it underscores the precise nature of genetic information transfer.
The synthesis kicks off at the transcription start site, and the building blocks used are ribonucleotide triphosphates (rNTPs). Like linking LEGO blocks, the phosphodiester bonds form between the nucleotides to extend the RNA strand. The energy for this comes from the rNTPs themselves, as they lose two phosphate groups during the binding process. This step-by-step assembly of the RNA strand is an essential concept as it underscores the precise nature of genetic information transfer.
Transcription Elongation
After the initiation of RNA synthesis, RNA polymerase moves along the DNA, a process known as transcription elongation. It's similar to a factory assembly line, with the enzyme operating in a 3' to 5' direction on the DNA, while elongating the RNA molecule in the 5' to 3' direction.
During this phase, the DNA double helix reforms behind the advancing enzyme, and the newly synthesized RNA peels away. This can be likened to laying down tracks for an advancing train; as the train passes over, the ground returns to its former state. It is critical that the RNA remains complementary to the template DNA to ensure the genetic code is accurately reflected in the RNA message.
During this phase, the DNA double helix reforms behind the advancing enzyme, and the newly synthesized RNA peels away. This can be likened to laying down tracks for an advancing train; as the train passes over, the ground returns to its former state. It is critical that the RNA remains complementary to the template DNA to ensure the genetic code is accurately reflected in the RNA message.
Transcription Termination
All good things must come to an end, and so does transcription. RNA polymerase advances until it encounters a stop signal – the termination sequence. This sequence prompts the enzyme to halt and release the completed RNA molecule, much like a car stopping at a red light and its passengers getting off.
Following transcription, the RNA strand typically undergoes modifications before it carries out its role in the cell. Conceptually, for students, the RNA is not yet ready to deliver its message immediately after transcription; it must first be processed and 'dressed up' into a mature form ready for translation. The termination phase ensures that RNA polymerase disengages properly from the DNA, wrapping up the synthesis process neatly.
Following transcription, the RNA strand typically undergoes modifications before it carries out its role in the cell. Conceptually, for students, the RNA is not yet ready to deliver its message immediately after transcription; it must first be processed and 'dressed up' into a mature form ready for translation. The termination phase ensures that RNA polymerase disengages properly from the DNA, wrapping up the synthesis process neatly.