Molecular Biology: Central Dogma Overview

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Questions and Answers

What is the role of the initiator box in transcription?

It is responsible for protein synthesis.

It binds to ribosomes for protein assembly.

It marks the start site of transcription. (correct)

It is where translation begins.

What must be added to the 5' end of eukaryotic mRNA before it can be translated?

A ribosome

A 5' cap (correct)

A poly(A) tail

Introns

What modification is done to eukaryotic mRNA at the 3' end?

Methylation of cytosines

Removal of exons

Addition of a poly(A) tail (correct)

Addition of a ribosomal binding site

What is the main difference between eukaryotic and bacterial mRNA processing?

Eukaryotic mRNA is processed before translation. Signup and view all the answers

During translation, what are codons on mRNA made up of?

Groups of three bases Signup and view all the answers

What role do tRNA molecules play in protein synthesis?

They transport amino acids to the ribosome. Signup and view all the answers

Which of the following statements about exons and introns is correct?

Exons are the regions that code for proteins. Signup and view all the answers

How many codons correspond to the amino acid valine?

Four Signup and view all the answers

What does the central dogma of molecular biology describe?

Information flow from DNA to RNA to protein Signup and view all the answers

Which step comes first in the process of gene expression?

Transcription Signup and view all the answers

What role does RNA polymerase play in transcription?

It synthesizes RNA from the DNA template Signup and view all the answers

How does gene expression vary depending on conditions in an organism?

Certain genes are activated only under specific circumstances Signup and view all the answers

What happens after transcription is completed?

The newly made RNA is released Signup and view all the answers

What are housekeeping genes responsible for?

Encoding proteins used all the time Signup and view all the answers

What is the result of transcription in the gene expression process?

An RNA copy of the gene is made Signup and view all the answers

Which of the following best describes translation?

The assembly of amino acids to form proteins Signup and view all the answers

What is the primary final product encoded by a gene?

Protein Signup and view all the answers

What is an open reading frame (ORF)?

A stretch of DNA that encodes a protein without stop codons Signup and view all the answers

What is the role of the promoter in gene transcription?

To initiate RNA polymerase binding and transcription Signup and view all the answers

Which part of the gene is located just after the promoter?

Transcription Start Site Signup and view all the answers

What does the 5' untranslated region (5' UTR) contain?

Ribosome binding site and regulatory elements Signup and view all the answers

What is the direction of RNA nucleotides addition during transcription?

5' to 3' Signup and view all the answers

What happens if a gene is expressed constitutively?

The gene is transcribed at all times Signup and view all the answers

Which type of RNA molecules are translated into proteins?

mRNA Signup and view all the answers

What occurs after peptidyl-transfer during protein synthesis?

The free tRNA in the P site is released and the ribosome moves. Signup and view all the answers

Which modification of polypeptides is NOT necessary for forming a functional protein?

Addition of a signal peptide post-translation Signup and view all the answers

Which type of RNA is primarily responsible for carrying the genetic information for protein synthesis?

mRNA Signup and view all the answers

What is the primary role of ribosomal RNA (rRNA) in cells?

To catalyze the formation of peptide bonds. Signup and view all the answers

What is a common structural characteristic of tRNA?

It has a cloverleaf structure. Signup and view all the answers

What percentage of total RNA in cells does rRNA constitute?

75% Signup and view all the answers

Which of the following is NOT a type of covalent modification that polypeptides may undergo?

Decarboxylation Signup and view all the answers

What is the size range of tRNA molecules in terms of ribonucleotides?

73 to 93 ribonucleotides Signup and view all the answers

What is the role of the coding strand in RNA synthesis?

It is identical in sequence to the newly formed RNA. Signup and view all the answers

What triggers RNA polymerase to stop transcribing in bacteria?

The formation of a hairpin structure in RNA. Signup and view all the answers

How are operons structured in prokaryotes?

They are composed of multiple genes transcribed as a single mRNA. Signup and view all the answers

Which RNA polymerase is responsible for transcribing protein-coding genes in eukaryotes?

RNA polymerase II Signup and view all the answers

What is a key characteristic of monocistronic mRNA in eukaryotes?

It contains only one cistron. Signup and view all the answers

What are the three different regions required for RNA polymerase II binding to eukaryotic promoters?

Initiator box, TATA box, and upstream elements. Signup and view all the answers

Why is the hybrid molecule DNA/RNA less stable during transcription?

Only two hydrogen bonds form between A and U. Signup and view all the answers

Which type of RNA is transcribed by RNA polymerase III?

tRNA and small RNA Signup and view all the answers

Study Notes

The Central Dogma of Molecular Biology

The flow of genetic information in cells occurs in the following sequence: DNA → RNA → protein.
The process of making RNA from DNA is called transcription.
The process of making protein from RNA is called translation.

Transcription Expresses Genes

Gene expression involves creating an RNA copy of the information stored in DNA.
Transcription involves the following steps:
- Unwinding the DNA double helix.
- Separating the DNA strands at the gene's start.
- Synthesizing a complementary RNA molecule using the DNA template strand, catalyzed by RNA polymerase.
- Terminating transcription at the end of the gene.
- The newly formed RNA detaches from the DNA, and the DNA returns to its supercoiled state.
Housekeeping genes are continuously expressed to produce essential proteins.
Other genes are expressed only under specific conditions, like the presence of lactose triggering the expression of lactose metabolism genes in E. coli.
Some genes encode proteins, and their transcripts are messenger RNA (mRNA), which are translated into proteins.
Other RNA molecules, like tRNA, rRNA, and snRNA, are used directly and not translated into proteins.
Large-subunit rRNA is a ribozyme that catalyzes enzymatic reactions.
The coding region of a gene is known as a cistron or structural gene, and it may encode a protein or a non-translated RNA.
An open reading frame (ORF) is a continuous sequence of DNA or RNA that encodes a protein without any stop codons.

Promoter and Transcription Start Site

Each gene has a promoter region upstream of the coding sequence.
RNA polymerase recognizes the promoter and initiates transcription.
Constitutively expressed genes have promoters that closely match the consensus sequence.
Genes expressed under specific conditions require activator proteins or transcription factors to bind to the promoter before RNA polymerase recognizes it.
The transcription start site is where RNA polymerase begins adding nucleotides.
Between the transcription start site and the ORF is the 5' untranslated region (5'UTR), containing regulatory elements controlling translation.
The ORF follows, containing no stop codons.
After the ORF is the 3' untranslated region (3'UTR).
The termination sequence marks the end of transcription.

Making the RNA

In bacteria, the sigma subunit of RNA polymerase recognizes the promoter, initiating transcription.
The core enzyme then forms a transcription bubble, separating the DNA strands.
The template strand (DNA template) is used by RNA polymerase to synthesize complementary mRNA.
RNA polymerase adds nucleotides in the 5' to 3' direction based on the template strand sequence.
The newly formed RNA molecule binds to the template strand via hydrogen bonds between complementary base pairs.
The coding strand of DNA is complementary to the template strand and has a sequence identical to the RNA.

Transcription Stop Signals

RNA polymerase continues transcribing until encountering a termination signal.
The Rho-independent terminator is a DNA sequence with two inverted repeats separated by six bases, followed by a stretch of adenines.
During transcription, the inverted repeats form a hairpin structure.
This structure causes RNA polymerase to pause.
When the stretch of adenines is transcribed into uracils, the DNA/RNA hybrid destabilizes due to A/U base pairs having only two hydrogen bonds.
RNA polymerase stutters and falls off the template strand, ending transcription.

Variations in Number of Genes on an mRNA

In prokaryotes, genes are closely spaced, and those involved in the same metabolic pathway are often clustered together.
Operons are clusters of genes sharing a common promoter and transcribed as a single large mRNA containing multiple structural genes or cistrons.
These transcripts are called polycistronic mRNA.
In eukaryotes, genes are often separated by large noncoding DNA sequences.
Each eukaryotic mRNA contains only one cistron and is called monocistronic mRNA.

Complexity of Eukaryotic Transcription

Eukaryotes have three RNA polymerases, each transcribing different types of genes:
- RNA polymerase I: transcribes genes for large ribosomal RNA.
- RNA polymerase III: transcribes genes for tRNA, 5S rRNA, and other small RNA molecules.
- RNA polymerase II: transcribes protein-coding genes and is the most studied.
Eukaryotic promoters have a distinct structure, requiring three regions for RNA polymerase II:
- Initiator box: the transcription start site.
- TATA box: located about 25 base pairs upstream of the initiator box.
- Upstream elements: vary between genes and bind transcription factors, regulating gene expression.

Processing of Eukaryotic mRNA

Bacterial mRNA can be translated without any processing, with transcription and translation occurring simultaneously.
Eukaryotic RNA undergoes several processing steps before exiting the nucleus for translation.
The 5' end of eukaryotic mRNA receives a cap: a GTP added in reverse orientation and methylated at position 7 of the guanine base.
The 5' cap is essential for binding to the ribosome during translation and protects the mRNA from degradation.
A poly(A) tail, a long stretch of adenines, is added to the 3' end of eukaryotic mRNA.
The poly(A) tail is crucial for mRNA stability and transport from the nucleus.
Introns, noncoding sequences present within eukaryotic genes, are removed from the pre-mRNA through splicing.
Exons, the coding sequences, are joined together to form mature mRNA.

Translating the Code into Proteins

Ribosomes use messenger RNA (mRNA) to synthesize proteins.
Translation involves converting the genetic language of nucleic acids into the amino acid sequence of proteins.
mRNA is read in three-base groups called triplets or codons.
Each codon codes for one amino acid.
The genetic code is redundant, with multiple codons corresponding to the same amino acid.
Transfer RNA (tRNA) molecules are small RNAs that recognize individual codons on mRNA and carry the corresponding amino acids.
During translation, tRNA molecules deliver amino acids to the ribosome, where they are linked together to form a polypeptide chain.
The polypeptide chain is synthesized in the 5' to 3' direction, following the sequence of codons on the mRNA.
After amino acid addition, a series of reactions called translocation occurs, involving the movement of the ribosome to the next codon on the mRNA.

Protein Synthesis at the Ribosome

Ribosomes are the cellular machinery where protein synthesis occurs.
They contain both ribosomal RNA (rRNA) and proteins.
rRNA plays a critical role in protein synthesis, including binding mRNA, tRNA, and initiation, elongation, and termination factors.
The ribosome has two subunits, a large subunit and a small subunit.
The small subunit binds mRNA, and the large subunit contains peptidyl transferase activity, which links amino acids together.

Post-translational Control

After release from the ribosome, polypeptides undergo various modifications to become fully functional proteins.
Modifications include limited proteolysis, proteolytic modification for activating enzymes, and removal of signal peptides.
Polypeptides also undergo chemical modifications of amino acids by adding acetyl, methyl, phosphate, hydroxy, and carboxyl groups, as well as glycosylation and acylation.

Roles of RNA

Three main types of cellular RNA are essential for translation:
- Messenger RNA (mRNA): carries the genetic information for protein synthesis.
- Ribosomal RNA (rRNA): a major component of ribosomes, essential for protein synthesis.
- Transfer RNA (tRNA): delivers amino acids to the ribosome during translation.

Ribosomal RNA (rRNA)

Ribosomes are abundant in cells actively engaged in protein synthesis.
Ribosomes contain significant amounts of rRNA, making it the most abundant type of RNA.
rRNA accounts for about 75% of total RNA.
Ribosomes carry out various activities necessary for protein synthesis, including:
- Peptidyl transferase activity
- Codon-directed binding of aminoacyl-tRNAs
- mRNA binding
- Binding initiation, elongation, and termination factors
- GTPase activity

Transfer RNA (tRNA)

Transfer RNAs (tRNAs) are the smallest type of RNA.
They are synthesized in the cytoplasm.
tRNAs play a crucial role in translating the genetic code at the ribosome.
All tRNAs have common features, such as:
- Single-stranded molecules containing 73-93 ribonucleotides.
- A cloverleaf structure, with a specific anticodon region that binds to the corresponding codon on mRNA.

Messenger RNA (mRNA)

mRNA structure is essential for ribosome interaction.
The coding region of mRNA contains codons corresponding to the protein's amino acid sequence.
mRNA is translated into protein by ribosomes, following the genetic code to synthesize the desired polypeptide chain.

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Description

This quiz explores the Central Dogma of Molecular Biology, detailing the processes of transcription and translation. It highlights the flow of genetic information from DNA to RNA and then to proteins, as well as the specifics of gene expression and the role of housekeeping genes. Test your understanding of these fundamental concepts in molecular biology.