Biology Chapter: Transcription Process

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Questions and Answers

What is the role of the branch site during intron splicing?

To facilitate the formation of a lariat structure (correct)

To cleave the 5’ end of the intron

To enhance stability of the mRNA

To serve as a donor site for the intron

Which component is part of the spliceosome responsible for intron removal?

Protein-encoding genes

Eukaryotic ribosomes

mRNA

Small nuclear RNAs (snRNAs) (correct)

How does the addition of a poly(A) tail contribute to mRNA functionality?

Increases mRNA stability and aids in nuclear export (correct)

Provides recognition sites for spliceosome binding

Facilitates the initiation of transcription

Contributes to the degradation of mRNA

Which sequence element marks the initiation site of transcription?

Promoter Signup and view all the answers

What is the main purpose of the splice donor and acceptor sites?

To cleave the intron from pre-mRNA Signup and view all the answers

Which of the following best describes the role of the spliceosome?

It catalyzes the splicing of introns from pre-mRNA Signup and view all the answers

During mRNA transcription, which sequence represents the transcribed RNA from the DNA sequence 5’–GATTCGAAGCT–3’?

5’–GAUUCGAAGCU–3’ Signup and view all the answers

What is one key difference between RNA and DNA?

RNA has ribose instead of 2-deoxyribose. Signup and view all the answers

What happens during step 2 of the intron splicing process?

The intron is looped out as a lariat Signup and view all the answers

Which type of RNA is most abundant in eukaryotic cells?

Ribosomal RNA (rRNA) Signup and view all the answers

What is the primary function of transfer RNA (tRNA)?

To bind to specific amino acids. Signup and view all the answers

What percentage of total RNA does messenger RNA (mRNA) typically represent?

5% Signup and view all the answers

What sequence length characterizes microRNA (miRNA)?

Approximately 22 nucleotides. Signup and view all the answers

Which of the following statements about ribosomal RNA (rRNA) is accurate?

It is required for protein synthesis. Signup and view all the answers

Which type of RNA is involved in regulating gene expression?

MicroRNA (miRNA) Signup and view all the answers

What is the role of the anticodon found in tRNA?

To bind to mRNA. Signup and view all the answers

Which statement about the genetic code is true?

Only mRNAs are translated into proteins. Signup and view all the answers

What is the first step in the process of translational initiation?

Assembly of ribosomal subunits with initiator aminoacyl-tRNA Signup and view all the answers

Where is ribosomal RNA (rRNA) synthesized in eukaryotic cells?

Nucleolus Signup and view all the answers

Which factors affect the regulation of translational initiation?

Heme availability in reticulocytes Signup and view all the answers

During the elongation phase of translation, what is formed by the action of peptidyl-transferase?

Peptide bonds between amino acids Signup and view all the answers

In which direction is mRNA translated during protein synthesis?

5'-to-3' direction Signup and view all the answers

What happens when the ribosome encounters a termination codon?

Eukaryotic release factor binds to the A site Signup and view all the answers

Which of the following is a role of eukaryotic initiation factors (eIFs)?

Regulates the rate of translation initiation Signup and view all the answers

What is the role of the methylguanosine cap binding complex (CBC) in translational initiation?

Facilitates binding of mRNA to the ribosome Signup and view all the answers

What is the primary function of the eukaryotic release factor (eRF)?

To release the polypeptide chain at termination codons Signup and view all the answers

Which sequence element defines the 5’ end of an intron?

Splice donor site Signup and view all the answers

What role does histone acetylation play in transcription?

Activation Signup and view all the answers

Which sequence element determines the site of transcription initiation?

Promoter Signup and view all the answers

What is the genetic code primarily responsible for?

Translating codons into proteins Signup and view all the answers

How many different codons are possible with the four nucleotides in mRNA?

64 Signup and view all the answers

What characterizes the genetic code as 'degenerate'?

An amino acid may be specified by more than one codon Signup and view all the answers

Which of the following codons serves as a termination signal?

UAA Signup and view all the answers

How is the directionality of mRNA sequence described?

5’-to-3’ direction Signup and view all the answers

What is the cause of Tay-Sachs disease?

Lack of hydrolytic enzyme in lysosomes Signup and view all the answers

Which disease is characterized by the accumulation of all acid hydrolases due to defective phosphorylation of mannose?

I cell disease Signup and view all the answers

What distinguishes regulated secretion from constitutive secretion?

Regulated secretion is hormone-dependent, while constitutive is not Signup and view all the answers

Which of the following options is a common feature of lysosomal storage diseases?

Accumulation of specific substrates due to enzyme deficiencies Signup and view all the answers

What is the typical lifespan of a child with the infantile form of Tay-Sachs disease?

4 years Signup and view all the answers

What is a characteristic clinical feature of Tay-Sachs disease?

Cherry Red Spot in the retina Signup and view all the answers

What type of secretion involves vesicles that bud off and continuously release contents outside the cell?

Constitutive secretion Signup and view all the answers

Which condition results from mutations in the HEXA gene?

Tay-Sachs disease Signup and view all the answers

Study Notes

Transcription

Transcription: the process of copying genetic information from DNA to RNA
Difference between DNA and RNA: the sugar in RNA is ribose, unlike deoxyribose in DNA, RNA has uracil instead of thymine, RNA is single stranded
Types of RNA: ribosomal RNA (rRNA), transfer RNA (tRNA), messenger RNA (mRNA), microRNA (miRNA)
Ribosomal rRNA: accounts for ~80% of total RNA, synthesized in the nucleolus, exists in multiple copies in the genome, 4 eukaryotic rRNAs: 28S, 18S, 5.8S and 5S, associate with proteins to form ribosomes, essential for protein synthesis
Transfer tRNA: accounts for ~15% of total RNA, synthesized in the nucleus, carries specific amino acids, at least one tRNA per amino acid, required for protein synthesis
Messenger mRNA: accounts for ~5% of total RNA, synthesized in the nucleus, carries genetic information from DNA to the cytosol, also referred to as transcripts, heterogeneous in size and sequence
MicroRNA miRNA: ~22 nucleotides in length, bind to 3’UTR of specific mRNAs, regulate gene expression by repressing protein production or triggering mRNA degradation
Splicing of introns: three-step process leading to removal of introns, requires spliceosome assembly
Spliceosome: composed of pre-mRNA, five small nuclear RNAs and proteins (snRNPs), ~50 proteins
Splice donor and acceptor sites: define the beginning and end of an intron
Branch site: contains an Adenosine (A) with an -OH group
Step 1: spliceosome recognizes donor and acceptor sites, brings the ends of the intron together
Step 2: hydroxyl group on the branch site A attacks the donor site, cleavage of intron at 5’ end, formation of a phosphodiester bond and lariat-like structure
Step 3: the cleaved 3’ end of exon 1 attacks the 5’ end of exon 2, intron is removed, exon ends are joined by a phosphodiester bond, spliceosome disassembles, mature mRNA is formed
Nuclear pores: function in exporting mature mRNA from the nucleus
Prokaryotes: continuous, fast transcription
Eukaryotes: transcription is more complex
Poly (A) tail addition: essential for stability and nuclear export, protects the mRNA from endonuclease activity
5' end of an intron: defined by a splice donor site
Histone acetylation: required for activation of transcription

Translation

Genetic code: set of rules used by ribosomes to translate DNA/RNA information into proteins, a link between nucleotide sequence and amino acid sequence
Codon: a sequence of three nucleotides that specifies a given amino acid
Codons: written and read in 5’-to-3’ direction, 64 different codons
61/64 codons: encode the 20 amino acids
3/64 codons: are termination signals, UAG, UGA, UAA
Specific: a codon always codes for the same amino acid
Universal: conserved across species, very few variations in mitochondrial code, specific yeast and bacteria types
Degenerate (or redundant): an amino acid can be specified by more than one codon
Initiation: assembly of ribosomal subunits, initiator aminoacyl-tRNA, eukaryotic initiation factors (eIFs), and mRNA
Regulation of Translational Initiation: rate-limiting step involves binding of the 5’ m7Gp Cap by the methylguanosine cap binding complex (CBC)
eIFs: control points for regulating initiation, globin synthesis in reticulocytes is regulated by heme availability
Elongation: ribosome moves along the mRNA in a multistep process, involves binding of incoming aminoacyl-tRNA to the A site, peptide bond formation, translocation of the ribosome alongside mRNA
Termination: ribosome reaches a termination codon, binding of eukaryotic release factor (eRF) to the A site, release of polypeptide chain from ribosome, dissociation of the ribosome-mRNA complex + recycling of components
STOP CODONS: UAA, UAG, UGA

Protein Trafficking

Lysosomal storage diseases: accumulation of undegraded substances due to defective lysosomal enzymes
I cell disease (GNPTAB mutations): impaired targeting of all acid hydrolases to lysosomes due to defective phosphorylation of mannose
Hurler & Hunter syndromes (IDUA or IDS mutations): Mucopolysaccharidosis types I & II
Tay-Sachs disease (HEXA mutations): accumulation of glycolipids (GM2 gangliosides) in the brain due to a lack of the hydrolytic enzyme in lysosomes, progressive deterioration of nerve cells, death by age 4 (infantile form), characteristic "Cherry Red Spot" in the retina, over 120 different mutations, single nucleotide insertions/deletions, splice site mutations, missense mutations
Secreted proteins: reach the Golgi, lack a retention signal for the Golgi, lysosomes, or plasma membrane, secreted outside the cell, encapsulated in transport vesicles, bud off from the Trans-Golgi Network, two options: constitutive secretion and regulated secretion
Constitutive secretion: fuses with the plasma membrane and is secreted continuously, transports large molecules from cells
Regulated secretion: stored in the cytoplasm and only secreted at specific times, a discontinuous process, occurs in response to signals

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Description

This quiz explores the transcription process, detailing the key differences between DNA and RNA. It covers the various types of RNA, including their functions, synthesis, and significance in protein synthesis. Test your understanding of ribosomal RNA, transfer RNA, messenger RNA, and microRNA.