Cell Biology Exam 1 Chp5-8

Overview of Translation

Translation is the process by which ribosomes synthesize proteins using mRNA as a template

Key Components of Translation

mRNA

Ribosome Sites

The ribosome has three sites: A (aminoacyl)

Aminoacyl-tRNA Synthetases

Enzymes that attach the correct amino acid to its corresponding tRNA

Specificity of Aminoacyl-tRNA Synthetases

Each amino acid has a specific synthetase to ensure accuracy in protein synthesis

Importance of tRNA Charging

Correct charging prevents errors in the polypeptide chain and ensures fidelity of translation

Initiation (Translation)

Begins with the initiator tRNA carrying methionine binding to the P site of the small ribosomal subunit

Start Codon Recognition

The ribosome scans the mRNA for the start codon (AUG) to begin translation at the correct site

Elongation (Translation)

Ribosome moves along mRNA

Termination (Translation)

Occurs when a stop codon is reached

Polyribosomes (Polysomes)

Clusters of ribosomes translating the same mRNA simultaneously

Protein Regulation

Controlled by mechanisms like proteasomes and ubiquitin-mediated degradation to manage protein longevity

Misfolded Proteins

Targeted for degradation to maintain cellular quality control

Post-Translational Modifications (PTMs)

Chemical changes after translation (e.g.

Reversibility of PTMs

PTMs can be reversible or irreversible

Cofactors in Protein Function

Some proteins need cofactors (metal ions like zinc/iron or organic molecules like vitamins) to function

Hemoglobin Cofactor Example

Hemoglobin requires heme as a cofactor to bind oxygen effectively

Protein Subunit Interaction

Many proteins (e.g.

Protein Production in Eukaryotes

Transcription occurs in the nucleus

translation occurs in the cytoplasm

showing compartmentalization

Eukaryotic vs. Prokaryotic Translation

Eukaryotic translation is more complex

Importance of Studying Translation

Understanding translation and post-translational modifications explains how cells control protein synthesis and function

Nucleosomes are not present in which chromosomes?

Prokaryotic chromosomes

What do histone modifications do to chromatin?

Recruit proteins and change histone–DNA interactions

What does heterochromatin formation cause?

Gene silencing

How many chromosomes are in a human gamete?

23

Which chemical group is at the 3’ end of DNA?

Hydroxyl group (–OH)

Which groups can modify histones?

Acetyl

What does phosphorylation of histone serine add?

A negative charge

Do histone–DNA interactions depend on sequence?

No (False)

How many telomeres per human nucleus?

92

Why can RNA fold into 3D shapes?

Because it is single-stranded

Which nucleic acid binds to a poly-T column?

mRNA (poly-A tail)

How do eukaryotic ribosomes find the start codon?

By scanning from the 5’ cap

What are polyribosomes?

Multiple ribosomes translating the same mRNA

What amino acid does AUG code for?

Methionine (start codon)

What modification activates RNA Pol II tail?

Phosphorylation

Must introns be removed precisely?

Yes (False if saying imprecise removal is fine)

What does the A site of the ribosome do?

Binds incoming tRNA

Which transcription factor binds first at the promoter?

TFIID

What does the sigma factor do in bacteria?

Recognizes promoters

What recognizes stop codons?

Protein release factors

Do miRNAs

Noncoding RNAs incorporated into a protein complex called RISC, which searches mRNAs in the cytoplasm for sequences complementary to the miRNA and targets them for degradation or translation inhibition

What does the Mediator complex do?

Links transcription regulators to RNA Pol II

Examples of post-translational control?

Subunit interactions

How many genes are typically expressed per cell?

5

Which is NOT a housekeeping gene product?

Hemoglobin

Which complex holds miRNA for gene silencing?

RISC

What triggers RNA interference (RNAi)?

Double-stranded RNA (dsRNA)

What enzyme cuts dsRNA into siRNAs?

Dicer

Are miRNAs made from rRNAs?

No (False)

Which polymerase transcribes rRNA?

RNA Pol I

Which polymerase transcribes mRNA & miRNA?

RNA Pol II

Which polymerase transcribes tRNA & 5S rRNA?

RNA Pol III

What is the role of tRNA?

Adaptor between mRNA and amino acids

What is the role of miRNA?

Regulates gene expression

What is the role of snRNA?

Functions in RNA splicing

What is the role of rRNA?

Forms ribosome core and catalyzes protein synthesis

What is the role of siRNA?

Provides antiviral defense and gene silencing

Chromosomal Packing

Organization of DNA within the nucleus for efficient storage and regulation of genetic material

Eukaryotic Chromosome Structural Changes

Chromosomes undergo structural changes throughout the cell cycle

Nucleosome

Basic unit of chromatin

Specialized DNA Sites

Regions essential for replication

Chromatin Forms

Euchromatin (active) and heterochromatin (inactive)

Histone Modifications

Chemical changes such as acetylation and methylation that regulate chromatin structure and function

Reversibility of Histone Modifications

Modifications can be reversed to allow dynamic chromatin changes in response to cellular signals

Heterochromatin Spreading

Facilitated by histone tail modifications

X Chromosome Inactivation

Model for dosage compensation and epigenetic regulation in female mammals

Chromatin-Remodeling Complexes

Reposition DNA around nucleosomes using energy to allow access to genetic information

Constitutive Heterochromatin

Always compact

Quiescent Euchromatin

Regions not actively transcribed but can be activated under certain conditions

Facultative Heterochromatin

Can switch between active and inactive states for flexible gene regulation

Active Euchromatin

Transcriptionally active genes where DNA is accessible to transcription machinery

Dynamic Chromatin

Essential for cellular differentiation and response to environmental changes

Mosaic Populations (X-Inactivation)

Result from random X-chromosome inactivation in females

Chromosomal Composition

Human chromosomes consist of 23 pairs

Karyotype

Technique to visualize chromosomes and identify abnormalities or structural variations

Centromeres and Telomeres

Specialized chromosome regions critical for stability and replication

Telomeres per Human Genome

92 telomeres protect chromosome ends from degradation and fusion

Nucleolus

Prominent interphase nucleus structure involved in ribosome biogenesis

Ribosome Biogenesis

Nucleolus produces rRNA and assembles ribosomal proteins into functional ribosomes

Human Ribosome Composition

Composed of four rRNAs and over 80 proteins

Nucleolus Function

Regulates ribosome production to control cellular metabolism and growth

DNA in Interphase vs Mitotic Chromosomes

Less compact in interphase