Ch 17

These flashcards cover key concepts, processes, and terminology related to the connection between genes and proteins, as discussed in the lecture.

What led Archibald Garrod to propose that genes dictate phenotypes through enzymes?

Archibald Garrod suggested that genes control phenotypes by coding for enzymes that regulate biochemical processes in the body, leading to observable traits linked with metabolic functions.

What were Beadle and Tatum's experiments with Neurospora, and what did they discover about genes controlling metabolism?

Beadle and Tatum used the mold Neurospora to show that specific genes are responsible for the production of specific enzymes, leading to the formulation of the 'one gene-one enzyme' hypothesis, establishing a clear link between genes and metabolic functions.

What are the differences between the 'one gene-one enzyme' hypothesis and the 'one gene-one polypeptide' hypothesis? Why was the original hypothesis revised?

The 'one gene-one enzyme' hypothesis proposed that each gene encodes a single enzyme. However, it was revised to the 'one gene-one polypeptide' hypothesis, acknowledging that genes may also produce polypeptides that are not enzymes, thus broadening the role of genes.

How does RNA differ from DNA?

RNA is typically single-stranded, contains uracil instead of thymine, and has ribose sugar, whereas DNA is double-stranded and consists of deoxyribose sugar.

How does information flow from gene to protein? Is the central dogma ever violated?

Information flows from DNA to RNA (transcription) and then to protein (translation). While the central dogma states this flow, exceptions exist, such as reverse transcription in retroviruses.

What is the distinction between transcription and translation?

Transcription is the process of synthesizing RNA from a DNA template, while translation is the decoding of mRNA to synthesize polypeptides.

Where do transcription and translation occur in bacteria and eukaryotes?

In bacteria, both processes occur in the cytoplasm, whereas in eukaryotes, transcription occurs in the nucleus and translation occurs in the cytoplasm.

Define 'codon' and explain its relationship to the amino acid sequence in a polypeptide.

A codon is a sequence of three nucleotides on mRNA that specifies a particular amino acid, establishing a direct relationship between the linear sequence of codons and the corresponding amino acids in a polypeptide chain.

What early techniques were used to identify amino acids corresponding to triplets UUU, AAA, GGG, and CCC?

Early techniques included polypeptide synthesis experiments and using artificial mRNA to determine which amino acids were specified by these codons, leading to the mapping of the genetic code.

Why do polypeptides begin with methionine when synthesized?

Polypeptides start with methionine because the codon AUG serves as the start codon in mRNA, which signals the initiation of translation.

What does it mean that the genetic code is redundant and unambiguous?

The genetic code is redundant because multiple codons can specify the same amino acid, but it is unambiguous since each codon corresponds to only one amino acid.

What is the significance of the reading frame during translation?

The reading frame determines how the triplet codons are grouped during translation; shifting the reading frame alters the resulting polypeptide, potentially leading to dysfunction.

What is the evolutionary significance of a nearly universal genetic code?

The nearly universal genetic code suggests that all living organisms share a common evolutionary ancestor, which reflects evolutionary relationships and provides insight into genetics across species.

How does RNA polymerase recognize where transcription begins? What are the roles of the promoter, terminator, and transcription unit?

RNA polymerase binds to the promoter sequence to initiate transcription, elongates the RNA strand, and stops at a terminator sequence, all of which are part of the transcription unit.

What are the major steps involved in transcription?

Transcription involves three major steps: initiation (binding of RNA polymerase to the promoter), elongation (synthesizing RNA), and termination (RNA polymerase dissociates at the terminator).

How is RNA modified after transcription in eukaryotic cells?

After transcription, RNA undergoes processing that includes 5' capping, polyadenylation at the 3' end, and splicing to remove introns.

What are ribozymes and what properties enable them to function?

Ribozymes are RNA molecules that can catalyze biochemical reactions, functioning due to their three-dimensional structure, the presence of functional groups, and the ability to form base pairs with other nucleic acids.

What is the significance of introns in RNA processing?

Introns play a functional role in gene expression regulation and alternative splicing, contributing to genetic diversity and evolutionary processes.

How does alternative RNA splicing increase protein diversity?

Alternative RNA splicing enables different combinations of exons from the same gene to be included in the final mRNA, allowing a single gene to produce multiple protein products.

Describe the structure and function of tRNA. How does it relate to protein synthesis?

tRNA (transfer RNA) has a cloverleaf structure that carries specific amino acids to the ribosome, where its anticodon pairs with mRNA codons during translation, facilitating the addition of amino acids to the growing polypeptide chain.

What is the significance of wobble in tRNA?

Wobble refers to the flexibility of base pairing at the third position of a codon, allowing tRNA to recognize multiple codons that specify the same amino acid, thus enhancing the efficiency of protein synthesis.

How is tRNA linked to its specific amino acid?

tRNA is attached to its corresponding amino acid by the enzyme aminoacyl-tRNA synthetase, which ensures correct pairing between tRNA and the amino acid based on the tRNA's anticodon.

Describe the structure and function of ribosomes. What role do they play in protein synthesis?

Ribosomes are large complexes composed of ribosomal RNA and proteins that facilitate the translation of mRNA into polypeptides by binding tRNAs and catalyzing peptide bond formation.

Why can a ribosome be considered a colossal ribozyme?

A ribosome can be regarded as a colossal ribozyme because it is primarily RNA-based and catalyzes the formation of peptide bonds between amino acids during protein synthesis.

What are the stages of translation and the associated requirements for each?

Translation involves initiation (assembling the ribosome, mRNA, and first tRNA), elongation (adding amino acids as specified by the mRNA codons), and termination (release of the completed polypeptide). Key enzymes like peptidyl transferase, protein factors, and energy sources such as GTP are required.

What is the significance of polyribosomes?

Polyribosomes consist of multiple ribosomes simultaneously translating the same mRNA strand, increasing the efficiency of protein synthesis by producing many copies of a protein simultaneously.

What determines the primary structure of a protein and how is it modified before becoming functional?

The primary structure of a protein is determined by the sequence of amino acids dictated by the mRNA. Polypeptides undergo folding and post-translational modifications (like glycosylation) to become fully functional proteins.

How do ribosomes determine if they are free in the cytosol or attached to the rough endoplasmic reticulum?

Ribosomes are free in the cytosol when synthesizing cytosolic proteins but become bound to the rough endoplasmic reticulum when synthesizing proteins that will be secreted or sent to the endomembrane system, guided by the presence of a signal peptide.

Define point mutations. What are base-pair substitutions and insertions? Provide examples and significance.

Point mutations are changes in a single nucleotide in the DNA sequence. Base-pair substitutions replace one nucleotide with another (e.g., A to G), while insertions add an extra nucleotide (e.g., ATG to ATCG). These mutations can lead to alterations in protein function.

What is the difference between missense and nonsense mutations?

Missense mutations result in a change in one amino acid in the protein, potentially altering its function, while nonsense mutations introduce a premature stop codon, leading to a truncated and usually nonfunctional protein.

Why are insertions or deletions typically more harmful than substitutions?

Insertions and deletions often cause frameshift mutations, altering the entire downstream reading frame and resulting in a drastically different and usually nonfunctional protein, whereas substitutions may only affect one amino acid.

Define mutation. Provide examples of physical and chemical mutagens.

A mutation is a change in the DNA sequence that can affect gene function. Examples of physical mutagens include UV radiation, while chemical mutagens can include substances like benzene or certain alkylating agents.

How do gene expressions compare among bacteria, archaea, and eukarya? Which is most similar to archaea?

Gene expression mechanisms vary; bacteria employ simpler processes, while archaea share traits with both bacteria and eukarya. However, archaeal gene expression is generally more similar to eukaryotic processes due to the presence of introns and similar transcription factors.

Discuss the historical evolution of the concept of a gene.

The concept of a gene has evolved from early theories proposing heredity factors to modern views of genes as sequences of DNA responsible for specific traits, influenced significantly by research in genetics, molecular biology, and genomics.