Biology 189 – Unit 5: Inheritance, DNA & Gene Regulation

156 Question-and-Answer style flashcards covering key terminology, mechanisms, historical experiments, and regulatory concepts from the Biology 189 Unit 5 Inheritance & Chromosomes study material. The collection spans Mendelian genetics, DNA structure and replication, transcription, RNA processing, translation, protein modification, mutations, and gene regulation in both prokaryotes and eukaryotes.

How many chromosomes are in a human somatic cell, and how are they inherited?

46 chromosomes total; 23 are inherited from the mother and 23 from the father.

What does the term diploid (2n) mean?

A cell containing two complete sets of chromosomes, one set from each parent.

What does the term haploid (n) mean?

A cell containing a single set of chromosomes (e.g., human gametes with 23 chromosomes).

Define homozygous.

Having two identical alleles for a given gene (e.g., AA or aa).

Define heterozygous.

Having two different alleles for a given gene (e.g., Aa).

Define gene.

A DNA sequence that encodes information to produce a functional product (usually a protein or functional RNA).

Define allele.

Alternative versions of a gene that account for variations in a trait.

What is a homologous pair of chromosomes?

Two chromosomes (one maternal, one paternal) that carry the same genes at the same loci but may carry different alleles.

Define locus.

The specific, fixed position of a gene on a chromosome.

Define phenotype.

The observable traits or characteristics of an organism, resulting from genotype and environment.

Define genotype.

The genetic makeup of an organism—the specific set of alleles it possesses.

What is the purpose of a Punnett square?

A diagram that predicts the genotypic and phenotypic ratios of offspring from a particular genetic cross.

Explain complete dominance.

One allele completely masks the effect of the other in heterozygotes; only the dominant phenotype is expressed.

Explain codominance.

Both alleles are fully expressed in the heterozygote (e.g., AB blood type, roan cattle).

Explain incomplete dominance.

The heterozygote displays an intermediate phenotype between the two homozygotes (e.g., pink snapdragons).

Explain sex-linked (X-linked) dominance or recessiveness.

The allele is located on the X chromosome; its expression depends on sex and whether the allele is dominant or recessive.

How does genotype determine phenotype?

Genotype directs protein/RNA production, which influences biochemical pathways that generate observable traits.

State Mendel’s Law of Segregation.

Allele pairs separate during gamete formation and reunite at fertilization.

State Mendel’s Law of Independent Assortment.

Allele pairs of different genes segregate independently during gamete formation (if on separate chromosomes).

State Mendel’s Law of Dominance.

In a heterozygote, one allele may mask the expression of another recessive allele.

Cross: non-rolling mother (rr) × heterozygous father (Rr). What are offspring ratios?

Genotypes: 50 % Rr, 50 % rr; Phenotypes: 50 % rollers, 50 % non-rollers (1 : 1).

Cross: roan bull × roan cow (codominant Rr × Rr). Predicted offspring?

25 % RR (red), 50 % Rr (roan), 25 % rr (white). Phenotypic ratio 1 red : 2 roan : 1 white.

Cross: red (RR) × white (rr) snapdragons. Offspring outcomes?

100 % Rr genotypes; all exhibit pink phenotype due to incomplete dominance.

Color-blindness cross – carrier mother × color-blind father. Chances offspring color-blind?

Overall 50 % of children; 50 % of sons, 50 % of daughters will be color-blind.

What is polygenic inheritance?

A trait controlled by many genes, each with small additive effects (e.g., skin color, height).

Define epistasis.

Interaction in which one gene masks or modifies the expression of another gene.

What did Frederick Griffith discover (1928)?

Transformation principle—non-virulent bacteria became virulent after exposure to heat-killed virulent cells.

What did Avery, MacLeod & McCarty show (1944)?

DNA—not protein—was the transforming principle in Griffith’s experiments.

What was Chargaff’s contribution (1947)?

Discovered that [A]=[T] and [G]=[C] in DNA—Chargaff’s rules.

Contribution of Wilkins & Franklin (1950)?

X-ray diffraction images (Photo 51) revealing DNA’s helical structure and uniform diameter.

Contribution of Hershey & Chase (1952 classic publication).

Demonstrated DNA (not protein) is the genetic material using T2 bacteriophage labeling.

Contribution of Watson & Crick (1953).

Proposed the double-helix model of DNA with antiparallel strands and specific base pairing.

State Chargaff’s rules.

In DNA, %A ≈ %T and %G ≈ %C; purine pairs with a specific pyrimidine.

Name the purines in DNA.

Adenine (A) and Guanine (G).

Name the pyrimidines in DNA.

Cytosine (C) and Thymine (T).

What three components form a nucleotide?

A 5-carbon sugar, a phosphate group, and a nitrogenous base.

Differentiate the 3′ and 5′ ends of DNA.

5′ end has a free phosphate on carbon 5 of deoxyribose; 3′ end has a free hydroxyl on carbon 3.

How are nucleotides linked in DNA?

Phosphodiester bonds connect the 3′-OH of one nucleotide to the 5′-phosphate of the next.

Define antiparallel.

The two DNA strands run in opposite 5′→3′ directions.

Define double helix.

Two complementary, antiparallel DNA strands twisted into a spiral shape.

Define base pair.

A hydrogen-bonded pair of complementary bases: A–T or G–C.

Define phosphodiester linkage.

Covalent bond between the phosphate of one nucleotide and sugar of the next in nucleic acids.

What is semiconservative replication?

Each daughter DNA molecule contains one parental and one newly synthesized strand.

What is an origin of replication?

Specific DNA sequence where replication begins.

Define replication fork.

Y-shaped region where parental strands are unwound and new strands are synthesized.

What is bidirectional synthesis?

Replication proceeds in both directions away from each origin.

Distinguish leading versus lagging strands.

Leading strand synthesized continuously toward the fork; lagging strand synthesized discontinuously away from the fork.

What are Okazaki fragments?

Short DNA fragments synthesized on the lagging strand.

What are telomeres?

Repetitive DNA sequences at chromosome ends protecting genes from erosion.

What is proofreading in DNA replication?

DNA polymerase’s 3′→5′ exonuclease activity that removes misincorporated nucleotides.

What is mismatch repair?

Post-replication system that detects and replaces mismatched bases not caught by proofreading.

Role of helicase in replication.

Unwinds and separates the two DNA strands at the fork.

Role of single-strand binding proteins.

Stabilize unwound DNA strands, preventing re-annealing.

Role of topoisomerase.

Relieves supercoiling ahead of the replication fork by cutting and rejoining DNA.

Role of RNA primase.

Synthesizes short RNA primers needed for DNA polymerase to start synthesis.

Role of DNA polymerase III (prokaryotes).

Main enzyme that extends DNA from primers on leading and lagging strands.

Role of DNA polymerase I.

Removes RNA primers (5′→3′ exonuclease) and fills gaps with DNA.

Role of endonuclease/exonuclease enzymes.

Cut nucleic acids internally (endo) or remove terminal nucleotides (exo) during repair/removal.

Role of DNA ligase.

Seals nicks by forming the final phosphodiester bond, joining Okazaki fragments.

Why do chromosomes shorten each replication cycle?

DNA polymerase cannot fully replicate the 3′ ends of lagging strands once primers are removed.

How do stem cells avoid telomere shortening?

Telomerase extends telomeres by adding repeats, preventing critical shortening.

How does DNA polymerase proofread mismatches?

It stalls, shifts the strand into its exonuclease site, removes the wrong base, then resumes synthesis.

What did Archibald Garrod propose?

‘Inborn errors of metabolism’—genes encode enzymes; linkage between genes and metabolic pathways.

What did Marshall Nirenberg accomplish?

Deciphered the first codons of the genetic code (poly-U codes for phenylalanine).

What do genes ultimately code for?

Functional products—primarily proteins, or functional RNAs such as rRNA, tRNA, snRNA.

Define transcription.

Synthesis of RNA using a DNA template.

What is the DNA template strand?

The strand of DNA read by RNA polymerase to build complementary RNA.

What is RNA polymerase?

Enzyme that catalyzes RNA synthesis 5′→3′ without a primer.

What are transcription factors?

Proteins that assist RNA polymerase in binding and initiating transcription.

Define promoter region.

DNA sequence where RNA polymerase and transcription factors assemble to begin transcription.

What is the TATA box?

A promoter element (~-25 bp) rich in T and A that helps position RNA polymerase II.

What is the transcription initiation complex?

RNA polymerase + general transcription factors assembled at the promoter.

Define elongation (transcription).

RNA polymerase moves along DNA, adding complementary RNA nucleotides.

What does “processive” mean for RNA polymerase?

It can synthesize a long RNA strand without dissociating from DNA.

Define termination (transcription).

Release of RNA transcript and detachment of polymerase at a termination signal.

Define pre-mRNA.

The initial, unprocessed RNA transcript produced in eukaryotes.

Define mature mRNA.

Processed RNA that has 5′ cap, poly-A tail, and introns removed; ready for export and translation.

Define snRNA.

Small nuclear RNA that forms part of spliceosome; involved in RNA splicing.

Define tRNA.

Transfer RNA that carries specific amino acids to the ribosome during translation.

List the four RNA bases (full names).

Adenine, Cytosine, Guanine, Uracil.

Which DNA bases pair with RNA bases?

RNA A pairs with DNA T; RNA U pairs with DNA A; RNA C pairs with DNA G; RNA G pairs with DNA C.

What is a polyadenylation signal?

AAUAAA sequence in RNA that signals cleavage and addition of poly-A tail.

In which direction does RNA polymerase synthesize RNA?

5′→3′ direction, adding to the 3′-OH of the growing RNA chain.

Where does transcription occur in eukaryotes?

In the nucleus.

What is the 5′ cap and its function?

Modified G nucleotide added to 5′ end; protects RNA and aids ribosome binding.

What is the poly-A tail and its function?

~50–250 A residues added to 3′ end; stabilizes mRNA and aids nuclear export.

What does UTR stand for and where are they found?

Untranslated regions at 5′ and 3′ ends of mRNA flanking the coding sequence.

Define intron.

Non-coding sequence removed from pre-mRNA during splicing.

Define exon.

Coding sequence that remains in mRNA and is expressed.

What composes a spliceosome?

snRNA + proteins (snRNPs) that remove introns and join exons.

What is an snRNP?

Small nuclear ribonucleoprotein particle—snRNA bound to proteins within spliceosome.

Define ribozyme and give an example.

RNA molecule with catalytic activity; e.g., rRNA peptidyl transferase or self-splicing introns.

Role of consensus sequences in splicing.

Defined nucleotide motifs at intron ends that guide spliceosome recognition and cutting.

Where does RNA processing occur?

In the nucleus of eukaryotic cells.

Difference between pre-mRNA and mature mRNA.

Pre-mRNA contains introns and lacks cap/tail; mature mRNA is capped, tailed, intron-free.

Define codon.

A three-nucleotide mRNA sequence specifying an amino acid or stop signal.

How many nucleotides are in one codon?

Three.

What is the start codon sequence and its amino acid?

AUG; codes for methionine (Met).

Name the stop codons and do they code an amino acid?

UAA, UAG, UGA; they do not code for any amino acid—signal termination.

What does redundancy of the genetic code mean?

Most amino acids are coded by more than one codon.