Molecular Biology & Genetics – Final Exam Review

A comprehensive set of 97 question-and-answer flashcards covering key concepts in molecular biology and genetics for final exam preparation.

What is an allele in genetics?

A specific version or variant of a gene (e.g., brown vs. blue eye‐color allele).

What does the term locus refer to?

The physical location of a gene on a chromosome.

What are homologous chromosomes?

A pair of chromosomes, one from each parent, that carry the same genes at the same loci.

What is the centromere?

The constricted region that joins sister chromatids and attaches to spindle fibers during division.

What is a telomere?

The protective end region of a chromosome that prevents degradation and fusion.

Define genotype.

The genetic makeup of an organism, usually written as allele pairs (e.g., AA, Aa, aa).

Define phenotype.

The observable physical or biochemical traits produced by the genotype (e.g., blue eyes, tall height).

What is a dominant allele?

An allele that expresses its phenotype even when only one copy is present (heterozygous condition).

What is a recessive allele?

An allele that is expressed only when two copies are present (homozygous condition).

What is codominance?

A pattern where both alleles in a heterozygote are fully expressed (e.g., AB blood type).

What is incomplete dominance?

A pattern where the heterozygote displays an intermediate, blended phenotype (e.g., pink flowers).

What is pleiotropy?

A situation where one gene influences multiple, seemingly unrelated traits.

What does homozygous mean?

Having two identical alleles for a particular gene (AA or aa).

What does heterozygous mean?

Having two different alleles for a particular gene (Aa).

In a monohybrid cross of Aa × Aa, what is the expected genotype ratio?

1 AA : 2 Aa : 1 aa.

In an Aa × Aa cross where A is dominant, what is the phenotype ratio?

3 dominant trait : 1 recessive trait.

What is the typical phenotype ratio for a dihybrid cross (AaBb × AaBb)?

9 : 3 : 3 : 1.

Why is a test cross performed?

To determine the unknown genotype of an organism showing a dominant phenotype by crossing it with a homozygous recessive individual.

If a test cross yields all dominant offspring, what is the unknown parent's genotype?

Homozygous dominant (AA).

Why do X-linked recessive traits appear more often in males than females?

Males have only one X chromosome, so a single recessive allele is expressed, whereas females need two copies.

What pedigree clue suggests an autosomal dominant inheritance?

The trait appears in every generation and affects both sexes equally.

What pedigree clue suggests an autosomal recessive inheritance?

The trait can skip generations and often appears among siblings of unaffected parents (carriers).

What pedigree clue indicates X-linked recessive inheritance?

More males are affected and there is no male-to-male transmission of the trait.

Who transmits mitochondrial (mtDNA) disorders to offspring?

Only mothers pass mitochondrial DNA to all their children.

Give an example of labile (constantly dividing) cells.

Epidermal skin cells, GI tract lining, or bone-marrow cells.

Give an example of stable (quiescent) cells.

Hepatocytes (liver), renal tubule cells, or smooth muscle cells that divide after injury.

Give an example of permanent (non-dividing) cells.

Neurons, cardiac muscle cells, and skeletal muscle cells.

Name the three stages of interphase.

G₁ (growth), S (DNA synthesis), and G₂ (preparation for mitosis).

What key event occurs during S phase?

DNA replication that doubles each chromosome into two sister chromatids.

What happens during G₂ phase?

The cell checks DNA for errors and prepares proteins needed for mitosis.

What is the primary purpose of mitosis?

To produce two genetically identical diploid cells for growth and tissue repair.

How many chromosomes does a human cell have just before mitosis?

46 chromosomes (as duplicated sister chromatids).

After mitosis, how many cells and chromosomes result?

Two cells, each with 46 chromosomes (diploid).

What is the primary purpose of meiosis?

To create four genetically unique haploid gametes for sexual reproduction.

How many cells and what ploidy result after meiosis II?

Four non-identical haploid cells, each with 23 chromosomes.

During which substage of meiosis does crossing-over occur?

Prophase I (specifically the pachytene stage).

What separates in anaphase I versus anaphase II of meiosis?

Anaphase I separates homologous chromosome pairs; anaphase II separates sister chromatids.

Define euploidy.

Having the normal, complete set of chromosomes for the species (e.g., 46 in humans).

Define aneuploidy.

Having an abnormal number of chromosomes (e.g., 45 or 47).

Turner syndrome (45,X) is an example of which chromosomal abnormality?

Monosomy—missing one chromosome of a pair.

Down syndrome (47, +21) is an example of which abnormality?

Trisomy—three copies of chromosome 21.

What is a Robertsonian translocation?

Fusion of two acrocentric chromosomes at their centromeres, forming one large chromosome and a tiny fragment.

Which chromosomes are most often involved in Robertsonian translocations?

Acrocentric chromosomes 13, 14, 15, 21, and 22.

What is the most common cause of Down syndrome?

Nondisjunction during meiosis leading to trisomy 21.

Which hand feature is classic for Down syndrome?

A single palmar (Simian) crease.

What is the typical survival outlook for infants with Edwards syndrome (trisomy 18)?

Most affected infants die within the first year of life.

Polydactyly is characteristic of which trisomy?

Patau syndrome (trisomy 13).

What is the normal male karyotype?

46, XY.

What is the function of the SRY gene?

It encodes testis-determining factor, initiating male sexual development.

What adult structures are derived from the mesonephric (Wolffian) duct?

Epididymis, vas deferens, seminal vesicles, and ejaculatory ducts (mnemonic: SEED).

What structures arise from the paramesonephric (Müllerian) duct?

Fallopian tubes, uterus, cervix, and upper one-third of the vagina.

How many Barr bodies are present in a normal female (46,XX) cell?

One inactive X chromosome, visible as a Barr body.

What is the genotype for Klinefelter syndrome?

47, XXY.

Which cancer risk is higher in Klinefelter patients?

Breast cancer (and some germ-cell tumors).

What is the genotype for Turner syndrome?

45, X (monosomy X).

Which classic heart defect is associated with Turner syndrome?

Coarctation of the aorta (also bicuspid aortic valve).

How is mitochondrial DNA inherited?

Exclusively from the mother (maternal inheritance).

Which sugar is found in RNA nucleotides?

Ribose.

How many hydrogen bonds connect A–T and C–G base pairs?

A–T has 2 bonds; C–G has 3 bonds.

Which bases are purines?

Adenine (A) and Guanine (G).

State the central dogma of molecular biology.

Information flows from DNA → RNA → Protein.

What is the main role of DNA polymerase III?

It adds nucleotides to the growing DNA strand in the 5′→3′ direction during replication.

What is the function of DNA ligase?

It seals nicks and joins Okazaki fragments on the lagging strand.

What does topoisomerase do during replication?

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

In which cell types is telomerase highly active?

Germ cells, stem cells, and most cancer cells.

Which strand is synthesized continuously toward the replication fork?

The leading strand.

How do origins of replication differ in prokaryotes versus eukaryotes?

Prokaryotes have one origin per chromosome; eukaryotes have multiple origins per chromosome.

What forms the core of a nucleosome?

An octamer of histone proteins (2 each of H2A, H2B, H3, and H4) wrapped by DNA.

Which type of chromatin is transcriptionally active?

Euchromatin (loosely packed, light-staining).

What is the function of the TATA box within a promoter?

It is a DNA sequence where transcription factors and RNA polymerase II assemble to start transcription.

What is the purpose of the 5′ m⁷G cap on eukaryotic mRNA?

Protects mRNA from degradation and aids ribosome binding for translation initiation.

What does RNA splicing remove from pre-mRNA?

Introns (non-coding sequences).

What three-letter codon serves as the universal start codon?

AUG (codes for methionine).

Name the three stop codons.

UAA, UAG, and UGA.

What is the role of the anticodon on tRNA?

It base-pairs with the complementary mRNA codon to ensure correct amino acid incorporation.

Where does translation occur in eukaryotic cells?

In the cytoplasm, on free ribosomes or ribosomes bound to rough endoplasmic reticulum.

What are Okazaki fragments?

Short DNA segments synthesized discontinuously on the lagging strand during replication.

What type of error is corrected by mismatch repair?

Incorrectly paired bases inserted during DNA replication.

What is the consequence of a nonsense mutation?

A premature stop codon, leading to a truncated, usually nonfunctional protein.

Which disease is commonly caused by a ΔF508 mutation in the CFTR gene?

Cystic fibrosis.

Which disease results from a Glu→Val substitution in β-globin?

Sickle cell anemia.

Expansion of CAG repeats causes which neurodegenerative disorder?

Huntington’s disease.

Loss of the paternal copy of chromosome 15 results in which syndrome?

Prader-Willi syndrome.

Bilateral vestibular schwannomas are diagnostic of which condition?

Neurofibromatosis type 2 (NF2).

What is the inheritance pattern of cystic fibrosis?

Autosomal recessive.

Why are sickle-cell carriers relatively protected from malaria?

Heterozygous (AS) red cells impede the life cycle of Plasmodium parasites.

Carcinomas originate from which basic tissue type?

Epithelial tissues (coverings and linings).

How does a gain-of-function mutation in an oncogene promote cancer?

It keeps cellular growth signals permanently activated (accelerator stuck on).

What is a driver mutation?

A genetic alteration that directly contributes to cancer initiation and progression.

During PCR, at approximately what temperature does DNA denaturation occur?

Around 95 °C to separate the DNA strands.

Why is Taq polymerase used in PCR?

It is heat-stable and remains active after repeated high-temperature cycles.

In gel electrophoresis, which DNA fragments migrate farthest?

The smallest (shortest) fragments travel the greatest distance through the gel.

What is the purpose of a DNA ladder in gel electrophoresis?

It provides fragment size markers for estimating the lengths of sample DNA bands.

In DNA fingerprinting, child bands that do not match the mother must come from whom?

The biological father.

List the three basic steps of one PCR cycle in order.

Denaturation → Annealing → Extension.

Which enzyme synthesizes RNA during transcription in eukaryotes?

RNA polymerase II (for mRNA).

What is the function of the 3′ poly-A tail on mRNA?

Enhances mRNA stability, aids nuclear export, and assists in translation termination.

What is an allele in genetics?

A specific version or variant of a gene (e.g., brown vs. blue eye‐color allele).

What does the term locus refer to?

The physical location of a gene on a chromosome.

What are homologous chromosomes?

A pair of chromosomes, one from each parent, that carry the same genes at the same loci.