Molecular Biology and Genetics: Key Concepts and Processes

What is gene expression?

The process by which genetic information in DNA is converted into functional gene products, primarily RNA and proteins.

What does the Central Dogma of Molecular Biology state?

DNA is transcribed to RNA, which is then translated into protein.

Who conducted experiments that laid the foundation for gene function understanding?

George Beadle and Edward Tatum.

What organism did Beadle and Tatum use for their experiments?

Neurospora crassa (bread mold).

What hypothesis did Beadle and Tatum propose?

The 'one gene-one enzyme' hypothesis, later refined to 'one gene-one polypeptide.'

What was the purpose of irradiating Neurospora in Beadle and Tatum's experiments?

To induce mutations for studying gene function.

What did Srb and Horowitz investigate?

The arginine biosynthesis pathway in Neurospora.

What are the three classes of mutants identified by Srb and Horowitz?

Class I (defect in Enzyme A), Class II (defect in Enzyme B), Class III (defect in Enzyme C).

What is the genetic code?

A triplet code where three consecutive nucleotides (a codon) specify a single amino acid.

What is a frameshift mutation?

A mutation caused by insertions or deletions that alters the reading frame of the genetic code.

What are the characteristics of the genetic code?

Redundancy (multiple codons for the same amino acid) and unambiguity (each codon specifies only one amino acid).

What is the start codon and its significance?

AUG, which codes for methionine and establishes the open reading frame (ORF).

What did Sutton and Boveri propose?

The Chromosome Theory of Inheritance, stating that genes are located on chromosomes.

What organism did Thomas Hunt Morgan use for his genetic studies?

Drosophila melanogaster (fruit flies).

What was significant about Morgan's white-eyed male fly?

It provided evidence for sex-linked inheritance patterns.

What is gene linkage?

The phenomenon where genes located on the same chromosome are inherited together.

What is the significance of crossing over during meiosis?

It allows for the exchange of genetic material between homologous chromosomes, creating recombinant gametes.

How is sex determined in humans?

By the sex chromosomes: XX for females and XY for males.

What is X-inactivation?

The random inactivation of one X chromosome in females, forming a Barr body.

What is incomplete dominance?

A genetic scenario where heterozygotes display an intermediate phenotype between the two homozygous phenotypes.

What is codominance?

A situation where both alleles in a heterozygote are fully and distinctly expressed.

What is pleiotropy?

When a single gene influences multiple, seemingly unrelated phenotypic traits.

What are lethal alleles?

Alleles that cause death when present in certain genotypes, affecting Mendelian ratios.

What is epistasis?

When the expression of one gene at one locus masks or modifies the expression of another gene at a different locus.

What is the significance of the SRY gene?

It is critical for initiating male development in humans.

What is the role of Punnett squares?

They are used to calculate genotypic and phenotypic frequencies for genetic traits.

What is polygenic inheritance?

Multiple genes contribute to a single phenotypic trait, resulting in continuous variation in phenotype.

Give an example of a trait that exhibits polygenic inheritance.

Human skin color or height.

What determines the phenotype of an organism?

The interaction between its genotype and the environment.

How does temperature affect the fur color of Himalayan rabbits?

Darker fur is produced in colder areas due to temperature-dependent phenotype.

What is nondisjunction?

The failure of chromosomes to separate properly during cell division, leading to aneuploidy.

What is an example of a condition caused by nondisjunction?

Trisomy 21, which causes Down syndrome.

What are deletions and duplications in chromosomes?

Deletions are the loss of a chromosomal segment, while duplications are the gain of an extra copy of a segment.

What is an inversion in chromosomal structure?

A chromosomal segment is reversed in orientation, potentially altering gene expression.

What is a translocation in genetics?

A segment of one chromosome moves to a different, non-homologous chromosome.

What is the Law of Segregation?

Allele pairs for a given trait separate during gamete formation, with each gamete receiving one allele.

What does the Law of Independent Assortment state?

Alleles of different genes assort independently during gamete formation.

What is a gene?

A heritable factor that determines a particular trait.

What is the difference between homozygous and heterozygous?

Homozygous has two identical alleles for a trait, while heterozygous has two different alleles.

What is a Punnett square used for?

To predict the genotypic and phenotypic ratios of offspring from genetic crosses.

What is the purpose of meiosis?

To produce haploid gametes essential for sexual reproduction and genetic diversity.

What are the key differences between mitosis and meiosis?

Mitosis results in two genetically identical daughter cells, while meiosis produces four genetically different haploid cells.

What occurs during Prophase I of meiosis?

Homologous chromosomes pair up and crossing over occurs, leading to genetic variation.

What is the outcome of Meiosis II?

Four genetically different haploid daughter cells (gametes) are formed.

What is the cell cycle?

The ordered sequence of events that a cell undergoes to grow and divide.

What happens during the G1 phase of the cell cycle?

Cell growth and normal metabolic functions occur.

What is the function of checkpoints in the cell cycle?

To act as molecular brakes, halting the cell cycle if conditions are unfavorable or errors have occurred.

What is the G1 checkpoint responsible for?

Checking for cell size, nutrients, growth factors, and DNA damage before committing to division.

What is random fertilization?

The process where any sperm can fertilize any egg, increasing offspring diversity.

What is aneuploidy?

An abnormal chromosome number resulting from nondisjunction.

What is the role of synapsis in meiosis?

Homologous chromosomes pair up to form tetrads during Prophase I.

What is the significance of the S phase in the cell cycle?

DNA synthesis (replication) occurs, duplicating chromosomes.

What does the G2 checkpoint assess?

Checks for DNA replication completion and DNA damage.

What is the purpose of the M checkpoint?

Ensures all chromosomes are properly attached to the mitotic spindle before anaphase.

What are Cyclin-Dependent Kinases (CDKs)?

Enzymes that activate or inactivate other proteins by phosphorylating them; they are inactive on their own.

What role do cyclins play in the cell cycle?

Regulatory proteins whose concentrations fluctuate cyclically to activate CDKs.

What triggers entry into mitosis?

The M-Cdk complex activated by M-cyclin.

What are external signals that influence checkpoint progression?

Growth factors, density-dependent inhibition, and anchorage dependence.

What internal signals affect checkpoint progression?

Cell size, nutrient availability, DNA integrity, and chromosome attachment to the spindle.

How is cancer related to cell cycle control?

Cancer is characterized by uncontrolled cell proliferation due to breakdown of cell cycle regulation.

What are proto-oncogenes?

Normal genes that promote cell growth and division; mutations can turn them into oncogenes.

What is the function of tumor suppressor genes?

They inhibit cell division, promote DNA repair, or induce apoptosis; mutations can lead to uncontrolled proliferation.

What is the role of the p53 gene?

A critical tumor suppressor that responds to DNA damage by halting the cell cycle or initiating apoptosis.

What are the main functions of cell division?

Reproduction, growth and development, and tissue renewal and regeneration.

What is the structure of eukaryotic chromosomes?

Linear DNA molecules tightly packaged with proteins into a complex called chromatin.

What happens to chromosomes during the S phase?

DNA replication occurs, forming sister chromatids.

What occurs during prophase of mitosis?

Chromosomes condense, and the mitotic spindle begins to form from centrosomes.

What is the significance of the prometaphase stage?

The nuclear envelope breaks down, and microtubules attach to kinetochores on chromosomes.

What happens during metaphase?

All chromosomes align at the metaphase plate, and kinetochores attach to microtubules from opposite poles.

What occurs during anaphase?

Cohesins are cleaved, sister chromatids separate, and are pulled towards opposite poles.

What happens during telophase?

Chromosomes arrive at the poles, decondense, and nuclear envelopes reform around the two sets.

What is cytokinesis?

The division of the cytoplasm, occurring concurrently with telophase.

How do cytokinesis differ in animal and plant cells?

In animal cells, a cleavage furrow forms; in plant cells, a cell plate forms and grows outward.

What was the significance of Griffith's Transformation Experiment?

It suggested that heritable material could be transferred between bacteria.

What did Avery, MacLeod, and McCarty's Experiment conclude?

Only DNA degradation prevented transformation, identifying DNA as the genetic material.

What was the key finding of the Hershey-Chase Experiment?

DNA, not protein, is the genetic material, as only labeled DNA entered bacterial cells.

What is the structure of DNA according to the Watson-Crick model?

A double helix with two antiparallel strands and specific base pairing between A-T and G-C.

What are Chargaff's rules?

In any DNA sample, the amount of adenine equals thymine, and the amount of guanine equals cytosine.

What defines the directionality of DNA strands?

DNA strands have directionality defined by their 5' (phosphate) and 3' (hydroxyl) ends.

What is DNA replication?

DNA replication is the process by which a DNA molecule is duplicated, ensuring that each daughter cell receives a complete set of genetic information.

What is the semi-conservative model of DNA replication?

The semi-conservative model, proposed by Watson and Crick and confirmed by Meselson and Stahl, states that each new DNA molecule consists of one parental strand and one newly synthesized strand.

What initiates DNA replication?

Replication begins at specific DNA sequences called origins of replication.

How does replication proceed from the origins?

Replication proceeds bidirectionally from each origin, forming replication bubbles.

Why do eukaryotic chromosomes have multiple origins of replication?

Eukaryotic chromosomes have multiple origins to speed up replication of their large genomes.

What is the role of helicase in DNA replication?

Helicase unwinds and separates the two DNA strands at the replication fork.

What does topoisomerase do during DNA replication?

Topoisomerase relieves the torsional strain (supercoiling) ahead of the replication fork caused by unwinding.

What is the function of single-strand binding proteins (SSBs)?

SSBs bind to the separated single DNA strands to stabilize them and prevent them from re-annealing.

What is the role of primase in DNA replication?

Primase synthesizes short RNA primers, providing a pre-existing 3'-OH group for DNA polymerase to start synthesis.

What is the function of DNA Polymerase III?

DNA Polymerase III synthesizes new DNA strands by adding nucleotides complementary to the template strand, synthesizing in the 5' to 3' direction.

What does DNA Polymerase I do?

DNA Polymerase I removes RNA primers and replaces them with DNA nucleotides.

What is the role of DNA ligase in DNA replication?

DNA ligase joins the newly synthesized DNA fragments (Okazaki fragments) on the lagging strand, forming a continuous strand.

What distinguishes the leading strand from the lagging strand?

The leading strand is synthesized continuously in the 5' to 3' direction, while the lagging strand is synthesized discontinuously in short fragments called Okazaki fragments.

What is the end replication problem?

The end replication problem occurs because DNA polymerase cannot fully replicate the 3' end of the lagging strand template, leading to progressive shortening of chromosomes.

What are telomeres?

Telomeres are repetitive, non-coding DNA sequences at the ends of eukaryotic chromosomes that act as buffers to postpone the erosion of genes.

What is the function of telomerase?

Telomerase is an enzyme that extends telomeres and is active in germ cells, embryonic cells, and often in cancer cells.

How does DNA replication achieve high fidelity?

DNA replication is accurate due to proofreading activity of DNA polymerases and mismatch repair mechanisms.

What is photosynthesis?

Photosynthesis is the process where light energy is converted into chemical energy to synthesize organic compounds.

What is chlorophyll?

Chlorophyll is the primary photosynthetic pigment responsible for absorbing light energy, containing a porphyrin ring with a magnesium atom.

What occurs during photoexcitation?

Photoexcitation occurs when chlorophyll absorbs a photon of light, boosting an electron to a higher energy orbital.

What are photosystems?

Photosystems are complexes of pigment molecules and proteins embedded in the thylakoid membrane that capture light energy.