Understanding Cell Division, Genetics, and Gene Expression
Bacteria vs. Archaea and Eukaryotes
What characteristic distinguishes bacteria from archaea and eukaryotes?
Bacteria lack histones associated with their chromosomes, unlike archaea and eukaryotes.
Cell Cycle and Mitosis
DNA Replication
In which cell cycle stage does DNA duplication occur?
DNA replication happens during the S phase.
Polyploidy
What term describes eukaryotic cells with more than two sets of genetic information?
These cells are called polyploid.
Telomeres
What is the function of telomeres?
Telomeres protect the ends of DNA molecules.
Centromeres
What is the constricted chromosome region where the kinetochore forms?
This region is the centromere.
Mitosis Products
What is the outcome of mitosis?
Mitosis produces daughter cells genetically identical to the parent cell.
Anaphase
Which stage of mitosis involves sister chromatids separating and moving to opposite poles?
Sister chromatids separate during anaphase.
G2/M Checkpoint
What must happen for a cell to pass the G2/M checkpoint?
The cell must have completely replicated and undamaged DNA.
Chromosomes and DNA Molecules in G2
A cell has 6 chromosomes in G1. How many chromosomes and DNA molecules will it have in G2?
It will have 6 chromosomes and 12 DNA molecules.
Meiosis and Genetic Variation
Homologous Chromosome Separation
In which stage of meiosis do homologous chromosomes separate?
Homologous chromosomes separate during anaphase I.
Sources of Genetic Variation
Meiosis generates genetic variation. What are the two sources?
- Random distribution of maternal and paternal chromosomes.
- Crossing over during prophase I.
Mitosis vs. Meiosis
Which statement does not accurately differentiate mitosis and meiosis?
Incorrect: Sister chromatids separate during mitosis but not during meiosis. (Sister chromatids separate in both processes.)
Cohesin’s Role
Which molecule holds sister chromatids together during mitosis and meiosis?
Cohesin holds sister chromatids together.
Ova Production
How many ova can be produced from two primary oocytes through meiosis?
Two ova can be produced.
Meiosis in Flowering Plants
What does meiosis in the male part of a flower produce?
It produces four microspores that will divide mitotically to form male gametophytes.
Mendelian Genetics
Homozygosity
What term describes an individual with two identical alleles at a gene locus?
This individual is homozygous.
Dominant Allele
When a homozygous round-seeded plant was crossed with a homozygous wrinkled-seeded plant, all progeny had round seeds. Why?
The allele for round seeds is dominant to the allele for wrinkled seeds.
Mendel’s Principle of Segregation
What process explains Mendel’s principle of segregation?
Meiosis explains the principle of segregation.
Phenotypic Ratio
What will be the phenotypic ratio in the offspring of an Aa individual crossed with an aa individual?
The phenotypic ratio will be 1:1.
Offspring Genotype
What type of offspring will result from a cross between an AABB individual and an aabb individual?
All offspring will be AaBb.
Genotype Ratio
What will be the genotype ratio in the offspring of two Aa parents crossed with each other?
The genotype ratio will be 1:2:1.
Independent Assortment
The principle of independent assortment involves at least how many different gene pairs?
It involves at least two different gene pairs.
Dominant Allele Assumption
If a plant has the genotype Aa, what can we assume?
We assume that the A allele is dominant to the a allele.
Sex Determination and Sex-Linked Inheritance
XX-XO System
In the XX-XO sex determination system:
Females have two X chromosomes, and males have one X chromosome.
Genie Sex Determination
With the genie sex-determination mechanism:
Sex is determined by genes on undifferentiated chromosomes. (Note: This statement seems unusual and may require further clarification.)
Male Phenotype in Humans
What typically determines the male sexual phenotype in humans?
The presence of the SRY gene on the Y chromosome.
White-Eyed Mutation in Drosophila
What phenotypes were observed in the offspring when Morgan crossed a white-eyed female Drosophila with a red-eyed male?
All males had white eyes, and all females had red eyes.
Y-Linked Inheritance
How is a mutant gene on the Y chromosome normally inherited in humans?
All sons of a father with the mutant gene will inherit it, but no daughters will.
X-Linked Red-Green Color Blindness
Red-green color blindness in humans is a recessive X-linked condition. What can we conclude?
A color-blind woman must have had a color-blind father.
Sex-Linked Inheritance in Chickens
In chickens, females are ZW, and males are ZZ. Congenital baldness is caused by a recessive sex-linked allele. What are the expected results of crossing a bald male with a normal female?
All male offspring will be bald, and all female offspring will be normal.
Barr Bodies
How many Barr bodies would be present in the somatic cells of a human male with the chromosome constitution XXXYY?
He would have two Barr bodies.
Gene Interactions
Incomplete Dominance
In a mouse species, fur color is determined by a single gene pair. BB individuals are black, bb individuals are white, and Bb individuals have gray fur with each hair being gray. What type of interaction is this?
This is an example of incomplete dominance.
Penetrance
What is penetrance?
Penetrance is the percentage of individuals with a specific genotype who express the expected phenotype.
Lethal Alleles
In chickens, the dominant Cr allele causes the creeper phenotype (short legs). However, the Cr allele is lethal in the homozygous condition (CrCr). The homozygous recessive genotype (crer) results in a normal individual. What will be the phenotype ratio among living offspring if two creepers are mated?
The phenotype ratio will be 2 creepers : 1 normal.
Codominance
A boy with blood type MN has the genotype LMLN. His red blood cells possess both the M and N antigens. What is the relationship between his two alleles?
His alleles exhibit codominance.
Genotype Combinations
A gene has three alleles. How many different genotypes are possible at this locus in a diploid organism?
Six different genotypes are possible.
Epistasis
What type of gene action occurs when one gene masks the effect of another gene at a different locus?
This is called epistasis.
Cytoplasmic Inheritance
In humans, mitochondrial genetic disorders are inherited only from the mother. Disease severity can vary greatly, even within a family. What form of inheritance is this?
This represents cytoplasmic inheritance.
Complementation
A deaf man and a deaf woman are both homozygous recessive for different autosomal mutant alleles causing deafness. If all their children have normal hearing, what has occurred within each child?
Complementation has occurred.
Pleiotropy
What is the phenomenon where one gene influences two or more seemingly unrelated phenotypes?
This is called pleiotropy.
Modified Dihybrid Ratios
Epistasis can modify dihybrid phenotypic ratios. Assume a modified ratio of 9:7 with gene pairs A and B. What would be a possible genotype for a phenotype within the 9 portion of the ratio?
A possible genotype is AaBB.
Human Genetics and Genetic Analysis
Adoption Studies
Which statement about adoption studies is incorrect?
Incorrect: Similarities between adopted persons and their adoptive parents can generally be attributed to genetic factors. (Similarities are more likely due to environmental factors.)
Autosomal Dominant Trait
A trait appears equally in men and women. Offspring can inherit it from either parent. All affected individuals have at least one affected parent. What type of trait is this?
This is an autosomal dominant trait.
α-Fetoprotein Levels
What might a significantly higher-than-normal α-fetoprotein level in a maternal blood screening test indicate?
It might indicate that the fetus has a neural tube defect.
Nondisjunction
What is the term for the failure of homologous chromosomes or sister chromatids to separate?
This is called nondisjunction.
Allopolyploidy
What type of polyploidy arises from hybridization between two different species?
This is called allopolyploidy.
Down Syndrome
Which statement about Down syndrome is true?
Trisomy of chromosome 21 is well-tolerated enough to allow for the development of fully developed individuals.
Prokaryotic and Eukaryotic Cells
Prokaryotic Cells
Relatively small (typically less than 10µm), lack a true cytoskeleton.
Eukaryotic Cells
Have a nuclear membrane surrounding DNA.
Both Cell Types
Contain DNA, use ribosomes to synthesize proteins.
Cat Genetics Example
In cats, curled ears (Cu) are dominant to normal ears (cu), and black color (G) is dominant to gray (g). These genes assort independently.
Cross 1: Homozygous Gray, Curled Ears x Homozygous Black, Normal Ears
Parental genotypes: ggCuCu x GGcucu
F1 genotypes: All GgCucu (black, curled ears)
Cross 2: Two F1 Cats
F1 genotypes: GgCucu x GgCucu
Expected F2 phenotype ratio:
- 9/16 black cats, curled ears
- 3/16 black cats, normal ears
- 3/16 gray cats, curled ears
- 1/16 gray cats, normal ears
Cross 3: F1 Cat x Stray Gray Cat with Normal Ears
F1 genotype: GgCucu
Stray cat genotype: ggcucu
Expected offspring phenotype ratio:
- 1/4 black cats, curled ears
- 1/4 black cats, normal ears
- 1/4 gray cats, curled ears
- 1/4 gray cats, normal ears
Central Dogma of Biology
What is the central dogma of biology?
The central dogma describes the flow of genetic information:
- Transcription: DNA is transcribed into RNA.
- Translation: RNA is translated into an amino acid sequence (protein).
Epistasis
What is epistasis?
Epistasis occurs when one gene influences or masks the effect of another gene.
Linkage
What is linkage?
Linkage describes the tendency of genes located close together on a chromosome to be inherited together. They do not assort independently.
Recombination Frequency and Genetic Maps
A recombination frequency of 5% translates to what distance on a genetic map?
It translates to a distance of 5 map units (m.u.).
Bacterial Genetics
Conjugation
Which method of gene transfer in bacteria requires physical contact between donor and recipient?
Conjugation requires physical contact.
F+ Bacterial Cell
What characterizes an F+ bacterial cell?
An F+ cell contains an F plasmid that carries genes for conjugation and may include some bacterial genes.
Plaque
What is the clear patch of lysed cells on a bacterial lawn called?
This clear patch is called a plaque.
Phage Cross Recombinant Classes
In a phage cross with T2 phage, one parent is h+r- and the other is h-r+. What are the recombinant progeny classes?
The recombinant classes are h+r+ and h-r-.
Conjugation with F+ Cell
Which type of bacterial cell can normally undergo conjugation with an F+ cell?
An F- cell can undergo conjugation with an F+ cell.
Avery, MacLeod, McCarty Experiment
In Avery, MacLeod, and McCarty’s experiment, heat-killed bacterial homogenates were treated with enzymes, and their ability to transform bacteria was tested. Under which condition would transformation not occur?
Transformation would not occur if the homogenates were treated with DNase, which degrades DNA.
Hershey-Chase Experiment
In the Hershey-Chase experiment, why was DNA confirmed as the genetic material?
The 32P label for DNA was found in the bacterial pellet, indicating that DNA had been injected into the bacteria.
Franklin and Wilkins’ Contribution
What was the major contribution of Franklin and Wilkins to the study of DNA?
They produced X-ray diffraction patterns of DNA, which were crucial in determining its structure.
Watson and Crick’s DNA Model
What was not shown by Watson and Crick’s model of DNA?
Their model did not show the bases facing outside for easy access. The bases are located on the inside of the double helix.
Fraenkel-Conrat and Singer’s Experiment
What did Fraenkel-Conrat and Singer demonstrate in their experiments with tobacco mosaic virus?
They showed that RNA could serve as the genetic material in some cases.
Deoxyribose Sugar
Why is the sugar in DNA called”deoxyribos”?
It is called deoxyribose because it is missing a 2′-OH group compared to ribose.
Z-DNA
What is a left-handed form of DNA?
A left-handed form of DNA is Z-DNA.
Hairpin Structure
What is a structure formed by a single-stranded DNA or RNA molecule with complementary sequences?
This structure is called a hairpin.
Genetic Screening Procedures
- Newborn screening: Identifies treatable genetic disorders soon after birth.
- Heterozygote screening: Identifies recessive disease carriers in a population.
- Prenatal diagnosis: Identifies genetic disorders in a developing fetus.
- Presymptomatic testing: Identifies a genetic disease in a healthy but high-risk individual.
Chromosomal Abnormalities
Turner Syndrome Mosaicism
How can mosaicism arise in individuals with Turner syndrome (45,X/46,XY)?
This mosaicism can occur if a cell loses a Y chromosome soon after fertilization in an XY embryo.
X-Linked Color Blindness Inheritance
Jill has normal color vision (XBXb), but her father is color blind (XbY). She marries Tom, who also has normal color vision (XBY). They have a daughter with Turner syndrome (45,X) who is color blind (Xb). How did the daughter inherit color blindness?
The daughter inherited the Xb chromosome from her mother. The Turner syndrome likely resulted from nondisjunction during meiosis in either the mother or father, leading to an ovum with only an Xb chromosome or a sperm lacking a sex chromosome.
Klinefelter Syndrome and Anhidrotic Ectodermal Dysplasia
Frank has Klinefelter syndrome (47, XXY). His mother has normal skin, but his father has anhidrotic ectodermal dysplasia, an X-linked condition where the skin lacks sweat glands. Frank has patches of normal skin and patches without sweat glands. Explain the inheritance pattern.
Frank received the X chromosome carrying the anhidrotic ectodermal dysplasia mutation from his father. The Klinefelter syndrome likely arose from nondisjunction during meiosis I in his father, resulting in a sperm carrying both an X and a Y chromosome.
Translocation Carrier and Down Syndrome
A woman is a translocation carrier of chromosome 21 on chromosome 14. She conceives six zygotes with a normal man. Predict the chromosome complements and viability of the zygotes.
Possible Outcomes:
- Viable, normal or translocation carrier: Two normal copies of 21, two normal copies of 14, or one normal 14, one normal 21, and one 21 to 14 translocation.
- Viable, with Down syndrome: One normal 14, two normal 21, and one 21 to 14 translocation.
- Inviable: One normal 14, one normal 21, two normal 14, one normal 21, one 21 to 14 translocation, or two normal 21, one normal 14.
Polyploidy
What is polyploidy?
Polyploidy is a chromosomal abnormality characterized by having at least three complete sets of homologous chromosomes.
Prokaryotic DNA vs. Plasmids
Plasmids
- Small, circular DNA molecules.
- Carry a small selection of additional genes.
- Replicate independently of the bacterial chromosome.
- Transferred during conjugation.
- Used as vectors in biotechnology.
Chromosomes
- The primary DNA molecule in prokaryotes.
- Usually circular.
- Located in the nucleoid region.
- Pulled to opposite poles during prokaryotic fission.
Bacterial Fertility Factors
- F- bacterium: Does not contain the F factor sequences; recipient of DNA during conjugation.
- F+ bacterium: Contains a plasmid with only the F factor.
- F’ bacterium: Contains a plasmid with the F factor and some genomic DNA.
- Hfr bacterium: Contains the F factor integrated into its chromosome.
Horizontal Gene Transfer in Bacteria
- Conjugation: DNA transfer through direct contact between donor and recipient cells via a pilus.
- Transduction: DNA transfer mediated by a bacteriophage.
- Transformation: Uptake and incorporation of exogenous DNA from the environment.
Conjugation Outcomes
Predict the outcomes of conjugation between these bacterial pairs:
- F+ x F- → F+, F+
- Hfr x F- → Hfr, F-
- F’ x F- → F’, F’
Role of F- Factor in Conjugation
What is the role of the F factor in conjugation?
- Contains genes for pilus formation, enabling cell-to-cell contact.
- Contains genes for replication and transfer of the F plasmid.
Lytic vs. Lysogenic Cycle
Lytic Cycle
- Phage DNA replicates independently of the host DNA.
- New phages are assembled.
- The bacterial cell lyses, releasing progeny phages.
Lysogenic Cycle
- Phage DNA integrates into the host chromosome (prophage).
- Phage DNA replicates along with the host DNA.
- The cell can switch to the lytic cycle under certain conditions.
Butterfly Sex Determination
How is sex determined in butterflies?
Butterflies use the ZW sex-determination system:
- Females: ZW (heterogametic)
- Males: ZZ (homogametic)
Inheritance of Z Chromosome
From which relatives could a female butterfly inherit her Z sex chromosome?
She could inherit it from her father or her paternal grandfather.
Chromosome Numbers in Mutants
Species 1 has 2n = 16 chromosomes. How many chromosomes per cell would be found in the following mutants?
- Monosomic: 15
- Autotriploid: 24
- Autotetraploid: 32
- Trisomic: 17
- Double monosomic: 14
- Nullisomic: 14
- Autopentaploid: 40
- Tetrasomic: 18
Griffith’s Transformation Experiment
- Live mouse + type IIR bacteria + heat-killed type IIIS bacteria: Mouse dies; live type IIIS bacteria recovered.
- Dead mouse + type IIIS bacteria: Mouse dies.
DNA Structure and Replication
Z-DNA
Z-DNA is a left-handed DNA helix.
Hydrogen Bonds in G-C Pairs
Complementary G-C base pairs are connected by three hydrogen bonds.
Antiparallel Strands
DNA strands are antiparallel, meaning they run in opposite directions. The 5′ end of one strand is opposite the 3′ end of the other strand.
Phosphodiester Bond
A phosphodiester bond is a strong covalent bond that links adjacent nucleotides in a DNA strand.
Okazaki Fragments
Okazaki fragments are short, discontinuous DNA segments synthesized on the lagging strand during DNA replication. They are necessary because DNA polymerase can only synthesize DNA in the 5′ to 3′ direction.
RNA Types and Functions
mRNA (Messenger RNA)
- Carries genetic information from DNA to ribosomes.
- Serves as a template for protein synthesis.
- Protected by a 5′ cap and a 3′ poly(A) tail.
tRNA (Transfer RNA)
- Transports amino acids to ribosomes during translation.
- Has a cloverleaf-like structure.
- Contains an anticodon that pairs with the mRNA codon.
rRNA (Ribosomal RNA)
- A structural and functional component of ribosomes.
- Catalyzes the formation of peptide bonds between amino acids during translation.
Transcription vs. Replication
Transcription
- Synthesis of RNA from a DNA template.
- Catalyzed by RNA polymerase.
- Occurs in the nucleus (in eukaryotes).
- Involves promoters, terminators, and introns/exons.
Replication
- Synthesis of DNA from a DNA template.
- Catalyzed by DNA polymerase.
- Occurs in the nucleus (in eukaryotes).
- Involves primers, Okazaki fragments, and leading/lagging strands.
Both
- Involve nucleic acids and base pairing.
- Essential for genetic information flow.
Introns and Exons
- Introns: Non-coding sequences within a gene that are transcribed but removed during RNA processing (splicing).
- Exons: Coding sequences within a gene that are transcribed and joined together to form the mature mRNA.
Spliceosome
A complex of RNA and protein molecules that catalyzes the removal of introns from pre-mRNA.
Replication, Transcription, and Translation Components
Replication
- DNA polymerase
- Okazaki fragments
- Primer
Transcription
- RNA polymerase
- Promoter
- Exons and introns
Translation
- Ribosomes
- Codons and anticodons
- Aminoacyl-tRNAs
Operon Regulation
Operator Mutation
What happens if a mutation at the operator site prevents the repressor from binding?
- Repressible operon: The operon will always be transcriptionally active.
- Inducible operon: The operon will always be transcriptionally active.
Lac Operon Regulation
- lacl: Encodes the lac repressor protein.
- Lac promoter: Site where RNA polymerase binds to initiate transcription.
- Lac operator: Binding site for the lac repressor.
Lac Operon Expression
Conditions for highest lac operon expression:
- High lactose levels (inducer)
- Low glucose levels (catabolite repression)
Attenuation
A regulatory mechanism in some bacterial operons where transcription termination is controlled by the formation of RNA secondary structures (e.g., trp operon).
Mutations
Types of Mutations
- Germ-line mutation: Occurs in gametes and can be passed on to offspring.
- Somatic mutation: Occurs in body cells and is not inherited.
- Missense mutation: Results in a different amino acid in a protein.
- Nonsense mutation: Introduces a premature stop codon.
- Silent mutation: Does not change the amino acid sequence.
- Frameshift mutation: Alters the reading frame of a gene.
- Conditional mutation: Affects the phenotype only under certain conditions.
Base Analogs
Mutagenic chemicals that resemble DNA bases and can be incorporated into DNA, leading to errors during replication.
UV Light and DNA Damage
UV light can cause pyrimidine dimers, which are covalent bonds between adjacent pyrimidine bases (thymine or cytosine) in DNA.
Ames Test
?A test to screen for mutagenic chemicalsWhich of the examples listed is not a DNA repair mechanism?Insertion sequence repairHow do mismatch repair enzymes in E.coli differentiate between the old and new strands of DNA?Methylation of the adenine in the GATC sequence of the old strandHow does direct repair return DNA errors to their original sequences?It removes functional groups or bonds from the altered nucleotide to restore the original structureWhich of the statements describes purines and pyrimidines in DNA molecules?Pyrimidines consist of a one-ring structure and pyrimidines form hydrogen bonds with purinesProkaryotic DNA structure: DNA is circular, condensed only by DNA supercooling, arranged in a single looped structure, and does not have telomere regionEukaryotic DNA structure: DNA is linear, condensed around histone proteins, arranged multiple chromosomes, has repeating telomere regionAs DNA is replicated, both continuous and discontinuous replication occur. Discontinuous replication is the result of which specific feature of DNA?Antiparallel strandsSynthesis of a short RNA sequence that intimates DNA synthesisDNA topisomerase relaxes the supercooled DNAHelicase unwinds the DNA double helixSSB protein prevents the reannealing of DNA strandsDNA polymerase III synthesizes new DNADNA ligaments connects the separated DNA segmentsTransfer RNA (tRNA)- have a distinct cloverleaf-like structure, are attached to specific amino acids has anticodons, is the smallest RNA molecule, transports amino acidsMessenger RNA (mRNA)- protected by a 5’-cap, carry instructions required to generate a protein, carries genetic information from nucleus to cytoplasm,serves as a template for protein synthesis Ribosomal RNA (rRNA)- transcribed in the nucleotide and associates with proteins, catalyze the formation of peptide bonds, combines with protein to form ribosomesTranscription- acts on only one strand, catalyzed by RNA polymeraseReplication- catalyzed by DNA polymerase, a primer is requiredBoth- localized in the nucleus in eukaryotic cellsIntron- region of a gene or transcript that is spliced out during RNA maturationExon-region of a gene or transcript that remains in the mature RNA transcriptSpliceosome- protein complex that removes regions of pre-messenger RNAReplication- DNA polymerase, Okazaki fragments, primerTranscription- RNA polymerase, promoter, exons and intronsTranslation- ribosomes, codons and anti-codons, aminoacyl-tRNAsA mutation at the operator site of an operon prevents the repressor from bindingWhat effect will this mutation have on transcription in a repressive operon?The operon will always be transcriptionally activeWhat effect will mutation have on transcription in an inducible operon?The operon will always be transcriptionally activeWhat is the function of lacl in the regulation of the lac operon? It encodes a repressor that, in the absence lactose, binds to the lac operator and blocks expression of the structural genesA mutation in the lac promoter region that causes this region to no longer function would lead to-Lack of expression of the lac structural genes in the presence of lactoseWhat is the function of the lac operator? It is bound by the lac repressor protein