Genetics Exam 3
Under which of the following conditions would a lac operon produce the greatest amount of B-galacatosidase? The least? Explain your reasoning. 1) lactose present, no glucose present 2) no lactose present, glucose present 3) lactose present, glucose present 4) no lactose present, no glucose present
A lac operon would produce the greatest amount of B-galactosidase in condition 1 and the least amount in condition 2 because in the absence of lactose, the repressor binds to the lac operator site and inhibits transcription, and high levels of glucose decreases transcription since RNA polymerase can't bind as well to the promoter.
What is the difference between a missense mutation and a nonsense mutation? A silent mutation and a neutral mutation?
A missense mutation changes a sense codon into a different sense codon resulting in the incorporation of a different amino acid in the protein. A nonsense mutation changes a sense codon into a nonsense codon resulting in premature termination of translation. A silent mutation changes a sense codon into a synonymous codon leaving amino acid sequence of protein unchanged. A neutral mutation changes amino acid sequence of a protein without altering its ability to function.
What is the difference between a transition and a transversion? Which type of base substitution is usually more common?
A transition results from purine to purine or pyrimidine to pyrimidine base substitutions. A transversion results from purine to pyrimidine or pyrimidine to purine base substitutions. Transitions are more common because they are spontaneous.
How do base analogs lead to mutations?
Base analogs have structures similar to those of the nucleotides and can be incorporated into the DNA in the course of replication. Many analogs tend to mispair which can lead to mutations. DNA replication is required for the base analog induced mutations to be incorporated into the DNA.
Briefly describe expanding nucleotide repeats.
Expanding nucleotide repeat result when a DNA insertion mutation increases the number of copies of a trinucleotide repeat sequence, could be due to errors in replication or unequal recombination.
List the stages of interphase and the major events that take place in each stage.
G1 Phase: growth and development of the cell S Phase: synthesis of DNA G2 Phase: preparation for division
The blob operon produces enzymes that convert compound A into compound B. The operon is controlled by a regulatory gene S. Normally, the enzymes are synthesized only in the absence of compound B. If gene S is mutated, the enzymes are synthesized in the presence and absence of compound B. Does gene S produce a repressor or an activator? Is this operon inducible or repressible?
Gene S produces a repressor in a negative repressible operon.
What are the major results of meiosis?
Meiosis comprises two cell divisions thus resulting in the production of 4 new cells. The chromosome number of a haploid cell produced by meiosis is half the chromosome number of the original diploid cell. Cells produced by meiosis are genetically different from the original cell and one another.
What causes errors in DNA replication?
Mispairing due to tautomeric shifts in nucleotides and mispairing through wobble caused by flexibility of the DNA molecule (most likely cause) cause errors in DNA replication.
Outline the process of male gamete formation in plants. Outline the process of female gamete formation in plants.
1) Diploid microsporocytes undergo meiosis to produce 4 haploid microspores 2) Each microspore undergoes mitosis to produce an immature pollen grain with 2 haploid nuclei 3) The tube nucleus directs the growth of a pollen tube that makes up the male gametophyte 4) The generative nucleus divides mitotically to produce 2 sperm cells 1) Diploid megasporocytes undergo meiosis to produce 4 haploid megaspores 2) The nucleus of the surviving megaspore divides mitotically 3 times to produce 8 haploid nuclei that make up female gametophyte 3) Cytoplasm divides producing separate cells, one becomes egg, two become polar nuclei and other is partitioned into separate cells 4) Double fertilization takes place when two sperm cells of pollen grain enter embryo sac 5) One sperm cell fertilizes egg cell producing diploid zygote 6) Other sperm cell fuses with two nuclei enclosed in single cell to form triploid endosperm which stores food
Outline the process of spermatogenesis in animals. Outline the process of oogenesis in animals.
1) Spermatogonia (2n) divide mitotically 2) Spermatogonium (2n) enter prophase I becoming primary spermatocyte 3) Primary spermatocyte (2n) produces two secondary spermatocytes 4) Secondary spermatocytes (1n) undergo meiosis II producing 2 haploid spermatids each 5) Spermatids (1n) mature into sperm 1) Oogonia (2n) divide mitotically 2) Oogonium (2n) enter prophase I becoming primary oocyte 3) Primary oocyte (2n) completes meiosis I producing a large secondary oocyte and a small polar body which disintegrates 4) Secondary oocyte (1n) completes meiosis II producing ovum and second polar body which also disintegrates 5) Ovum (1n) is mature female gamete capable of being fertilized
What is a checkpoint? How are checkpoints important in the cell cycle?
A checkpoint regulates progression through the cell cycle by ensuring that all cellular components are present and in good working order to prevent damaged cells or missing chromosomes.
What is catabolite repression? How does it allow a bacterial cell to use glucose in preference to other sugars?
Catabolite repression regulates glucose levels because when glucose is available, genes that participate in the metabolism of other sugars are repressed. When high levels of glucose are available, the amounts of cAMP and CAP are lowered which decreases transcription because the RNA polymerase can't bind to the promoter until the CAP has formed a complex with cAMP and then binded to DNA.
Name three essential structural elements of a functional eukaryotic chromosome and describe their functions.
Centromere: attachment point for spindle microtubules Telomere: natural ends of chromosome that protect and stabilize the chromosome Origins of replication: sites where DNA synthesis begins
What two processes unique to meiosis are responsible for genetic variation? At what point in meiosis do these process take place?
Crossing over takes place in Prophase I and is the exchange of genetic material between nonsister chromatids. Independent assortment takes place in Anaphase I and is the random distribution of chromosomes into different combinations.
How do insertions and deletions arise?
Insertions and deletions arise from strand slippage which occurs when one nucleotide strand forms a small loop (insertion on newly synthesized strand, deletion on template strand), or from misaligned pairing causing unequal cross over which results in one DNA molecule with an insertion and the other with a deletion.
What types of mutations are produced by ionizing and UV radiation?
Ionizing and UV radiation alter the structures of bases and distort the configuration of DNA blocking replication from thymine dimers.
List some similarities and differences between mitosis and meiosis. Which differences do you think are most important and why?
Mitosis: single cell division produces 2 genetically identical cells, chromosome number of daughter cells and original cells stays the same, daughter cells and original cells are genetically identical, homologous chromosomes do not synapse, individual chromosomes line up at metaphase plate, sister chromatids separate in anaphase Meiosis: two cell divisions usually produce 4 cells, daughter cells are haploid and have half the chromosomal complement of original diploid cell, daughter cells are genetically different from one another and original cell because of crossing over and separation of homologous pairs, homologous chromosomes synapse in Prophase I, homologous pairs line up at metaphase plate but it's individual chromosomes in meiosis II, homologous chromosomes separate in anaphase but sister chromatids separate anaphase II
Explain why mutations in the lacI gene are trans in their effects, but mutations in the lacO gene are cis in their effects.
Mutations in the lacI gene are trans because the lacI gene encodes the lac repressor protein which can attach to any operator and effect the expression of genes on the same or different molecules of DNA. Mutations in the lacO gene are cis because the lacO gene encodes the operator which affects the binding of RNA polymerase to DNA and therefore affects the expression of genes only on the same molecule of DNA.
What is the difference between positive and negative control? What is the difference between inducible and repressible operons?
Positive control stimulates transcription (gene expression). Negative control inhibits transcription (gene expression). Inducible operons are normally off and transcription can be turned on. Repressible operons are normally on and transcription can be turned off.
List some genetic differences between prokaryotic and eukaryotic cells.
Prokaryotic: no nucleus, small cell diameter 1-10 um, one circular DNA molecule, no histones complexed to DNA, no membrane bound organelles Eukaryotic: has nucleus, large cell diameter 10-100 um, multiple linear DNA molecules, DNA is complexed with histones, has membrane bound organelles
What are the stages of meiosis and what major events take place in each stage?
Prophase I: chromosomes condense, homologous chromosomes synapse, crossing over takes place, nuclear envelope breaks down, mitotic spindle forms Metaphase I: Homologous pairs of chromosomes line up on the metaphase plate Anaphase I: the two chromosomes of each homologous pair separate and move toward opposite poles Telophase I: chromosomes arrive at spindle poles Cytokinesis: cytoplasm divides to produce two cells, each having half the original number of chromosomes Prophase II: chromosomes condense, the spindle forms, nuclear envelope disintegrates Metaphase II: individual chromosomes line up on metaphase plate Anaphase II: sister chromatids separate and move as individual chromosomes toward spindle poles Telophase II: chromosomes arrive at spindle poles, the spindle breaks down and a nuclear envelope reforms Cytokinesis: cytoplasm divides
List the stages of mitosis and the major events that take place in each stage.
Prophase: chromosomes condense and mitotic spindle forms Prometaphase: nuclear envelope disintegrates, spindle microtubules anchor to kinetochores Metaphase: chromosomes align on the metaphase plate Anaphase: sister chromatids separate becoming individual chromosomes that migrate toward spindle poles Telophase: chromosomes arrive at spindle poles, nuclear envelope reforms Cytokinesis: cytoplasm divides
Briefly describe the lac operon and how it controls the metabolism of lactose.
The lac operon consists of three structural genes lacZ, lacY, and lacA which encode B-galactosidase, permease, and thioglactoside transacetylase. All three genes share a promoter and operator region. Upstream from the lactose operon, the lacI gene encodes the lac operon repressor which binds at the operator region and inhibits the transcription of the lac operon by preventing RNA polymerase from initiating transcription. When lactose is present, the enzyme B-galactosidase converts it into allolactose which binds to the lac repressor and inactivates it so RNA polymerase can initiate transcription.
How are the events that take place in spermatogenesis and oogenesis similar? How are they different?
The processes begin similarly with diploid cells dividing mitotically then entering meiosis and producing diploid cells that complete meiosis I an divide. However, oogenesis begins to differ because cytokinesis is unequal in the secondary cell and only one of the secondary cells undergoes meiosis II, so oogenesis only produces one single mature gamete instead of four games like spermatogenesis.
A mutation prevents the catabolite activator protein (CAP) from binding to the promoter in the lac operon. What will the effect of this mutation be on the transcription of the operon?
This mutation will cause the lac operon to decrease transcription because the RNA polymerase will bind poorly to the lac promoter.
A cell in G1 of interphase has 12 chromosomes. How many chromosomes and DNA molecules will be found per cell when this original cell progresses to the following stages? a) G2 of interphase b) Metaphase I of meiosis c) Prophase of mitosis d) Anaphase I of meiosis e) Anaphase II of meiosis f) Prophase II of meiosis g) After cytokinesis following mitosis h) After cytokinesis following meiosis II
a) 12, 24 b) 12, 24 c) 12, 24 d) 12, 24 e) 12, 12 f) 6, 12 g) 12, 12 h) 6, 6
A mutation at the operator site prevents the regulator protein from binding. What effect will this mutation have in the following types of operons? a) regulator protein is a repressor in a repressible operon b) regulator protein is a repressor in an inducible operon
a) The operon will never turn off and transcription will be constitutive. b) The result will be constitutive expression and transcription will take place all the time.
Indicate whether each of the following cells is haploid or diploid. a) Primary spermatocyte b) Microsporocyte c) First polar body d) Oogonium e) Spermatid f) Megaspore g) Ovum h) Secondary oocyte i) Spermatogonium
a) diploid b) diploid c) haploid d) diploid e) haploid f) haploid g) haploid h) haploid i) diploid
For each of the following types of transcriptional control, indicate whether the protein produced by the regulator gene will be synthesized initially as an active repressor, inactive repressor, active activator, or inactive activator. a) negative control in a repressible operon b) positive control in a repressible operon c) negative control in an inducible operon d) positive control in an inducible operon
a) inactive repressor b) active activator c) active repressor d) inactive activator
