Unit 2- ch. 7, 8, 24, 10-12 Flashcards
What does the interference tell us about the effect of one crossover on another?
-positive interference indicates that a crossover inhibits or interferes
with the occurrence of a second crossover nearby.
-a negative interference suggests that a crossover event can stimulate additional crossover events in the same region of the chromosome.
Explain how to determine, using the numbers of progeny from a three-point cross, which of three linked loci is the middle locus.
Compare the two double-crossover phenotypes with the alleles of the two non recombinant phenotypes. whatever letter is different is the middle locus.
What is the relationship between recombination frequency and a centiMorgan?
A recombination frequency of 1% is equal to one centiMorgan, a unit of distance for
genetic maps based on observed recombination frequencies
What is the difference between genes in coupling configuration and genes in repulsion? How does the arrangement of linked genes (whether they are in coupling or repulsion) affect the results of a genetic cross?
Coupling: the non-recombinants look like parents
Repulsion: the recombinants look like parents
Why is the frequency of recombinant gametes always half the frequency of crossing over?
each crossover involves two of the four strands and generates two recombinant gametes. The remaining two strands that were not involved in the crossover generate two non-recombinant gametes.
In a testcross for two genes, what types of gametes are produced with (a) complete linkage, (b) independent assortment, and (c) incomplete linkage?
(a). only non-recombinant gametes will be
produced
(b). will result in 50% of the gametes being
recombinant and 50% being non-recombinant
(c). greater than 50% of the gametes produced are
non-recombinant and less than 50% of the gametes are recombinant
How do you determine if genes are assorting independently?
Chi-square test and look for 9:3:3:1 ratio, if not this ratio assume genes are linked
Def: Quantitative genetics
traits with phenotypes that can be measured numerically
Differentiate between discontinuous and continuous traits
Continuous: exhibits a continuous range of phenotypes (height)
Discontinuous: exhibits only a few easily distinguished phenotypes (tall or dwarf plant)
Explain why some characteristics are continuous
Because they are considered quantitative because they are determined by multiple genetic and environmental factors
Describe truly continuous traits
Define meristic characteristic
Characteristic whose phenotype varies in whole numbers, such as number of vertebrae, but may be caused by continuous genetic variation.
why are meristic characteristics considered to be quantitative traits even though there is not an infinite number of phenotypes
the underlying determination of the characteristic is still quantitative
Define threshold characteristic
Characteristic that has only two phenotypes (presence and absence) but whose expression depends on an underlying susceptibility that varies continuously.
why are threshold characteristics considered to be quantitative traits even though there are only 2 phenotypes (you have it or you do not have it)
they also are determined by multiple genetic and environmental factors
Define karyotype
The complete set of chromosomes possessed by an organism
Define chromosome rearrangement, aneuploidy, and polyploidy
What are the 4 types of chromosome rearrangements?
Metacentric: two equal arms
Submetacentric: 1 long arm 1 short arm
Acrocentric: long arm and a knob
Telocentric: centromere is close to end of chromosome
Know that duplications cause unbalanced gene dosage
Know the effects of an inversion
-genetic material is not lost nor gained
-pronounced phenotypic effects
-when gene position is altered expression may be altered = position effect
Define Robertsonian translocation
Translocation in which the long arms of two acrocentric chromosomes become joined to a common centromere, generating a metacentric chromosome with two long arms and another chromosome with two very short arms.
Define monosomy, trisomy, nullisomy, and tetrasomy
Monosomy: 2n-1
Nullisomy: 2n-2
Trisomy: 2n+1
Tetrasomy: 2n+2
Explain how Robertsonian translocations can lead to aneuploidy
the small chromosome generated by a Robertsonian translocation may be lost in mitosis or meiosis
Explain how nondisjunction can lead to aneuploidy
the failure of homologous chromosomes or sister chromatids to separate in meiosis or mitosis
leads to some gametes or cells that contain an extra chromosome and other gametes or cells that are missing a chromosome
Understand why sex chromosome aneuploidy is more tolerated in humans than autosomal aneuploidy?
autosomal aneuploidies resulting in live births are less common because there is no mechanism of dosage compensation for autosomes
–>most embryos with autosomal aneuploidies are spontaneously aborted
–> aneuploidy of sex chromosomes is better tolerated than aneuploidy of autosomes (EX: turner or Klinefelter result from aneuploidy of sex chromosomes)
Explain how the two types of Down Syndrome arise
- Primary down syndrome- (3 full copies of chromosome 21) arises from spontaneous nondisjunction during egg formation: about 75% of the nondisjunction events that cause Down syndrome are maternal in origin, most arising in meiosis I
- Familial down syndrome- 4% of people with Down syndrome are not trisomic for a complete chromosome 21. instead, they have 46 chromosomes, but an extra copy of chromosome 21 is attached to another chromosome through translocation parents are carriers of chromosomes that have undergone a Robertsonian translocation (chromosome 14 and 21)
Define genetic mosaicism and explain how it arises
nondisjunction in a mitotic division may generate patches of cells in which every cell has a chromosome abnormality and other patches in which every cell has a normal karyotype
this type of nondisjunction leads to a region of tissues with different chromosome constitutions, a condition known as genetic mosaicism
EX: 50% of those with Tuner syndrome are mosaics, possessing some 45 X cells and some normal 46 XX cells
Define polyploidy and differentiate between autopolyploidy and allopolyploidy
polyploidy: any organism that has more than two sets of chromosomes
Autopolyploidy: all chromosome sets are from a single species
Allopolyploidy: chromosome sets are from two or more species
Define Chargaff’s rules
DNA from different organisms varies greatly in base composition
within each species, there is some regularity in the ratios of the bases: the amount of adenine is always equal to the amount of thymine (A = T) and the amount of guanine is always equal to the amount of cytosine (G = C)
Describe Griffith’s experiment
disease-causing in smooth (S) form and nonvirulent rough form (R)
-Griffith observed that small amounts of living type IIIS bacteria injected into mice cause them to die
-When injected with IIR bacteria, they lived
-Griffith knew that boiling killed all bacteria and destroyed their virulence; when he injected large amounts of heat-killed IIS bacteria, mice lived and no IIIS bacteria recovered from their blood
surprise:
-when injected mice with small amount of living type IIR and large amount of heat-killed type IIIS bacteria (both nonvirulent)
-expected the mice to live but they died
-live type IIIS bacteria observed in blood
Griffith’s experiment and the conclusion that was drawn
Type IIR bacteria had somehow been transformed, acquiring the genetic virulence of the dead type IIIS bacteria, and this transformation had produced a permanent genetic change in the bacteria
He theorized that some substance in the dead bacteria might be responsible. He called this substance the transforming principle
Describe the Avery, MacLeod, McCarty experiment and the conclusion that was drawn from it
succeeded in isolating and partially purifying the transforming substance
found that enzymes capable of destroying DNA eliminated the biological activity of the transforming substance
Conclusion: the transforming principle - and therefore genetic information - resides in DNA
Describe the Hershey-Chase experiment
grew one batch of E. coli containing 32 P and infected bacteria with T2 phage so all the progeny phages would have DNA labeled with 32 P
grew one batch of E. coli containing 35 S and infected these bacteria with T2 phage so that all the progeny phages would have proteins labeled with 35 S
then infected separate batches of unlabeled E. coli with 35 S and 32 P progeny phages
after allowing time for the phages to infect the E. coli cells, they placed the cells in a blender and sheared off the now-empty phage protein coats from the cell walls
Hershey-Chase experiment conclusion that was drawn from bacteria infected by phages labeled with 35 S
most of the radioactivity was detected in the phage protein coats, and little was detected in the cells. when new phages emerged from the cells, they contained almost no 35 S
results indicate that the protein component of a phage does not enter the cell and is not transmitted to progeny phages
Hershey-Chase experiment conclusion that was drawn from bacteria infected by phages labeled with 32 P
when removed the phage protein coats, the bacteria were radioactive
new progeny emerged and many of those phages emitted radioactivity demonstrating that DNA from the infecting phages had been passed on to the progeny phages
Overall conclusion of hershey and chase
DNA, not protein, is the genetic material of the phages
Describe the Fraenkel-Conrat and Singer experiment
a few viruses, use RNA, not DNA, as their genetic material
demonstrated by Fraenkel-Conrat and Singer who worked with tobacco mosaic virus (TMV) which infects and causes disease in tobacco plants
Fraenkel-Conrat and Singer experiment conclusion that was drawn from it
found that after separating DNA and protein of TMV, could remix the DNA and protein of different strains of TMV and obtain intact, infectious viral particles
these results showed that RNA carries the genetic information in TMV
Define nucleotide and know the 3 parts that make up a nucleotide
Nucleotide: the primary structure of DNA consists of a string of nucleotides joined together by phosphodiester linkages
3 parts:
1. sugar
2. phosphate group
3. nitrogen-containing base
Describe the structure of DNA
-2 polynucleotide strands twisted together (double helix)
-strands antiparallel (run in opposite direction)
-sugar-phosphates on outside (phosphodiester bonds)
-bases on inside of helix
-bases are complementary and joined by hydrogen bonds
–> 2 bonds between A and T
–> 3 bonds between G and C
Describe the structure of RNA
single-stranded
bases = G, C, A, U (you do not have to draw the structure of the bases)
sugar = ribose
Define chromatin
eukaryotic DNA in the cell is closely associated with proteins. This complex of DNA and protein is called chromatin.
The two basic types of chromatin are euchromatin and heterochromatin
Define euchromatin
undergoes the normal process of condensation and decondensation in the cell cycle. constitutes the majority of the chromosomal material and is where most transcription takes place
Define heterochromatin
remains in a highly condensed state throughout the cell cycle, even during interphase
Differentiate between euchromatin and heterochromatin
Euchromatin: various condensing throughout cell cycle—where most transcription takes place
Heterochromatin: remains very condensed throughout cell cycle—what are some areas of a chromosome that contain heterochromatin?
-all chromosomes have permanent heterochromatin
Define histone proteins and know the 5 different types of histone proteins
the most abundant proteins in chromatin are the histones, which are small positively charged proteins of five major types:
H1, H2A, H2B, H3, and H4
Understand why it is important for histone proteins to have a positive charge
all histones have a high percentage of arginine and lysine, positively charged amino acids that give histones a net positive charge
these positive charged attract the negative charges on the phosphates of DNA; this attraction holds the DNA in contact with the histones
Describe the makeup of the nucleosome with respect to the histone proteins and the DNA
-the simplest level of chromatin structure, the nucleosome; the basic structural unit of DNA packaging in eukaryotes
-Nucleosome: octamer of 8 histones + DNA wrapped around histones
- octamer = 2 H2A, 2 H2B, 2 H3, 2 H4
- DNA wraps around octamer core ~2 times
- H1 clamps DNA in place
Understand how mitochondrial and chloroplast DNA are inherited differently than nuclear DNA
new mitochondria and chloroplasts arise by the division of existing organelle; these divisions take place throughout the cell cycle and are independent of mitosis and meiosis
Understand the endosymbiotic theory
propose that mitochondria and chloroplast were once free-living bacteria that became internal inhabitants of early eukaryotic cells
Describe replicative segregation
when a heteroplasmic (two distinct varieties of DNA within the cytoplasm) cell divides, the organelles segregate randomly into the two progeny cells in a process called replicative segregation and chance determines the proportion of mutant organelles in each cell
when replicative segregation takes place in somatic cells it may create phenotypic variation within a single organism: different cells of the organism may possess different proportion of mutant and wild-type sequences
most of the replicated cells are heteroplasmic
Describe the Meselson & Stahl experiment and understand how they interpreted their results
overview:
to determine which of the three models of replication applied to E.coli cells, Meselson and Stahl needed a way to distinguish old and new DNA
What they did:
used two isotopes of nitrogen 14N and 15N
grew culture of E. coli with 15N and then switched the bacteria that contained 14N
1. bacteria synthesized before change was all 15N
2. any DNA synthesized after switch was 14N
to distinguish between 15N and 14N used equilibrium density gradient centrifugation
Results:
found that DNA from bacteria grown in 15N produced a single band at the position expected of DNA containing only 15N
DNA from bacteria transferred to 14N and produced a band intermediate between the expected DNA containing only 15N/14N
Results were inconsistent with conservative replication model (predict one heavy and one light band)
a single band intermediate is predicted by semiconservative/dispersive model)
Using the Meselson & Stahl experiment design, predict the results if DNA were replicated in a conservative fashion or a dispersive fashion.
to distinguish between two models, Meselson and Stahl grew bacteria containing only 14N for second generation
two bands equal intensity appeared (one intermediate and one at the position expected of DNA containing only 14N)
Meselson and Stahl’s results were exactly as expected for semiconservative replication and were incompatible with those predicted for both conservative and dispersive replication
Briefly describe the 3 models of semiconservative replication
- Theta Model—common in bacteria, results in 2 circular DNA molecules
- Rolling Circle Model—some viruses and bacteria, results in multiple circular DNA molecules
- Linear Model—eukaryotic chromosomes, larger linear chromosomes in eukaryotic cells contain too much DNA to be replicated from a single origin
Define dNTP
the raw materials from which new DNA molecules are synthesized
consist of deoxyribose sugar and a base (nucleoside) attached to three phosphate groups
List the 4 substrates required for DNA replication
- dATP
- dTTP
- dGTP
- dCTP
Explain how dNTPs are added to a growing DNA chain
nucleotides are added to the 3’ OH group of the growing nucleotide strand
3’ OH group of the last nucleotide on the strand attacks the 5’ phosphate group of the incoming dNTP
Know the direction of DNA synthesis
-nucleotides added to 3’ OH of growing chain, so chain grows in a 5’ to 3’ direction
-template strand is read in a 3’ to 5’ direction because it is antiparallel to the new strand
Explain how 5’ to 3’ replication can occur simultaneously on two antiparallel strands
because the two single-stranded DNA templates are antiparallel and strand elongation is always 5’ → 3’, if synthesis on one template proceeds from right to left, synthesis on the other strand proceeds left to right
Define: Okazaki fragments
short lengths of DNA produced by discontinuous replication of lagging strand
Okazaki fragments on the lagging strand are linked together to create a continues new DNA molecule
Define: continuous DNA synthesis
DNA unwinds, template strand exposed in 3’ → 5’ direction and allows the new strand to by synthesized continuously in 5’ → 3’
new strand, which undergoes continuous replication is called the leading strand
Define: discontinuous DNA synthesis
the other template strand is exposed in the 5’ → 3’ direction
the synthesis must proceed 5’ → 3’ that is in the direction OPPOSITE that of unwinding
runs out of template
–> DNA synthesis must start anew at replication fork until it runs into the previously replicated segment of DNA
–>process repeated
newly made strand that undergoes discontinuous replication is lagging strand
Explain in detail how replication occurs in bacteria
four stage: initiation, unwinding, elongation and termination
Explain in detail how replication occurs in bacteria- Initiation:
single origin of replication
initiator proteins bind to origin and cause a short section of DNA to unwind
unwinding allows helicase and other single-strand binding proteins to attach to the polynucleotide strand
Explain in detail how replication occurs in bacteria- Unwinding:
DNA synthesis requires a single-stranded template, so double-stranded DNA must be unwound before DNA synthesis can take place
Helicase:
breaks H bonds between the bases
cannot initiate the unwinding
once bound to single-stranded DNA, helicase moves in 5’ → 3’ direction
SSBs:
after DNA unwound by helicase, single-strand-binding proteins attach tightly to exposed single-stranded DNA
protect single-stranded nucleotide chains and prevent formation of secondary structures
bind to any single-stranded DNA
Gyrase:
control supercoiling of NDA
type II topoisomerase (create double-strand breaks)
reduce strain that builds up ahead of replication fork
Explain in detail how replication occurs in bacteria- Elongation DNA Polymerase:
requires 3’ OH in order to add a nucleotide
cannot initiate DNA synthesis on bare template
DNA polymerase III:
synthesizes DNA from primers by adding nucleotides to 3’ end
5’→ 3’ polymerase activity (DNA synthesis)
3’→ 5’ exonuclease activity (proofreading)
–> correct errors
High processivity: can add many nucleotides without releasing the template
DNA polymerase I:
removes and replaces primers
5’ → 3’ polymerase activity
3’ → 5’ exonuclease activity
5’ → 3’ exonuclease activity (remove primers and replace with DNA nucleotides by synthesizing in 5’ → 3’ direction)
Explain in detail how replication occurs in bacteria.
Elongation Primase
creates primer (RNA) to initiate DNA synthesis
–> Primase = RNA polymerase; does not require 3’ OH to start the synthesis of nucleotide strand
-Catalyzes the formation of a phosphodiester bond without adding another nucleotide to the strand
Explain in detail how replication occurs in bacteria.
Termination
Usually occurs when replication forks run into each other
State if telomerase is present in germ cells, somatic cells, and cancer cells
-present in germ cells and some somatic cells
-BUT most somatic cells have little or no telomerase activity and chromosomes in these cells progressively shorten with each cell division
telomerase plays a role in cancer
- cancer cells have capacity to divide indefinitely and telomerase is expressed in 90% of all cancer
Explain how the telomeres are replicated with the help of telomerase
the protruding end of telomere can be extended by
telomerase (protein and RNA component)
telomerase extends the 3’ end of the chromosome without the use of a complementary DNA template
