genetics exam 2: ch. 8-12

types of chromosome mutations

rearrangements, aneuploidy, polyploidy

chromosomal rearrangements

caused by double-stranded breaks in DNA and crossover events; duplication, deletion, inversion, translocation

duplication

A chromosome mutation where a segment of DNA is copied, resulting in two copies of that segment

tandem duplication

A type of duplication where the duplicated segment is adjacent to the original segment on the chromosome.

displaced duplication

A type of duplication where the duplicated segment is located at a different site on the chromosome, either on the same chromosome or on a different chromosome; not the same as translocation

reverse duplication

A type of duplication where the duplicated segment is inverted and placed back into the chromosome

effects of chromosome duplications on meiosis

if one chromosome has a duplication, the duplicated region must loop out to allow the chromosome to align with its homologous chromosome, which can lead to unequal crossing over and increase the risk of genetic disorders.

effects of chromosome duplications on phenotype

Chromosome duplications can lead to gene dosage imbalances, which may affect gene expression and produce phenotypic changes, potentially resulting in developmental abnormalities or increased susceptibility to diseases.

unequal crossover

occurs when chromosomes do not align properly during recombination, leading to the exchange of unequal segments between homologous chromosomes; can result in duplication and deletion of genetic material that may contribute to various genetic disorders or phenotypic variations.

unbalanced gene dosage

refers to an abnormality in the number of copies of a gene due to chromosomal duplications or deletions, leading to an altered amount of gene product/protein that can cause developmental problems

deletions

loss of a chromosomal segment; during pairing, normal chromosome loops out so homologous sequences of the chromosomes can align

effects of deletions

imbalances in gene product/loss of protein product, pseudodominance, haploinsufficiency

pseudodominance

expression of a normally recessive gene if the dominant gene is deleted in a heterozygote; effect of deletion

haploinsufficiency

a single gene is not sufficient to produce a wild-type phenotype; one wild-type gene is present, but mutant phenotype

inversion

a chromosomal rearrangement where a segment of a chromosome is inverted; depends on involvement of centromere

effects of inversions in meiosis in homozygous individuals

no problems arise during meiosis (no DNA gained or lost), but can still have phenotypic consequences

effects of inversions on meiosis in heterozygous individuals

homologous sequences align only if the two chromosomes form an inversion loop; reduced recombination in a paracentric inversion, abnormal gametes formed in a pericentric inversion

paracentric inversion

does not include centromere; results in recombinant gametes that are missing genes and will not produce viable offspring

process of paracentric inversion

inversion loops forms in prophase I —> single crossover results in unusual structure (1 chromatid is dicentric and one is acentric) —> chromatids segregate improperly, resulting in gametes: 1 normal nonrecombinant, 2 nonviable recombinant, 1 nonrecombinant with paracentric inversion

pericentric inversion

includes centromere; results in nonviable recombinant gametes that are either missing genes or have too many copies

process of pericentric inversion

inversion loop —> crossing over within inversion —> 2 chromatids have too many copies of some genes and no copies of others, resulting in gametes: 1 normal nonrecombinant, 2 nonviable recombinant, 1 nonrecombinant with pericentric inversion

nonreciprocal translocation

genes move from one non-homologous chromosome to another without reciprocal exchange

reciprocal translocation

two-way exchange of segments between non-homologous chromosomes

Robertsonian translocation

the fusion of two acrocentric chromosomes at their centromeres, resulting in one larger chromosome and a smaller fragment that is usually lost; often results in deletions

effects of translocations in meiosis

leads to different methods of segregation that can result in nonviable gametes

alternate segregation

a type of segregation during meiosis that leads to viable gametes

adjacent-1 segregation

a type of segregation that occurs during meiosis resulting in gametes that contain unbalanced chromosome combinations, often leading to nonviable gametes

adjacent-2 segregation

a type of segregation during meiosis that results in gametes containing two copies of one chromosome and none of another, often leading to nonviable gametes; rare because centromere do not separate

aneuploidy

change in the number of individual chromosomes

causes of aneuploidy

deletion of centromere during mitosis and meiosis, Robertsonian translocation (lose chromosome), nondisjunction during mitosis and meiosis

nullisomy

type of aneuploidy that results from the loss of both members of a homologous pair (2n-2)

monosomy

type of aneuploidy that results from the loss of a single chromosome (2n-1)

trisomy

type of aneuploidy that results from the gain of a single chromosome (2n+1)

tetrasomy

type of aneuploidy that results from the gain of 2 homologous chromosomes (2n+2)

nondisjunction during meiosis I

chromosomes do not separate evenly during anaphase I —> one daughter cell has an extra chromosome while the other has none —> zygotes are ¼ trisomic and ½ monosomic

nondisjunction during meiosis II

chromatids do not separate evenly during anaphase II —> results in two gametes with the normal chromosome number and one gamete with an extra chromosome and another with none, leading to ½ normal, ¼ trisomic, and ¼ monosomic zygotes.

nondisjunction during mitosis

sister chromatids do not separate evenly during anaphase —> results in two daughter cells with an abnormal chromosome number —> somatic clone of monosomic cells, somatic clone of trisomic cells

effects of aneuploidy on plants (example: Jimson weed)

each mutant is trisomic for a different chromosome pair, leading to different phenotypes

effects of aneuploidy in humans

various genetic disorders, can occur in sex chromosomes and autosomes

sex chromosome aneuploids

Turner syndrome (XO), Klinefelter syndrome (XXY)

autosomal aneuploids

trisomy 21/Down syndrome; most common human aneuploid

primary down syndrome

a form of Down syndrome resulting from an extra copy of chromosome 21; 75% nondisjunction in meiosis II; more likely with older mothers

familial down syndrome

a form of Down syndrome caused by a Robertsonian translocation between chromosomes 14 and 21, which can be inherited

familial down syndrome carrier

an individual who has 45 chromosomes and a Robertsonian translocation that results in chromosome 21 long arm attached to chromosome 14 long arm, short armed chromosome is lost

individual affected by familial down syndrom

individual with 46 chromosomes who inherited the 14/21 long arm chromosome

uniparental disomy

a genetic phenomenon where an individual inherits two copies of a chromosome from one parent and none from the other, due to trisomy that lost one chromosome early in development

polyploidy

change in the number of sets of chromosomes

autopolyploidy

a type of polyploidy where all chromosome sets are from a single species

autotetraploid (4n) cell

type of autopolyploidy that results from nondisjunction in mitosis

autotriploid (3n) cell

type of autopolyploidy resulting from nondisjunction in meiosis I that produces a 2n gamete, fuses with 1n gamete

autotriploid during meiosis

3n cell goes through meiosis —> unbalanced gametes, no viable offspring/sterile (many seedless fruits are autotriploid)

allopolyploidy

type of polyploidy that involves hybridization between two different species, resulting in a genome composed of both parent species and unbalanced/nonviable gametes (sterile)

allotetraploid

type of allopolyploidy resulting from nondisjunction at an early mitotic cell division

significance of polyploidy

increase in cell size, larger plant attributes, evolution/may give rise to new species

chromosomal instability and cancer cells

chromosomal instability is a general feature of cancer cells; deletions, inversions, translocations

chronic myelogenous leukemia

type of cancer caused by a reciprocal translocation between chromosomes 9 and 22; more active protein leads to increased, unregulated cell division

advantages of using bacteria and viruses for genetic studies

can rapidly reproduce many progeny, haploid genome allows for all mutations to be expressed directly, asexual reproduction simplifies the isolation of genetically pure strains, growth in lab is easy, small genomes (90% encodes proteins), easy manipulation of genetic material, can be genetically engineered to produce substances of commercial value

prototrophic bacteria

wild-type bacteria, able to synthesize all amino acids and nucleotides essential for growth; can grow on minimal media without additional nutrients

auxotrophic bacteria

mutant bacteria that cannot synthesize certain essential compounds, requiring additional nutrients to grow on minimal media; can only grow on complete media without additional nutrients

minimum medium

a growth medium that contains only the essential nutrients needed for the growth of prototrophic organisms, allowing for the study of auxotrophic mutants by providing a limited environment

complete medium

a growth medium that contains all essential nutrients, including amino acids, vitamins, and other growth factors required for the growth of auxotrophic organisms

bacterial growth in the lab

under controlled conditions, bacteria form colonies of genetically identical cells; colony phenotypes can be studied; mutant bacteria can be isolated based on their nutritional requirements

the bacterial genome

mostly single, circular DNA molecule/chromosome; plasmids and episomes

plasmids

extra chromosome-like, small circular DNA molecules that can replicate independently of the bacterial chromosome, often carrying genes that provide advantageous traits such as antibiotic resistance

plasmid replication

independent of chromosome replication; begins at ori site, strands separate and replication takes place in both directions, eventually producing 2 circular DNA molecules

episomes

freely replicating plasmids; F factor

F factor

“fertility” factor; controls mating and gene exchange between bacteria, containing a number of genes that regulate transfer into the bacterial cell, replication, and insertion into the bacterial chromosome

horizontal gene transfer in bacteria

asexual and leads to recombination; conjugation, transformation, transduction

conjugation

direct transfer of genetic material from one bacterium to another through direct contact by a sex pilus; DNA of donor cell replicates and transfers to recipient —> crossover in recipient —> recombinant chromosome

F+

bacterial cell that contains the F factor and can initiate conjugation to transfer genetic material to other bacteria (donor)

F-

bacterial cell that lacks the F factor and cannot initiate conjugation; it can receive genetic material from F+ cells during the process (recipient)

conjugation between F+ and F-

F+ transfers genes to F-, F- becomes F+ (result: F+/F+)

Hfr cells

high-frequency strains of bacteria with the F factor integrated into the bacterial chromosome (result of crossover in F+ between F factor and chromosome), conjugation donor cell

transfer of bacterial genes from Hfr cell to F- cell

in Hfr donor cell, F is nicked and 5’ end moves into the F- cell —> transferred strand replicates (F factor plus bacterial genes) —> crossover between donated Hfr chromosome and F- chromosome —> may lead to recombination of alleles —> linear chromosome is degraded —> Hfr/F- cells

why does the F- almost never become F+ or Hfr in conjugation with an Hfr cell?

the F- cell almost never becomes F+ or Hfr because the F factor is nicked in the middle in the initiation of strand transfer, placing part of F at the beginning and part at the end of the strand to be transferred

F’ cells

bacteria cells in which the F plasmid carries some bacterial genes; results from crossing over within the Hfr chromosome

conjugation between F’ and F-

F’ donor cell transfers its F factor that contains bacterial genes into the F- cell —> F- becomes F’ (results: F’/F’)

merozygotes

bacteria that are partially diploid due to the presence of an F' plasmid, which carries extra genes alongside their chromosomal genes.

how can we map bacterial genes using conjugation?

bacterial genes can be mapped by interrupting conjugation at regular intervals; the transfer times indicate the order and relative distances between genes, with less time meaning the gene is closer to the origin

direction of gene transfer in Hfr

different Hfr strains of a given species of bacteria have the F factor integrated into the bacterial chromosome at different sites and in different orientations, resulting in different directions of transfer; orientation of F determines the direction

antibiotic resistance

the ability of bacteria to survive and thrive in the presence of antibiotics; caused by actions of genes located on R plasmids that can be transferred naturally (conjugation)

transformation

the process by which a bacterium takes up DNA from the environment/medium, leading to recombination between introduced genes and the bacterial chromosome

competent cells

cells that have the ability to take up DNA (transformation)

transformants

cells that receive genetic material

cotransformed cells

cells that are transformed by two or more genes simultaneously during the transformation process, resulting in the uptake of both genetic materials

process of transformation

one strand of a DNA fragment in the environment enters the recipient cell —> pairs with the bacterial chromosome —> recombination —> remainder of single-stranded DNA fragment is degraded, recipient cell expresses genes from the newly acquired DNA —> recipient cell replicates and divides —> one of the resulting cells is transformed, the other is not

how can we map bacterial genes via transformation?

by measuring the rate of cotransformation of genes; higher rate = closer distance between genes (inversely proportional); cotransformation cannot occur between genes that are far apart

transduction

the process by which a bacteriophage transfers genes to bacteria; usually occurs between the same or related species

virus

a replicating structure that consists of DNA/RNA (single/double stranded, linear/circular) and a protein coat

bacteriophage

a type of virus that infects bacteria

virulent phage

type of bacteriophage that reproduces through the lytic cycle, killing the host cell

temperate phage

inactive prophage; phage DNA integrates into bacterial chromosome through the lysogenic cycle

lytic cycle

phage binds to bacterium —> phage DNA enters host cell —> host DNA digested —> phage DNA replicates —> host cell transcribes and translates phage DNA, producing phage proteins —> assemble new phages —> phage-encoded enzyme causes cell to lyse, releasing new phages

lysogenic cycle

phage binds to bacterium —> phage DNA enters host cell —> phage DNA integrates into bacterial chromosome and becomes a prophage —> prophage replicates, can continue through many cell divisions —> prophage may separate and cell will enter lytic cycle

generalized transduction

type of transduction where any gene may be transferred; bacteria are infected with phage —> bacterial chromosome is fragmented —> some bacterial genes incorporate into phages —> cell lysis releases transducing phages —> transducing phage infects another bacterium, transferring genes from the other bacterium —> recombination —> produce a transduced bacterial cell

specialized transduction

type of transduction where only a few specific genes are transferred

how can we map bacterial genes via generalized transduction?

by measuring the rate of cotransduction between different genes; genes that are close together on the chromosome are more likely to be cotransduced

how can we determine the position of a gene on a phage chromosome?

by analyzing the frequencies of recombination between that gene and known marker genes; the closer the genes are, the higher the recombination frequency.

retrovirus

RNA virus that is integrated into the host genome; ex: HIV and AIDS

reverse transcriptase

enzyme that catalyzes the transcription of RNA into DNA; essential for retroviruses to replicate/incorporate their RNA into the host DNA