Chapter 16: Genetics
Replication of DNA
before undergoing nuclear division DNA is replicated in S phase of cell cycle #mutations occur during DNA replication
chromatin becomes less compact, opening to allow replicating factors
one strand of double helix cut, two strands separate
form “bubble” called replicon
free nucleotides diffuse to regions of single-stranded DNA & pair with base
ribonucleotides polymerized into short pieces of primer RNA
aprox. 10 nucleotides long
Mutations
any change in DNA
point mutation: smallest mutation affecting the least DNA
single base converted to another
deletion: piece of DNA lost
insertion: addition of extra DNA
inversion: DNA repaired & put back backwards
Causes:
mutagen: something that causes mutations
includes certain chemicals, ultraviolet light, x-rays, & radiation
deletions: can be caused by one strand of DNA w/ self-complimentary sequence forms small loop
DNA polymerase passes by loop w/o reading the bases in it
insertions: piece of foreign DNA present after cutting may be accidentally incorporated
transposable elements: pieces of DNA that readily change positions from chromosome to chromosome
2 basic forms
insertion sequences: contain genes that code for enzymes involved cutting insertion sequence out & splicing it elsewhere
transposon: carries genes that code for proteins not in transposition
only few thousand base pairs long
similar to insertion seq. except may be longer
Effects:
effects & significance depends on nature, position, & extent
changes to introns: unimportant generally, will not be incorporated into final mRNA
changes to exons: unimportant if changed codon is similar to previous, or located in noncritical location
the larger the mutation, the higher chance that a critical part of DNA is affected
somatic mutations: mutations occurring in cells that never lead to sex cells
Monohybrid Crosses
Other Aspects of Inheritance
Dihybrid Crosses
not very important, affect small area & not passed to offspring
DNA Repair
cannot be too efficient or too inefficient
Incomplete Dominance: neither parental trait dominates the other
Crossing Heterozygote with Self:
cross: sexual reproduction between two individuals
only single character analyzed; inheritance of other traits not considered
parental generation: parents of offspring
F1: offspring of cross, first filial generation
F2: offspring of F1 generation
homozygous: has 2 identical alleles for a gene
heterozygous: has 2 different alleles for a gene
offspring of cross resembles each parent somewhat
plant’s own pollen used to fertilize it’s own eggs
called selfing
can also be selfed by being crossed w/ another plant w/ same genotype
Punnett Square: helps analyze & understand results of a cross
all types of one gamete arranged on top, all types of the other gamete arranged on the left side #punnett can be used on dihybrid cross as well
Complete Dominance: dominant allele completely masks traits of recessive allele
Test Crosses:
cross involving plant in question & homozygous recessive for the trait
2 genes studies and analyzed simultaneously
Independent Assortment:
if the 2 genes are on separate chromosomes, the alleles of one move independently of the other
3:1 Ratio: offspring of heterozygous parents, 75% dominant 25% recessive
9:3:3:1 Ratio:
9/16 of plants dominant for both traits
3/16 of plants dominant first trait recessive second
3/16 of plants recessive first trait dominant second
1/16 of plants recessive for both
Crossing Over
exchange of genetic mat. by nonsister chromatids of homologous chromosomes
Linkage:
when two genes occur close together on a chromosome, considered linked
most prominent exception to Mendel’s Law of Independent Assortment
biparental inheritance: alleles of both parents transmitted to progeny equally
during fertilization sperm cell loses most of cytoplasm, only nucleus enters egg
maternal inheritance: zygote obtains all plastid & mitochondrial genomes from maternal parent
aka uniparental inheritance
conifers: eggs destroy plastids, sperm do not
Lethal Alleles: phenotypic result of mutation can kill the plant
variegation: pretense of spots/sectors that are white/red/orange on otherwise green plant
act as substrates for DNA polymerase
enters and adds depxyribo
genetically inherited lethals most often recessive, must be homozygous to affect plant #homozygous means it has 2 of same alleles
too efficient: all cells of individual would be completely identical, making sexual reproduction useless
too inefficient: cells become not functional due to lack of repair
replication fork: as DNA uncoils & separates it takes on forked appearance
recombinant chromosomes: formed from crossing-over of homologous chromosomes & recombination alleles
rate of crossing-over directly proportional to physical space b/t genes on chromosome