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6.6 Meiosis and Genetic Variation
KEY CONCEPT
Independent assortment and crossing over during
meiosis result in genetic diversity.
6.6 Meiosis and Genetic Variation
Sexual reproduction creates unique combinations of
genes.
• Sexual reproduction creates unique combination of genes.
– independent assortment of chromosomes in meiosis
– random fertilization of gametes
• Unique phenotypes may give a reproductive advantage to
some organisms.
6.6 Meiosis and Genetic Variation
Crossing over during meiosis increases genetic diversity.
• Crossing over is the exchange of chromosome
segments between homologous chromosomes.
– occurs during prophase I of meiosis I
– results in new combinations of genes
6.6 Meiosis and Genetic Variation
• Chromosomes contain many genes.
– The farther apart two genes are located on a
chromosome, the more likely they are to be separated
by crossing over.
– Genes located close together on a chromosome tend to
be inherited together, which is called genetic linkage.
• Genetic linkage allows the distance between two genes to
be calculated.
8.3 DNA Replication
KEY CONCEPT
DNA replication copies the genetic information of a
cell.
8.3 DNA Replication
Replication copies the genetic information.
• A single strand of DNA serves as a template for a new
strand.
• The rules of base pairing direct
replication.
• DNA is replicated during the
S (synthesis) stage of the
cell cycle.
• Each body cell gets a
complete set of
identical DNA.
8.3 DNA Replication
Proteins carry out the process of replication.
• DNA serves only as a template.
• Enzymes and other proteins do the actual work of
replication.
– Enzymes unzip the double helix.
– Free-floating nucleotides form hydrogen bonds
with the template strand.
nucleotide
The DNA molecule unzips
in both directions.
8.3 DNA Replication
– DNA polymerase enzymes bond the nucleotides
together to form the double helix.
– Polymerase enzymes form covalent bonds between
nucleotides in the new strand.
new strand
nucleotide
DNA polymerase
8.3 DNA Replication
• Two new molecules of DNA are formed, each with an
original strand and a newly formed strand.
• DNA replication is semiconservative.
original strand
Two molecules of DNA
new strand
8.3 DNA Replication
Replication is fast and accurate.
• DNA replication starts at many points in eukaryotic
chromosomes.
There are many origins of replication in eukaryotic chromosomes.
• DNA polymerases can find and correct errors.
8.7 Mutations
KEY CONCEPT
Mutations are changes in DNA that may or may not
affect phenotype.
8.7 Mutations
Some mutations affect a single gene, while others affect
an entire chromosome.
• A mutation is a change in an organism’s DNA.
• Many kinds of mutations can occur, especially during
replication.
• A point mutation substitutes one nucleotide for another.
mutated
base
8.7 Mutations
• Many kinds of mutations can occur, especially during
replication.
– A frameshift mutation inserts or deletes a nucleotide in
the DNA sequence.
8.7 Mutations
• Chromosomal mutations affect many genes.
• Chromosomal mutations may occur during crossing over
– Chromosomal mutations affect many genes.
– Gene duplication results from unequal crossing over.
8.7 Mutations
• Translocation results from the exchange of DNA segments
between nonhomologous chromosomes.
8.7 Mutations
Mutations may or may not affect phenotype.
• Chromosomal mutations tend to have a big effect.
• Some gene mutations change phenotype.
– A mutation may cause a premature stop codon.
– A mutation may change protein shape or the active site.
– A mutation may change gene regulation.
blockage
no blockage
8.7 Mutations
• Some gene mutations do not affect phenotype.
– A mutation may be silent.
– A mutation may occur in a noncoding region.
– A mutation may not affect protein folding or the active
site.
8.7 Mutations
• Mutations in body cells do not affect offspring.
• Mutations in sex cells can be harmful or beneficial to
offspring.
• Natural selection often removes mutant alleles from a
population when they are less adaptive.
8.7 Mutations
Mutations can be caused by several factors.
• Replication errors can cause
mutations.
• Mutagens, such as UV ray and
chemicals, can cause mutations.
• Some cancer drugs use
mutagenic properties to kill
cancer cells.
11.1 Genetic Variation Within Population
KEY CONCEPT
A population shares a common gene pool.
11.1 Genetic Variation Within Population
Genetic variation in a population increases the chance
that some individuals will survive.
• Genetic variation leads to phenotypic variation.
• Phenotypic variation is necessary for natural selection.
• Genetic variation is stored in a population’s gene pool.
– made up of all alleles in a population
– allele combinations form when organisms have offspring
11.1 Genetic Variation Within Population
• Allele frequencies measure genetic variation.
– measures how common allele is in population
– can be calculated for each allele in gene pool
11.1 Genetic Variation Within Population
Genetic variation comes from several sources.
• Mutation is a random change in the DNA of a gene.
– can form new allele
– can be passed on to
offspring if in
reproductive cells
• Recombination forms new combinations of alleles.
– usually occurs during meiosis
– parents’ alleles
arranged in new
ways in gametes
11.1 Genetic Variation Within Population
Genetic variation comes from several sources.
• Hybridization is the crossing of two different species.
– occurs when individuals can’t find mate of own
species
– topic of current scientific research