6 6 Meiosis and Genetic Variation KEY CONCEPT

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 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 nucleotide DNA polymerase

8. 3 DNA Replication • Two new molecules of DNA are formed, each with an original strand a newly formed strand. • DNA replication is semiconservative. original strand Two molecules of DNA new strand

8. 3 DNA 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 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 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 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