Chapter 2 – Chromosomes and Sexual Reproduction

Basic Cell Types - Prokaryotic “before nucleus” Unicellular Simple structure –No internal membranes Eubacteria –“true bacteria” Archaea –“ancient bacteria” –More closely related to eukaryotes

Basic Cell Types - Eukaryotic “true nucleus” Unicellular or mutlicellular Large, complex –Have internal membranes

Genetic Material Prokaryotes –Single, circular chromosome May have small, accessory plasmids –Eubacteria DNA within cytoplasm (nucleoid region) –Archaea DNA may have associated protein (histones), but different from eukaryotic chromosomes Eukaryotes –Multiple, linear chromosomes –DNA with associated histone proteins Chromatin Chromatin arranged in specific complex to form chromosomes –Allows for packaging within a small nucleus

Genetic Material - Viruses Protein coat surrounding nucleic acid (DNA or RNA) Not classified as living organism –Dependent on host cell for reproduction –Evolved after cells –Closely related to host Similar genes

Cell Reproduction - Prokaryotic Binary fission Origin of replication –Initiation site of DNA replication 2 DNA molecules move to opposite ends of cells –Proteins bind near replication origins and anchor new DNA molecules to plasma membrane

Cell Reproduction - Prokaryotic New cell wall forms to produce 2 cells Identical to each other, and parent cell Asexual reproduction

Cell Reproduction - Eukaryotic Chromosomes –Each species has a characteristic number –Diploid cell/organism Has 2 copies of each chromosome Homologous chromosomes –Save same genes at same locus –May have different alleles –Haploid cell/organism One set/one copy of each chromosome

Chromosome Structure 3 components of a functional chromosome –Centromere Attachment point for microtubules Kinetochore – protein complex attached –Telomere Ends of linear chromosomes stabilize –Origins of replication 2 identical copies of DNA = sister chromatids –Held together by common centromere

Chromosome Classification Classified by location of centromere “p” arm and “q” arm Humans do not have telocentric chromosomes

Cell Cycle Interphase –Extended period of growth Mitotic phase –Mitosis –Cytokinesis Key areas are regulated at checkpoints

Interphase G 1 S G 2

Mitosis Nuclear Division 5 stages –Prophase –Prometaphase

Mitosis 5 stages con’t –Metaphase –Anaphase

Mitosis 5 stages cont –Telophase

Movement of Microtubules

Cytokinesis Division of cytoplasm –Animal cells – cleavage furrow –Plant cells – cell plate Usually occurs simultaneously with telophase End result of mitosis/cytokinesis is 2 identical cells –Asexual reproduction

Sexual Reproduction and Genetic Variation Meiosis –Creates gametes/sex cells/egg and sperm –Chromosome number is reduced by half Diploid to haploid Fertilization –Fusion of egg and sperm to restore diploid condition Forms diploid, single cell - zygote –Sexual reproduction 1 or 2 parents

Meiosis One DNA replication followed by two rounds of division Meiosis I –Reduces number of chromosomes Diploid to haploid Meiosis II –Separates sister chromatids

Meiosis I Prophase

Meiosis I Metaphase Anaphase

Meiosis I Telophase Interkinesis –Nuclear envelopes are re- formed and spindle breaks down Some cells skip to metaphase II –Cell may or may not split into two cells

Meiosis II Prophase Metaphase

Meiosis II Anaphase Telophase

Consequences of Meiosis End result is 4 haploid cells from one diploid cell –Each is genetically different Crossing over –Exchange between homologous chromosomes –Sister chromatids are not identical Random assortment –Shuffles maternal and paternal chromosomes in different combinations –Metaphase I

Crossing Over and Random Assortment

Separation during Mitosis Cohesin holds sister chromatids together –Established during S –Broken down during anaphase by separase Separase is inactive during Interphase and early mitosis

Separation during Meiosis Cohesin aids in formation of synaptonemal complex Anaphase I –Cohesin broken down by separase –Centromeric cohesin is protected by shugoshin Keeps sister chromatids together Metaphase II –Separase breaks down shugoshin Allows separation of sister chromatids

Meiosis in Animals

Alternation of Generations in Plants