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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
• G1
• S
• G2
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