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