Chapter 2: Reproduction and Chromosome

reproduction

the process by which new cells or organisms are produced 

• Requires the transmission of chromosomes from parent to offspring 

prokaryotes

• Prokaryotes: bacteria and archaea 

  • No nucleus → have nucleoid: where chromosome resides

  • No membrane-bound organelles

binary fission

•  asexual reproduction 

  • Mother cell: original cell that divides

  • Mother cell replicates DNA, FtsZ protein cause septum formation (tourniquet ring) → leads to 2 daughter cells

eukaryotes

• protists, fungi, plants and animals

  • Nucleus w/DNA (separates genetic content from rest of the cell) 

cytogenetics

• the field of genetics that involves the microscopic examination of chromosomes

diploid

2 copies of each chromosome

homologous chromosomes

have the same genes, but not always the same alleles

allele

sequence of DNA at a gene

asexual reproduction

pre-existing cell produces 2 new cells 

cell cycle

interphase (G1, S, G2) + mitosis (prophase, pro metaphase, metaphase, anaphase, telophase, cytokinesis)

Growth 1 phase (G1)

• cell prepares to divide

  • Restriction point: the point at which molecular changes have accumulated to commit the cell to proceed through cell division 

synthesis phase (S)

chromosomes are replicated 

dyad

pair of sister chromatids

centromere

DNA that is hidden beneath the kinetochore proteins

kinetochore

proteins attached to the centromere

Growth 2 phase (G2)

Cell accumulates the material for nuclear and cell division 

• Centrosomes and chromosomes are replicated

prophase

• Nuclear envelope starts disintegrating 

• Chromatids condense

• Mitotic spindle begins to form

Mitotic spindle apparatus

mechanism to organize and sort eukaryotic chromosomes

microtubule-organizing centers (MTOCs)

centrosomes in animals

spindle pole

a centrosome with 2 centrioles 

aster microtubules

position spindle by attaching to plasma membrane

polar microtubules

push spindle poles to opposite sides of cell

kinetochore microtubules

attach to chromosomes

pro metaphase

• Centrosomes move to poles

• MTs connect to kinetochores

• Mitotic spindle is formed

metaphase

• Chromatids align on metaphase plate

• Chromatids connected to both poles

anaphase

• Sister chromatids separate

• MTs shorten 

telophase

• Chromosomes reach the poles + decondense

• Nuclear envelope reforms

• Organelles separate 

cytokinesis

• Protein myosin pulls cell membrane together and pinches to constrict plasma membrane → cleavage furrow 

  • Animals

• Plants form cell plate

meiosis

• Produces unique, haploid gametes from original diploid cell

• 2 reductive divisions

prophase 1

5 stages (LZPDD)

Leptotene

• replicated chromosomes begin to condense 

  • chromosomes appear very thin and thread-like and consist of 2 chromatids 

Zygotene

• homologous chromosomes begin to join in pairs through synapsis: homologs binding together through a protein  

  • Forms synaptonemal complex: cohesions 

Pachytene

• pairing of homologous chromosomes (bivalents) is completed 

  • Crossing over: involves a physical exchange of chromosome pieces 

  • Forms chiasma: 4 chromatids are together, location of crossing over 

Diplotene

• synaptonemal complex has mostly disappeared, chromatids within bivalent pull slightly apart

  • Homologous chromosomes reveal their 4 chromatids → don’t separate completely and remain in contact via the chiasmata 

Diakinesis

• chromosomes shorten and orient themselves towards the equatorial plate → synaptonemal complex has completely disappeared 

  • Ends prophase I 

  • Bivalent formation 

metaphase I

• Pairs of sister chromatids form a double row at metaphase plate

• Arrangement of sister chromatids is random with regard to which parent they came from

anaphase

• The recombined homologous chromosomes, each with 2 chromatids, separate and migrate toward the poles of the cell

• Haploid cell

meiosis II

same phases as mitosis, Meiosis produces 4 haploid daughter cells that are NOT genetically identical

independent assortment

combinations of chromosomes in haploid cells are random = genetic diversity

Gameotogenesis

process of forming gametes

sexual reproduction

gametes make gametes w/half the amount of genetic material

• Gametes fuse with each other during fertilization to begin the life of a new organism

Spermatogenesis

• production of sperm in testes 

  • Mother cell goes through meiosis I to produce 2 haploid cells: secondary spermatocytes 

  • Meiosis II produces spermatides 

  • Maturation process to form haploid sperm cells

Oogenesis

• production of egg cells in ovaries 

  • Primary oocyte: diploid, arrested during prophase in embryonic development

  • During sexual maturity + ovulation → undergo meiosis I and form secondary oocyte 

  • Secondary oocyte: half are haploid, half are polar body (disengrates, gets rid of some chromosomes) 

    • Arrested in meiosis II unless fertilized 

  • Continues to meiosis II after contact w/sperm (sperm + egg nuclei fuse) 

  • Haploid egg cell and second polar bodies (so egg cell can become haploid)