Lecture Slides on Cellular Reproduction - General Biology | BIOL 1001, Study notes of Biology

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Chapter 9 - Part II

Cellular Reproduction

Biol 1001 Spring 2012

Cell Cycle Control During Mitosis



Many cells divide:

 (^) Frequently throughout life of organism  (^) In response to stimuli (e.g. damage to tissue) 

Cell division needs to be regulated

 Limit amount of mutated DNA passed on to daughter cells  (^) Ensures correct number of chromosomes passed on to daughter cells  (^) Prevents daughter cells from dying too early & becoming cancerous

3 Checkpoints for Regulation

 Each checkpoint uses a protein complex to regulate progression to next phase of cycle  G1 to S subphase  (^) Determines if DNA is intact & suitable for replication  G2 to mitosis  (^) Determines if DNA was accurately replicated  Metaphase to anaphase  (^) Determines if chromosomes are attached to spindle microtubules & aligned properly along cell’s equator  Progression to next phase is halted if checkpoint determines a problem

Defective G

1

to S Checkpoint

 (^) Primary cause of cancer  (^) Involves gene mutations  (^) Proto-oncogenes mutate into oncogenes into  (^) Proto-oncogenes = gene that promote mitosis  (^) Oncogene = gene that causes cancer  (^) Tumor suppressor genes become inactive  (^) Tumor suppressor genes prevents uncontrolled cell division & mutations from being passed on  (^) When inactive, DNA replication occurs & mutations copied  (^) Cancer cells divide rapidly & uncontrollably  (^) Mutated DNA passed on to daughter cancer cells  (^) Immune system will kill mutated cells if given chance

If asexual reproduction by mitosis is so easy, why do so many organisms undergo sexual reproduction?

Asexual vs Sexual Reproduction



Asexual reproduction by mitosis only

produces genetically identical offspring



Sexual reproduction produces genetically

unique offspring

 (^) Provides an evolutionary advantage 

DNA mutations are going to happen

 Most are neutral or harmful  (^) Some are beneficial

DNA Mutations

 Passed on to offspring  Homologous chromosomes have same genes, but may have slight variations in DNA  Alternate forms of genes (=alleles) produce differences in structure of function  (^) Ex: hair color, length of appendages  Sexual reproduction = (fusion of gametes from 2 parents)  (^) Increases possibility of bringing in different alleles  (^) May increase (or decrease) organisms chance of survival

Meiotic Cell Division



Prerequisite for sexual reproduction

 (^) Occurs in cells that give rise to gametes (egg & sperm)  Ex: testes & ovaries of animals 

End product = 4 haploid daughter cells

 (^) Each contains ½ parent cell’s genetic material  Not genetically identical to each other or parent cell 

Consists of

 (^) Meiosis-nuclear division  (^) 2 rounds of cytokinesis- cytoplasmic division

Meiosis



Diploid cell undergoes

 1 round of Interphase (DNA replication)  Produces 2 chromatids in each duplicated chromosome  Total of 4 chromatids  2 rounds of nuclear division  Meiosis I  2 diploid daughter cells  Meiosis II  4 daughter cells  2 rounds of cytokinesis  Cytokinesis I  (^) Cytokinesis II

Interphase

 Similar to Interphase before mitosis  G1  S  G  Duplication of chromosomes  sister chromatids  Sister chromatids attached at centromere

MEIOSIS I

Prophase I

 (^) 90% of meiosis devoted to this phase  (^) 2 homologous chromosome pairs join to form a tetrad (=pairs of sister chromatids)  (^) Consist of 1 maternal and 1 paternal homologues  (^) Line up side by side & joined by proteins  (^) Chiasmata = visible linkage between sister chromatids  (^) 2 or 3 may form between homologues  (^) “Crossing over” of DNA  (^) Mutual exchange of DNA segments  (^) Occurs between maternal chromatid & paternal chromatid  (^) Source of additional genetic variation

Crossing-over Process

2 Tetrad forms with chiasmata linkage sites 1 Maternal & paternal homologues line up 3 Crossing-over of DNA segments between non-sister chromatids Fig 9-

Prophase I – post crossing-over

 Tetrads become visible & detach from nuclear envelope  Nucleoli & nuclear envelope disappear  Centrioles move towards poles  Spindle microtubules form & attach to sister chromatid pairs of tetrads and kinetochores paired homologous chromosomes spindle microtubule chiasma Fig 9-15a

Metaphase I



Tetrads line up along equator of cell

 1 homologue of each tetrad faces pole  Random orientation  genetic variation 

Spindle microtubules attach at kinetochore

recombined chromatids kinetochores Tetrad Fig 9-15b

Anaphase I



Sister chromatids of duplicate homologue seperate

from tetrad & move toward opposite poles

 Sister chromatids remain attached & move together  End up with 1 copy of each chromosome at each pole 1 homologue consisting of sister chromatids Duplicate homologue Fig 9-15c

Telophase I & Cytokinesis I



Spindle microtubules disappear



Nuclear envelopes may reform (this varies)



Cytokinesis occurs  2 daughter cells formed

 (^) Note: Book says “haploid” daughter cells, but same amount of DNA as diploid parent cell since sister chromatids not separated Fig 9-15d

MEIOSIS II

Prophase II

 No Interphase occurs prior to Prophase II  Chromosomes recondense  Spindle microtubules form & attach to kinetochores of both Fig 9-15e

Metaphase II



Chromosomes line up along equator

 Sister chromatids face opposite poles Fig 9-15f

Anaphase II

 Sister chromatids seperate  Independent chromatids move toward opposite poles  (^) 1 chromatid from each chromosome at each pole Fig 9-15g Independent sister chromatids

Telophase II & Cytokinesis II



Spindle microtubules disintergrate



Nuclear envelopes reform at both poles

around chromosomes



Nucleoli reappear



Cytokinesis occurs  4 haploid daughter cells

 Each daughter cell is genetically different  Each daughter cell contains ½ of the original number of chromosomes = ½ of the DNA

Fig 9-15 h & i Telophase II Cytokinesis II

sister chromatids homologous chromosomes After meiosis I^ After meiosis II Replicated homologues prior to meiosis Fig 9-13

Meiosis Video