Meiosis  

Varia%on  -­‐  The  art  of  crossing  over  

Vocabulary  You  Need  to  Know!  

•  Chroma%d  •  Chroma%n  •  Sister  Chroma%ds  •  Chromosomes  •  Homologous  Chromosomes  •  Tetrad  •  Centromere    

•  Daughter  Cell  •  Parent  Cell  •  Haploid  •  Diploid  •  Crossing  Over  •  Gene%c  Recombina%on  •  Independent  Assortment  

 

Diploid  vs  Haploid  •  Diploid  – 2  copies  of  each  chromosome  – All  our  body  cells  are  diploid  – 2n  

•  Haploid  – one  copy  of  each  chromosome  – Our  sex  cells  (sperm,  egg)  are  haploid    – 1n    *n=  number  of  copies  of  chromosome*  

•  Each  synapsis  is  made  up  of  2  pairs  of  sister  chroma8ds  

•  This  matched  set  of  4  chroma8ds  is  called  a  tetrad  

MEIOSIS AND CROSSING OVER

Chromosomes  are  matched  in  homologous  pairs  

Chromosomes

Centromere

Sister chromatids

•  Crossing  over  is  the  exchange  of  corresponding  segments  between  two  non-­‐sister  chroma8ds  of  homologous  chromosomes  

•  Gene8c  recombina8on  results  from  crossing  over  during  prophase  I  of  meiosis    – This  increases  varia8on  further  

Crossing  over  further  increases  gene%c  variability  

•  How  crossing  over  leads  to  gene8c  recombina8on  

•  Nonsister  chroma8ds  break  in  two  at  the  same  spot  

•  The  2  broken  chroma8ds  join  together  in  a  new  way  

Tetrad (homologous pair of chromosomes in synapsis)

Breakage of homologous chromatids

Joining of homologous chromatids

Chiasma

Separation of homologous chromosomes at anaphase I

Separation of chromatids at anaphase II and completion of meiosis

Parental type of chromosome

Recombinant chromosome

Recombinant chromosome

Parental type of chromosome

Gametes of four genetic types

1

2

3

4

Coat-color genes

Eye-color genes

•  A  segment  of  one  chroma8d  has  changed  places  with  the  equivalent  segment  of  its  nonsister  homologue  

•  If  there  were  no  crossing  over  meiosis  could  only  produce  2  types  of  gametes  

Tetrad (homologous pair of chromosomes in synapsis)

Breakage of homologous chromatids

Joining of homologous chromatids

Chiasma

Separation of homologous chromosomes at anaphase I

Separation of chromatids at anaphase II and completion of meiosis

Parental type of chromosome

Recombinant chromosome

Recombinant chromosome

Parental type of chromosome

Gametes of four genetic types

1

2

3

4

Coat-color genes

Eye-color genes

Increasing  Gene%c  Diversity  

•  During  Meiosis  I  two  steps  are  key  in  increasing  gene%c  diversity  

1.   Crossing  over:  During  Prophase  I  – Homologous  chromosomes  exchange  pieces  of  DNA  

2.  Independent  assortment:  During  Anaphase  II  – Homologous  chromosomes  and/or  sister  chroma%ds  are  randomly    assigned  to  each  daughter  cell    

Crossing  Over  

Independent  Assortment  

Why  Meiosis?  •  Humans  are  DIPLOID:  23  pairs  of  chromosomes  

•  Sex  cells  are  HAPLOID:  contain  1  set  of  23  chromosomes  

•  Fer%liza%on  of  two  SEX  CELLS  (sperm  and  egg)  create  one  ZYGOTE  that  is  DIPLOID  

•  Sexual  reproduc%on  creates  a  zygote  that  is  gene%cally  different  than  both  its  parents    

Why  is  Meiosis  Important?  

•  First  and  foremost  it  allows  for  gene8c  variability  –  This  is  the  most  important  part  of  Meiosis.    You  get  chromosomes  from  both  parents.  This  is  what  allows  you  to  be  different  from  your  parents  or  siblings.  

•  Gene%c  variability  means  that  everything  isn’t  a  clone  of  the  next  

•  It  helps  maintain  the  chromosome  number  in  species  •  Meiosis  can  play  a  part  in  evolu%on  (via  natural  selec%on)  

•  THE  MULE  

Stages  •  The  stages  look  very  similar  to  the  stages  of  mitosis-­‐  a  few  differences:  

•  Meiosis  I  – Homologous  chromosomes  pair  (tetrad)  and  line  up  at  equator  

– Sister  chroma%ds  move  to  the  SAME  pole  of  the  cell   “Tetrad”  

Stages  •  Meiosis  II  – Each  daughter  cell  con%nues  to  divide  to  produce  two  haploid  cells  

– No  replica%on  of  DNA  between  Meiosis  I  and  Meiosis  II  

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