Meiosis to Mendel

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Meiosis to Mendel

• Chapter 9
• Read first few sections.

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Reproduction

• Asexual reproduction mitosis
• Produces clones genetically identical
  individuals
• What would happen if the environment changed?
• Genetic variability helps the population as a
  whole survive changes

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WHY SEX ?

• Sexual reproduction combines the DNA from two
  different individuals
• A gene is a section of a chromosome that carries
  instructions for a specific trait (protein)
• An allele is a different version of a gene
• The greater the number of different combinations
  of genes the more variation among individuals,
  and the greater the chance of survival of the
  species.

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Sex cell formation

• Sexually reproducing organisms need to produce
  specialized reproductive cells or gametes.
• Produced from germ cells in organs called gonads
• In females ovaries produce eggs or ova
• In males testes produce spermatozoa
• A mutation in one of your cells wont be passed
  on unless it is in the germ line cells

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Diploid vs. Haploid

• In animals, most plants, and many other
  organisms, each cell contains 2 sets of
  chromosomes.
• This is the diploid (2n) number of chromosomes.
• A pair of homologous chromosomes, one from each
  parent.
• Gametes contain only one member of each pair or
  the haploid (n) number of chromosomes

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• When a sperm and egg combine in the process of
  fertilization, or syngamy, a new diploid cell or
  zygote is formed.
• The gametes must be haploid, that is, have n
  number of chromosomes. If the gametes were
  diploid, every generation would have twice the
  number of chromosomes.
• To form haploid gametes, there needs to be a
  process other than mitosis this is called
  meiosis.

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Meiosis

• This is a two part process meiosis I and
  meiosis II, features TWO cell divisions.
• However, the DNA is only replicated once
• Meiosis I and II both use the same four stages of
  mitosis prophase, metaphase, anaphase and
  telophase

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Major differences between mitosis and meiosis

• Meiosis has two cell divisions.
• Meiosis has a step where homologous chromosomes
  swap pieces
• Increases genetic variation, new combos.
• In the first division, sister chromatids dont
  split apart! Homologs separate instead.
• In the second division, sister chromatids
  separate like in mitosis.

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During prophase I the homologous chromosomes pair
up in synapsis. This is the longest phase of
meiosis. Crossing over may occur further
increasing genetic variation.
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Homologs dont hang together

• In metaphase I the tetrads migrate to the center
  of the cell.
• In Anaphase I the centromeres do not break and
  one member of each homologous pair (2 sister
  chromatids) move to opposite ends of the cell
• Which homolog goes to which end of the cell
  occurs at random.
• Telophase occurs as in mitosis.

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Meiosis II

• There is no replication of chromosomes between
  telophase I and prophase II
• Meiosis II proceeds just like mitosis during
  anaphase the centromeres break and the two sister
  chromatids go to opposite poles.

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With only 2 pairs of chromosomes, a gamete could
have any one of 4 different combinations
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Cytokinesis

• Varies by which type of cell is being made
• If we are producing sperm, each of the four cells
  produced by meiosis II become sperm.

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• If we are making ova, cytokinesis is uneven and
  one cell takes nearly all the cytoplasm, leaving
  the other cell merely a package of discarded DNA
  called a polar body.
• In humans, the cell again divides unevenly, so at
  the end of meiosis II we have formed one ovum
  (egg) and three polar bodies. The polar bodies
  disintegrate.

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• The average woman produces one ovum every 28 days
• Males produce 300 million sperm/ day
• If less than 20 million / ml, a man is considered
  infertile.
• Fertilization is a group effort, but only one
  sperm penetrates the ovum.

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• Sometimes things go wrong.
• In anaphase I the separation of homologous
  chromosomes is called disjunction.
• When they do not separate it is called
  nondisjunction and the resulting gametes contain
  one too many or one too few chromosomes.
• Fertilization results in a zygote with 45 or 47
  chromosomes. This is an aneuploid (vs. euploid)
  number of chromosomes

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Chromosomal Abnormalities per 100,000 recognized
human pregnancies                                 
                                                  
    15,000                        
85,000                                            
                             spontaneous
abortions           live births ------------------
--------------------------------------------------
------------------------------------- Trisomy   A
Group           1                                 
                         0                        
                      0                        
2                                                 
         159                                      
    0                         3                   
                                      
53                                           
0             B Group          
4                                                 
         95                                       
     0                         5                  
                                       
0                                             
0   C Group           6-12                        
                             561                  
                        0     G group           
21                                                
        350                                       
   113                         22                 
                                      
424                                          0
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• Three copies of a chromosome is called trisomy
  Down syndrome is trisomy 21
• A zygote with one too few chromosomes does not
  usually develop. (monoploid)
• Extra copies of the sex chromosomes (vs. the
  autosomes) do not cause as much of a problem
• XX is female in humans (male in birds)
• XY is male in humans
• XXY , XYY, XXX, XO all happen
• Plants do much better with multiple chromosomes

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              15,000                        
85,000                                            
            spontaneous abortions       live
births sex chromosomes                        
XYY                               
4                                             
46                         XXY                    
            4                                     
         44                        
XO                           1350               
                         
8                         XXX                     
     21                                     
       44 Translocations            
Balanced                             
14                                           
164             Unbalanced                        
    225                                 
         52 Polyploid            
Triploid                            
1275                                        
0             Tetraploid                         
450                       
                   0 Other (mosaics,
etc)                      280                
                           49 Total  
                                   
7500                                         550
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Genetics

• A gene is a section of DNA that codes for
• Proteins for structures such as muscles -
  or for enzymes
• RNA molecules like rRNA and tRNA
• Genes have sections of DNA next to them
• that control whether they get used or not
  regulatory regions

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DNA is like magnetic recording tape song
information is encoded in the structure of the
tape magnetically. Many different songs are
recorded and each one can be played individually
(or not played). DNA is very long and thin like
recording tape.
DNA is like digital media also, because each song
can be specifically accessed at the beginning.
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• Each cell of an organism that reproduces sexually
  has two copies of each chromosome, and therefore
  has two copies of every gene one on each member
  of each pair of chromosomes(exception is the Y
  chromosome, which is smaller than the X).
• The two versions of each gene are called alleles.
  Alleles may be the same or different, depending
  on the traits of the parents.

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• All the genes that are contained on all the
  chromosomes of an individual make up that
  individuals Genome.
• The Genotype is all the genetic information the
  individual has for a particular trait.

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• Not all genes are expressed.
• Those traits that are expressed can be seen
  (physical traits) or measured (chemical traits)
  are the individuals phenotype.
• The phenotype is influenced by both the genotype
  and the environment.
• In diploid organisms (like us) an allele from one
  parent can mask the appearance of the other. So
  often genotype ? phenotype.
• We can still pass on a hidden trait and see it in
  our children.