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					IB Biology Review
 Chapter 13: Meiosis
      What is the goal of meiosis?

  To create haploid cells from diploid cells

Another way of thinking about it is
 Somatic cell -> gamete
                    Meiosis Vocab

Genes
    A discrete unit of hereditary information consisting of a
    specific nucleotide sequence in DNA
Somatic cell
    A diploid body cell
Gamete
     a haploid cell such as an egg or sperm
Asexual reproduction
    Type of reproduction that involves one parent producing
    two identical offspring by budding or binary fission
Sexual reproduction
    Type of reproduction in which two parents produce
    offspring with unique combinations of genes
               Meiosis Vocab Cont.

Diploid cell
    Cell with two sets of chromosomes (2n), one from each
    parent
Haploid cell
    Cell with one set of chromosomes; gamete
Autosome
    A chromosome that is not directly involved in determining
    sex
Sex chromosome
    One of the pair of chromosomes responsible for the sex of an
    individual
Zygote
    The diploid product of the union of haploid gametes in
    conception; a fertilized egg
                   Meiosis Vocab cont.
Synapsis
     The pairing of replicated homologous chromosomes during prophase I
     of meiosis
Crossing over
     The reciprocal exchange of exchange material between non-sister
     chromatids during prophase I of meiosis
Chiasma / Chiasmata (plural)
     The X-shaped, microscopically visible region where homologous
     chromatids exchange genetic material in crossing over
Tetrad
     A paired set of homologous chromosomes, composed of two sister
     chromatids
     Forms during prophase I of meiosis
Recombinant chromosome
     A chromosome created when crossing over combines the DNA from two
     parents into a single chromosome
Nondisjunction
     An error in meiosis where chromosomes fail to move apart correctly
     Can lead to an abnormal number of chromosomes in gametes
   What are the stages of meiosis?

Prophase I
Metaphase I
Anaphase I
Telophase I and cytokinesis I
Metaphase II
Anaphase II
Telophase II and cytokinesis II
 What happens during prophase I?

Chromosomes condense / coil / become shorter
and fatter
Homologous chromosomes pair up
Crossing over /chiasmata form
                 Crossing Over

 Similar pieces of DNA are
 exchanged between the
 mother’s and father’s
 chromosomes
 Occurs at chiasma

What is the point of
crossing over?
    To increase genetic diversity
    What happens during metaphase I?

 Pairs of homologous chromosomes move
 toward the equator



What is the equator called?
    Metaphase plate
What are the pairs of homologous chromosomes
 called?
    Tetrads
 What happens during anaphase I?
Half of chromosome tetrad moves to either
pole




Sister chromatids remained attached at
centromere
The cell then goes through telophase and split
via cytokinesis
What happens during Metaphase II?

(Two chromosomes already present, so prophase II
establishes microtubules)
Sister chromatids move towards the equator
What happens during Anaphase II?

Sister chromatids split
Chromosomes move towards opposite poles
What happens in telophase II and cytokinesis?

Chromosomes decondense / uncoil
Cell splits into two identical daughter cells
with one set of chromosomes (haploid)
What is the law of independent assortment?
 Chromosomes line up independently at the
 metaphase plate
 Means that which chromosome goes to which
 gamete is completely random
 50% chance a daughter cell will receive
 maternal and 50% chance it will receive
 paternal
 2n combinations, n = number of chromosomes
   In humans, 223 = 8 million different types of gametes
     What is random fertilization?

Any egg can combine with any sperm
Random combination of maternal and paternal
genes
8 million maternal gametes x 8 million paternal
gametes
  = 64 trillion different combinations in humans
What are three sources of genetic variation?

  Independent assortment
    2n different combinations for chromosomes
  Crossing over
    Mixes maternal and paternal genes
  Random fertilization
    Any of the gametes can be combined
          What is a karyotype?

 A picture of a person’s chromosomes arranged
 in pairs according to their size, shape, and
 patters

Is this a karyotype
  of a male or female?
  Female
  Two X chromosomes
  Males have X and Y
     Why are karyotypes useful?

Cells from a developing fetus can be taken from
the amniotic sac and karyotyped
Reveals gender and genetic disorders
For example, Downs Syndrome
  Occurs as a result of nondisjunction on
  chromosome 21
  Means individual receives 3 copies of chromosome
  21 instead of the usual 2
Mitosis vs. Meiosis
IB Exam Question
1. Describe the behavior of chromosomes in the
different phases of meiosis.
                                                     (8 marks)

  chromosomes condense / coil / prior to meiosis
  Chromosomes become shorter and fatter during prophase I
  (homologous) chromosomes pair up in prophase I;
  crossing over occurs in prophase I;
  movement of pairs of chromosomes to the equator in
  metaphase I;
  movement of half of chromosomes to each pole in anaphase I;
  Sister chromatids separate in anaphase II and chromatids move
  to opposite poles.
  decondensation / uncoiling in telophase II;
IB Exam Question
2. A cell with a diploid number of 12
chromosomes undergoes meiosis. What will be
the product at the end of meiosis?    (1 mark)

   A.      2 cells each with 12 chromosomes
   B.      4 cells each with 6 chromosomes
   C.      2 cells each with 6 chromosomes
   D.      4 cells each with 12 chromosome

      Correct answer: B
IB Exam Question

3. Explain how meiosis and fertilization
can give rise to genetic variety.    (7 marks)

  random orientation of pairs of chromosomes in Metaphase I
  In other words, maternal and paternal chromosome could go to
  either pole;
  2 n combination;
  In humans, this produces 2 23 combinations; e.g. over 8 million
  gametes with unique combinations of chromosomes
  crossing over in Metaphase I also adds diversity
  This is when there is an exchange of material between
  homologous
  In addition, fertilization brings together genes from two
  different parents;
  fertilization generates new combinations of genes / alleles and
  is called random fertilization
  This produces over 64 million unique zygotes in humans
  (ignoring crossing over)
IB Exam Question

4. Describe how sexual reproduction promotes
genetic variation within a species.    (4 marks)

  (segregation of alleles involves) meiosis;
  crossing over / chiasma formation in prophase I /
  meiosis (do not allow if wrong phase of meiosis
  given);
  random orientation / assortment of homologues at
  metaphase I;
  fertilization by chance / one of many male gametes;
  number of different gametes is 2n (ignoring crossing
  over);
  genes / alleles combined from two parents;
IB Exam Question

5. Explain how the inheritance of chromosome
21 can lead to Down’s syndrome.         (3 marks)

  non-disjunction;
  the failure of homologues / sister chromatids to
  separate during meiosis;
  anaphase I / anaphase II;
  two copies of chromosome 21 in gamete;
  fertilization leads to trisomy / trisomy 21;
IB Exam Question
6. Outline the differences between the
behaviour of the chromosomes in mitosis and
meiosis.                               (5 marks)


  two divisions in meiosis, only one in mitosis;
  meiosis results in haploid cells, mitosis in diploid cells;
  crossing over only occurs in meiosis;
  no S phase precedes meiosis II;
   chiasmata only form during meiosis;
  homologous chromosomes move to the equator in
  pairs only in meiosis;
IB Exam Question
7. What is a Karyotype and how it is used to
detect genetic abnormalities? (Include a
description of the procedure used)
     (5 marks)
  A Karyotype is a “picture” of a full set of Human
  Chromosomes
  It is used to detect whether the individual has the
  correct number of Chromosomes like in Down
  Syndrome when there’s one extra chromosome 21
  Or whether there is obvious physical damage to the
  chromosomes
  Fetal cell samples can be obtained from the amniotic
  sac during early fatal development
  These are obtained with a needle in a process called
  amniocentesis
  The cells are karyotyped and analyzed for
  abnormalities

				
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