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Meiosis. Chapter 11.4. Chromosome Review. Let’s review… What is a chromosome? Condensed chromatin (DNA + proteins) only visible during cell division!! How many chromosomes do you have in each of your cells? 46 (23 pairs) But any of your cells NOT have 46? SEX CELLS!.

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Chapter 11.4

chromosome review
Chromosome Review
  • Let’s review…
    • What is a chromosome?
      • Condensed chromatin (DNA + proteins)

only visible during cell division!!

    • How many chromosomes do you have in each of your cells?
      • 46 (23 pairs)
    • But any of your cells NOT have 46?
      • SEX CELLS!
chromosome number
Chromosome Number
  • What does it mean to have 23 pairs of chromosomes?
    • 23 chromosomes: one copy of each from mom & one from dad!
  • Cells that have a chromosome from each parent are DIPLOID (2N)!
    • Somatic cells (body cells)
    • “two sets”
chromosome number1
Chromosome Number
  • Some cells don’t have a pair of each chromosome!
    • Gametes (sex cells) of sexually reproducing organisms
    • These cells are HAPLOID (N)!
    • “one set”
    • Single set of genes!
    • Why do they only have half the

number of chromosomes?

Each gamete can only

contain one set of genes!



chromosome structure review
Chromosome Structure Review
  • Chromatid
    • One strand of a duplicated chromosome
    • Joined by a centromere to its sister chromatid
  • Sister chromatids
    • Two chromatids joined by a common centromere
    • Each carries identical genetic information
    • Together called a DYAD
  • The genetic information that we (and other sexually reproducing organisms) inherit comes from two cells: sperm and egg
  • Meiosis – cell division in which the chromosome number is cut in half
    • Gametes (sperm and egg) divide this way!
  • Germ cells in the testis and ovary

undergo meiosis and produce


meiosis reduces the number of chromosomes cell by half
Meiosis: Reduces the number of chromosomes/cell by half
  • Occurs in two steps:
    • Meiosis I
    • Meiosis II
  • Meiosis I and II each have prophase, metaphase, anaphase, and telophase stages

2N = 4

N = 2


2 dyads

  • DNA replication occurs during interphase before the beginning of meiosis I but not before meiosis II
  • Chromosomes will associate with the other member of its pair during Meiosis
    • Homologous chromosomes (also called a TETRAD)

1 tetrad

  • Meiosis I is a reductionaldivision: the number of centromeres is reduced by half after this division
  • Meiosis II is an equational division: the number of centromeres remains equal after this division
meiosis 1 prophase 1
Meiosis 1: Prophase 1
  • Prophase I
    • Chromosomes become visible
    • Homologous chromosomes pair up to form tetrads
    • consumes 90% of the time for meiosis
    • Crossing Over occurs
      • Results in Genetic Variation
      • New allele combinations!

Two major sources of genetic variation in Meiosis I

  • CrossingOver
    • Creates new combinations of mom and dad’s alleles
    • Think about chromosomal mutations!
  • IndependentAssortment
metaphase 1
Metaphase 1
  • As prophase I ends, tetrads attach to spindle fibers
  • Tetrads line up at center of cell
anaphase 1
Anaphase 1
  • Dyads pulled toward opposite poles by spindle fibers
  • Disjunction: separation of chromosomes
    • Nondisjunction leads to polyploidy! (extra chromosomes)
  • Note the exchange of information between paternal and maternal chromosomes
telophase 1
Telophase 1
  • Separated chromosomes cluster at opposite ends of cell
  • Nuclear membrane forms around each cluster
  • Cytokinesis follows and forms 2 new cells
This was a reductional division: number of centromeres reduced per cell.

Prophase I: 4 centromeres, therefore 4 chromosomes

Prophase II: 2 centromeres, therefore 2 chromosomes/cell

results of meiosis 1
Results of Meiosis 1
  • Two daughter cells
  • Neither with two complete sets of chromosomes (haploid)
  • Sets have been shuffled and independently assorted
  • Chromosomes differ between each other and the original cell
meiosis ii
Meiosis II
  • Two cells enter second meiotic division
  • Neither cell goes through DNA replication prior to this division!
prophase ii
Prophase II
  • Chromosomes (dyads) become visible
  • Do not form tetrads because they are already separated from homologous pair!
metaphase ii anaphase ii
Metaphase II & Anaphase II
  • Chromosomes (dyads) attach to spindle fibers and line up in center of cell
    • Remember…they aren’t paired with another chromosome!
  • Anaphase II – chromatids (monads) separate from each other at the centromere
    • move to opposite poles!
telophase ii
Telophase II

TelophaseII & Cytokinesis

  • Four genetically different haploid cells produced (N)
  • Each monad may be an entirely new combination of maternal and paternal genetic information
meiosis review
Meiosis Review
  • Meiosis II
  • Prophase II: Dyads reappear, no tetrads!
  • Metaphase II: Dyads line up in middle
  • Anaphase II: Monads pulled apart (separated at centromere)
  • Telophase II: four new genetically different haploid daughter cells with monads

Meiosis I

  • Prophase I: tetrads form (homologous chromosomes pair), crossing over occurs
  • Metaphase I: tetrads line up in middle
  • Anaphase I: Dyads pulled apart
  • Telophase I: two new genetically different haploid daughter cells with dyads
meiosis formation of gametes
Meiosis: Formation of Gametes
  • Meiosis results in two kinds of haploid, sexual gametes
    • Males produce sperm
    • Females produce eggs (usually only one of the four egg cells is used!)
  • Sperm fertilizes

egg to produce

2N zygote!

    • Goes through mitosis

& cell specialization

to form a new


mitosis vs meiosis a comparison
Mitosis vs. Meiosis: a Comparison
  • Both preceded by DNA replication
  • Both are methods of cell division
  • Both include Prophase, Metaphase, Anaphase, and Telophase
  • Both are followed by cytokinesis

Mitosis vs. Meiosis

contrasting meiosis mitosis
Contrasting Meiosis & Mitosis


  • Each daughter cell receives a complete set of chromosomes
  • Less genetic diversity
  • Doesn’t change the chromosome number of the original cell
  • Single cell division
  • Two genetically identical diploid daughter cells
  • Asexual reproduction
  • Makes somatic cells


  • Two alleles for each gene segregated and end up in different cells
  • Greater variety of possible gene combinations
  • Reduces the chromosome number by half
  • Two rounds of cell division
  • Four genetically different haploid daughter cells
  • Sexual reproduction
  • Makes gametes
gene linkage
Gene Linkage
  • Genes on different chromosomes assort independently
  • What about genes on the same chromosome?
    • Tend to be linked!
    • Chromosomes assort independently, but typically genes on the same

chromosome are inherited together

    • Especially when close together!
    • Crossing over causes some genes on

the same chromosome to assort


gene maps
Gene Maps
  • Frequency of crossing-over between genes during meiosis is used to determine genes’ locations
  • Farther apart, more likely that crossing over occurs between them
  • Close together, crossovers rare
  • Use frequency of crossing over to determine distances from each other and map genes’ locations on chromosomes!