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LG: How do we produce gametes? Date: 2/4/13. Attention: from now on, if you see any words or phrases inside this “[ ]” symbol, you need to know them by heart! . [ Mitosis] and [Meiosis]. Mitosis: - division of [ somatic cells] = [ body cells] Meiosis

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lg how do we produce gametes date 2 4 13

LG: How do we produce gametes? Date: 2/4/13

Attention: from now on, if you see any words or phrases inside this “[ ]” symbol, you need to know them by heart!

mitosis and meiosis
[Mitosis] and [Meiosis]
  • Mitosis:

-division of [somatic cells] = [body cells]

  • Meiosis

-division of [gametes] = [sex cells]

mitosis meiosis
Mitosis: Meiosis:

Each resulting cell still has chromosomes from mom & dad



Let’s Review!

asexual reproduction
[Asexual Reproduction]

One parent

Offspring [identical]=(clones)


No [variation]=difference

i nterphase
  • Interesting things happen!
  • Cell preparing to [divide]=multiply
  • [Genetic material] doubles (making copies)







Sister Chromatid

– each side of a replicated chromosome

  • [Chromosomes] pair up!
  • Chromosomes thicken and shorten

-become visible

-2 chromatids joined by a [centromere]

  • [Centrioles] move to the opposite sides of the nucleus (only in animal cells)
  • [Nuclear membrane] disappears
m etaphase
  • Chromosomes meet in the middle!
  • Chromosomes arrange at equator of cell
  • Become attached to [spindle fibres] by centromeres
  • [Homologous chromosomes] do not pair up
what are homologous chromosomes
What are homologous chromosomes?

Definition: chromosomes having the same genes at the same loci (location) but possibly different alleles (alternative forms of a gene). For example, two chromosomes may have genes encoding eye color, but one may code for brown eyes, the other for blue.

a naphase
  • Chromosomes get pulled apart
  • Spindle fibres contract pulling chromatids to the opposite poles of the cell
t elophase cytokinesis
  • Now there are two!
  • Nuclear envelope forms around chromosomes
  • Chromosomes uncoil
  • Cytokinesis
    • animals - pinching of plasma membrane
    • plants- elongates and the [cell plate] forms( future cell wall and cell membrane)
what is meiosis for
What is [meiosis] for?
  • Gametes for [Sexual Reproduction]
  • Two parents
  • Offspring all genetically different
  • Takes longer
  • Lots of variations
  • 4 [daughter cells] produced
  • Each daughter cell has half the chromosomes of the parent
  • 2 sets of [cell divisions] involved
meiosis reduction division
Meiosis = reduction division
  • Meiosis
    • special cell division in sexually reproducing organisms
    • reduce number of chromosomes
      • 2n (46)  1n (23)
      • [diploid]  [haploid]
        • Reduced by half
    • makes [gametes]
      • [sperm], [eggs]
you have body cells and gametes

sex cells (egg)

body cells

sex cells (sperm)

You have body cells and gametes.
  • Body cells are also called somatic cells.
    • Body cells cannot pass DNA on to offspring
  • Gametes are sex cells: egg and sperm.
    • Gametes have [DNA] that can be passed to offspring.
your cells have autosomes and sex chromosomes
Your cells have [autosomes] and [sex chromosomes].
  • Your body cells have 23 pairs of chromosomes.
  • Chromosome pairs 1-22 are autosomes.
    • Homologous pairs of chromosomes have the same structure.
    • For each homologous pair, one chromosome comes from each parent.
  • Sex chromosomes, X and Y, determine gender in mammals.
once again
Once again!

Homologous chromosomes

body cells are diploid gametes are haploid
Body cells are diploid; gametes are haploid.
  • Diploid (2n) cells have two copies of every chromosome.
    • Body cells are diploid.
    • Half the chromosomes come from each parent.
Haploid (n) cells have one copy of every chromosome.
    • Gametes are haploid.
    • Gametes have 22 autosomes and 1 sex chromosome.
    • [Fertilization] restores the diploid number
meiosis interphase
Meiosis Interphase

The chromosomes have replicated, and they become visible.


meiosis prophase i
Meiosis Prophase I
  • The chromosomes are completely condensed. In meiosis (unlike mitosis), the homologous chromosomes pair with one another
  • [Crossing over] occurs


crossing over
Crossing over
  • 3 steps
    • cross over
    • breakage of DNA
    • re-fusing of DNA
  • New combinations of traits


crossing over contd
Crossing over contd.
  • Crossing Over of genes occurs now
    • The exchange of genetic material between homologous chromosomes
    • Results in new genetic combinations of offspring.
    • This is the main advantage of sexual reproduction

Results in Variations!!

meiosis metaphase i
The homologous chromosomes attach to the spindle fibers. They are preparing to go to opposite poles. Meiosis Metaphase I


meiosis anaphase i
The homologous chromosomes (not chromatids) move to opposite ends of the cell. Meiosis Anaphase I


meiosis telophase i cytokinesis
The cell begins to divide into two daughter cells. It is important to understand that each daughter cell can get any combination of [maternal] and [paternal] chromosomes. Meiosis Telophase I & Cytokinesis


meiosis prophase ii
The cell has divided into two daughter cells. Meiosis Prophase II


meiosis metaphase ii
Just like in Mitosis, the chromosomes line up in the middle on the spindle fibers. Meiosis Metaphase II


meiosis anaphase ii
The two cells each begin to divide. As in Mitosis, the chromatids move to opposite ends of each cell. Meiosis Anaphase II


telophase ii cytokinesis
With the formation of four cells, meiosis is over. Each of these prospective gamete cells carries half the number of chromosomes of somatic cells. Telophase II &Cytokinesis


[Gametogenesis] is the production of gametes.
  • Gametogenesis differs between females and males.
    • Sperm become streamlined and motile.
    • Sperm primarily contribute DNA to an embryo.
    • Eggs contribute DNA, cytoplasm, and organelles to an [embryo].
    • During meiosis, the egg gets most of the contents; the other cells form [polar bodies].
  • Meiosis animation:


similarities between mitosis meiosis
Similarities between Mitosis & Meiosis
  • Both are forms of nuclear division
  • Both involve [replication]
  • Both involve disappearance of the nuclear membrane
  • Both involve formation of spindle fibers
Meiosis differs from mitosis in significant ways.
    • Meiosis has two cell divisions while mitosis has one.
    • In mitosis, homologous chromosomes never pair up.
    • Meiosis results in haploid cells; mitosis results in diploid cells.
mitosis vs meiosis1

1 division

daughter cells genetically identical to parent cell

produces 2 cells

2n  2n (diploid)

produces cells for growth & repair

no crossing over


2 divisions

daughter cells genetically different from parent

produces 4 cells

2n  1n (haploid)

produces gametes


Mitosis vs. Meiosis
this may help you remember these terms
This may help you remember these terms:

MITosis takes the cell and Makes It Two (diploid)

Meiosis has to do with sex

From the cell’s point of view:

mITosis results in IdenTical Cells

mEioSis results in Egg and Sperm (haploid)

Clyde Freeman Herreid – Dept. of Biological Sciences: Buffalo State Univ. of New York

putting it all together
















Putting it all together…

meiosis  fertilization  mitosis + development





mitosis &development




from one cell to many


From one cell to many
  • A fertilized egg goes through an early stage of development called an embryo.
specialized cells perform specific functions

Inner: intestines

Outer: skin cells

Middle: bone cells

Specialized cells perform specific functions.
  • Cells develop into their mature forms through the process of cell [differentiation].
  • Cells differ because different combinations of genes are expressed.
the value of sexual reproduction
The value of sexual reproduction
  • Sexual reproduction introduces genetic variation
    • genetic recombination during meiosis
      • independent assortment of chromosomes
        • random alignment of homologous chromosomes in Meiosis 1
    • crossing over
      • mixing of alleles across homologous chromosomes
    • random fertilization
      • which sperm fertilizes which egg?
  • Driving evolution
    • variation for natural selection
variation from random fertilization
Variation from random fertilization
  • Sperm + Egg = ?
    • any 2 parents will produce a zygote with over 70 trillion (223 x 223) possible diploid combinations
sexual reproduction creates variability

Michael & KirkDouglas

Baldwin brothers

Martin & Charlie Sheen, Emilio Estevez

Sexual reproduction creates variability

Sexual reproduction allows us to maintain both genetic similarity & differences.

words you must know
Words You Must know
  • Mitosis
  • Meiosis
  • Somatic cells
  • Gametes
  • Asexual reproduction
  • Identical
  • Variation
  • Interphase
  • Prophase
  • Genetic material
  • Chromosome
  • Centromere
  • Centrioles
  • Nuclear membrane
  • Metaphase
  • Spindle fibers
  • Homologous chromosomes


  • Crossing over
  • Maternal
  • Paternal
  • Gametogenesis
  • Polar bodies
  • Replication
  • Anaphase
  • Telophase/cytokinesis
  • Sexual reproduction
  • Cell division
  • Sperm
  • Egg
  • Autosomes
  • Sex chromosomes
  • Diploid (2n)
  • Haploid (n)
  • Fertilization