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Chapter 13 – Campbell’s 6th Edition
Meiosis and Sexual Life Cycles
Definitions
1) Heredity: transmission of traits from generation to the
next
2) Variation: slight differences in appearance
3) Genetics: scientific study of heredity and hereditary
variation
Genes
• Genes:coded information in the form of
hereditary units (lots of genes on each chromosome)
• Where else can you find DNA beside the nucleus of the cell?
MitochondriaChloroplast
Asexual Reproduction
• Asexual reproduction single individual is
sole parent and passes
copies of all its genes to its offspring
• Clone the offspring of
asexual individuals
Sexual reproduction
Two parents give rise to offspring with unique combinations of genes; results in more variation
Life Cyclesequence of stages from an organisms
conception to production of its own offspring
Animals
Fungi and some algaePlants and some algae
More Vocab
1) somatic cell:body cells; any cell other than sperm or ovum
2) karyotype:micrograph of the 46 human chromosomes in which they are lined up in pairs starting with the longest
1) homologous chromosomes:same length, centromere position, staining pattern; carry genes governing the same inherited characters
2) sex chromosomes:X and Y chromosomes
3) autosomes:all non-sex chromosomes (1 through 22)
Karyotype
Meiosis and Sexual Reproduction
• Sexual Reproduction – chromosomes of two parents combine to form offspring
23
23
+
46
Gametes
Fertilization
Zygote
(2N)
(N) + (N)
About Chromosomes
• (2N) = diploid – any cell with 2 complete sets of chromosomes
• (N) = haploid – a cell with 1 complete set of chromosomes
• Zygote (2N) – egg cell after fertilzation = egg + sperm– Each chromosome in egg has a matching chromosome from sperm
• Homologous Pairs – matching chromosomes
Homologous pairs
chromosomes occur in pairs one set from each parent
• Diploid or 2N • Cells that contain both sets
of chromosomes.• Haploid =1N• Sex cells have only half the
chromosome number
Just before Meiosis begins, during interphaseidentical sister chromatidsare attached at the centromere, and centrioles have been duplicated.
Centrioles
Meiosis: Prophase I
During Prophase I:
• chromosomes condense• nucleus breaks down• spindle fibers form• Homologous chromosomes form tetrads (fours)
Meiosis: Prophase I
the chromosomes condense
centrioles move to opposite poles of the parent cell and begin building the spindle microtubules.
the nuclear envelope breaks down
Meiosis: Prophase I
Homologous chromosomes pair up, forming tetrads.
Tetra =4
Meiosis: Prophase I
Homologous chromosomes pair up, forming tetrads...
Meiosis: Prophase I
…. and crossing over may occur.
Crossing over- pieces of DNA are exchanged with the matching homologous chromosome
Meiosis: Prophase I
centrioles begin building the spindle fibers.
Meiosis: Prophase I
Spindle fibers grab each chromosome at the centromere. Move the chromosomes towards the poles.
Meiosis: Metaphase I
During Metaphase I:Homologous chromosomes line up in the middle.
Equator = metaphase plate
Meiosis: Metaphase IEach pair is assortedindependently; this means that the maternal homolog in each pair randomly faces up or down.
So this pair could have assorted like this.
And this pair could have assorted like this.
Meiosis: Anaphase I
During Anaphase I:
Homologous pairs of separate.
Sister chromatids remain attached at the centromere.
Meiosis: Anaphase I
Meiosis: Anaphase I
Meiosis: Telophase I
During Telophase:
• spindle fibers break down •new nuclear membrane forms
the spindle fibers break down
Meiosis: Telophase I
Meiosis: Telophase I
new nuclear membrane forms around each group of chromosomes
Cytokinesis:
Splitting of the cytoplasm
Now these two haploid cells will begin Meiosis II; in this division, sister chromatids will separate.
Meiosis II is very much like Mitosis.
Meiosis: Prophase II
During Prophase II:
• nuclear membrane breaks
• spindle fibers reform
Meiosis: Prophase II
the nuclear membrane breaks down
Meiosis: Prophase II
centrioles move to opposite poles of the parent cell and begin building new spindle fibers
Meiosis: Prophase II
Meiosis: Prophase II
centrioles begin building the spindle fibers
Meiosis: Prophase II
The spindle fibersattach to the chromosomes at the centromere
Meiosis: Metaphase II
The spindle fibersmove the chromosomes to the center (metaphase plate) of the cell.
Meiosis: Anaphase II
Sister chromatids separate.
Meiosis: Anaphase II
Meiosis: Telophase II
During Telophase II:
• the spindle fiber break down •nuclear membrane reforms around each group of chromosomes
Meiosis: Telophase II
the spindle fibers break down
Meiosis: Telophase II
new nuclear membrane form around each group of chromosomes
Cytokinesis:
Result: Four non-identical haploid cells are formed.
Mitosis vs. Meiosis
A comparison of mitosis and meiosis
• Mitosis and Mieosis– Chromosomes replicate only once
• Mitosis– Cell divides once– Produces 2 identical daughter cells
• Meiosis– Cell divides twice– Daughter cells with half the number of chromosomes– Daughter cells are not all alike– Produces 4 cells (gametes)
Independent Assortment
Crossing Over