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AP BiologyChapter 10
Meiosis
Key Concepts:
Sexual reproduction entails meiosis, gamete formation, and fertilization
Germ cells of male and female animals produce gametes
Diploid cells have two of each type of chromosome, one from each parent
Meiosis divides the chromosome number in half
Key Concepts:
Gametes are haploid (n)
The union of gametes at fertilization results in a diploid number (2n)
During meiosis, chromosomes may exchange random segments by crossing over
Meiosis leads to variations in traits among offspring
Comparison of Asexual and Sexual Reproduction
Asexual
Single parent offspring inherit same genes as parent (number and type)
Sexual Produces genetic variation among offspring
Involves
Meiosis
Gamete formation
Fertilization
Chromosomes come in pairs, like
socks
Onehomolog
Homologouschromosomes(unreplicated)
Homologous Chromosomes
Cell has two of each chromosome
One chromosome in each pair from ♀ (mom) and one from ♂ (dad)
Paternal and maternal chromosomes carry the same genes but different alleles.
Sexual Reproduction Shuffles Alleles Through sexual reproduction,
offspring inherit new combinations of alleles, which leads to variations in traits.
This variation is traits is the basis for evolutionary change
Gamete Formation Gametes are sex cells (sperm, eggs) Arise from germ cells
anther ovary
Germ cells are diploid (2n) Gametes are haploid (n) Meiosis halves chromosome
number
Chromosome Number
Sum total of chromosomes in a
cell
Germ cells are diploid (2n)
Gametes are haploid (n)
Meiosis halves chromosome
numberHuman Karyotype
Meiosis: Two Divisions
Two consecutive nuclear
divisions Meiosis I
Meiosis II
DNA is not duplicated
between divisions
Four haploid nuclei form
A Chromosome Refresher…
Germ Cell at Interphase
A diploid cell (2n)
DNA duplicated
Ready for first division of meiosis
Meiosis I
Telophase IAnaphase IMetaphase I
spindle equator
one pair of homologous chromosomes
newly forming microtubules
Prophase I
Meiosis I Stepped Art
Meiosis I - Overview
Meiosis I
Prophase I Homologs pair
Crossing over occurs
Spindle forms
Nuclear envelope disappears
Metaphase I Pairs of homologous chromosomes are
at spindle equator
Random arrangement
Crossing over
Crossing Over
Effect of Crossing Over After crossing over, each
chromosome contains both maternal and paternal segments
Creates new allele combinations in offspring
Either the maternal of paternal member of a homologous pair can end up at either pole
The chromosome in a gamete are a mix of chromosomes from the two parents
Meiosis I – continued… Anaphase I
Duplicated chromosomes
pulled apart from homologue
toward opposite spindle poles
Sister chromatids remain attached
Telophase I Chromosomes arrive at poles
Usually followed by cytokinesis
Cells divides into two cells
Meiosis II - overview NO DNA duplication between I and II Sister chromatids are separated
Result is four haploid daughter cells
Meiosis II Prophase II
New spindle formed
Chromosomes move towards equator
Metaphase II Duplicated
chromosomes positioned at the equator
Anaphase II Sister chromatids are
moved apart
Telophase II Four nuclei with
haploid chromosome number (n) resulting in four cells
1 2 3
combinations possible
or
or
or
Stepped Art
Possible outcomes of the random alignment of only three pairs of chromosomes at metaphase I (without crossing over!!)
As a result of random alignment, the number of possible combinations in a gamete is 2n, where n=# of chromosome types
Random alignment
Results of Recombination
Oogenesis
Spermatogenesis
Fertilization
Male and female gametes unite and
nuclei fuse
Fusion of two haploid nuclei
produces diploid nucleus in the
zygote
Which two gametes unite is random Adds to variation among offspring
In Conclusion
Life cycle of sexually reproduction includes meiosis, gamete formation, and fertilization
A germ cell (2n) has two of each type of chromosome inherited from each parent. Each pair is homologous.
Chromosomes are duplicated in interphase.
Meiosis consists of two divisions
In Conclusion In meiosis I, each chromosome
moves away from homologous partner. In meiosis II, sister chromatids are separated
Meiosis produces haploid cells Crossing over, random allocation of
chromosomes, and the chance of gametes meeting at fertilization contribute to variation in traits among offspring