2007-2008 Meiosis & Sexual Reproduction Cell division/Asexual reproduction Mitosis ▫produce cells...
Transcript of 2007-2008 Meiosis & Sexual Reproduction Cell division/Asexual reproduction Mitosis ▫produce cells...
2007-2008
Meiosis & Sexual Reproduction
Cell division/Asexual reproduction•Mitosis
▫produce cells with same information identical daughter cells
▫exact copies clones
▫same amount of DNA same number of chromosomes same genetic information
Aaaargh!I’m seeingdouble!
Asexual reproduction•Single-celled eukaryotes
▫yeast (fungi)▫Protists
Paramecium Amoeba
•Simple multicellular eukaryotes▫Hydra
What are thedisadvantages ofasexual reproduction?
What are the advantages?
budding
budding
How about the rest of us?•What if a complex multicellular organism
(like us) wants to reproduce?▫joining of egg + sperm
•Do we make egg & sperm by mitosis?
46 46+ 92
egg sperm zygote
What if we did, then….
Doesn’t work!
No!
•Genetics▫Is the scientific study of heredity and variation▫Studies the passing of chromosomes from
parents to offspring•Meiosis creates gametes, sex cells containing
half the species chromosome number •Meiosis and Fertilization
maintain a species chromosome number during a sexual life cycle
Increase variation of individuals within a species
•Heredity▫Is the transmission of traits from one generation
to the next•Variation
▫Shows that offspring differ somewhat in appearance from parents and siblings Two parents give rise to offspring that have unique
combinations of genes inherited from the two parents
Figure 13.1
•We inherit▫One set of chromosomes from our mother
and one set from our father
Specific location of a gene
• gametes transmit genes from on generation to the next
• one chromosome contains hundreds to a few thousand genes
• each gene has a specific sequence of nucleotides
•Genes▫Are the units of heredity▫Are segments of DNA▫Contain specific sequence of nucleotides
•Genes program cells to synthesize specific enzymes and other proteins which create an organism’s inherited traits
Sets of Chromosomes in Human Cells• In humans
▫Each somatic cell has 46 chromosomes, made up of two sets
▫One set of chromosomes comes from each parent
• All 46 are visible during mitosis
• The two chromosomes
composing a pair of each type are called homologous chromosomes
Human female karyotype
46 chromosomes23 pairs
22 pairs of autosomes = chromosomes that do not control one’s sex
Sexchromosomes
Human male karyotype
46 chromosomes23 pairs
homologouschromosomes homologous = “same information”
single-stranded
•Homologous chromosomes▫Are the two chromosomes composing a pair▫Have the same characteristics or gene loci
Therefore, every cell has two copies of every gene
Allele: version of a gene Ex: Hitchhikers thumb, Tongue rolling
All cells have two alleles for every gene – one maternal and one paternal
homologouschromosomes
sister chromatidssingle-stranded double-stranded
homologouschromosomes
Figure 13.5
Key
Haploid (n)
Diploid (2n)
Haploid gametes (n = 23)
Ovum (n)
SpermCell (n)
MEIOSIS FERTILIZATION
Ovary Testis Diploidzygote(2n = 46)
Mitosis anddevelopment
Multicellular diploidadults (2n = 46)
•The Human Life Cycle
• At sexual maturity▫ The ovaries and testes
produce haploid gametes by meiosis
• During fertilization▫ These gametes, sperm and
ovum, fuse, forming a diploid zygote
▫ This starts the human life cycle
• The zygote (fertilized egg)▫ Develops into an adult
organism▫ Generates all of the somatic
cells of the organism
haploid
diploid
23
Meiosis reduces the number of chromosome sets from diploid to haploid▫Takes place in two sets of divisions, meiosis I
and meiosis II▫Results in 4 daughter cells with half the
chromosome number of the parent
2346
egg
sperm
46
meiosis 46
fertilization
23
23gametes
zygote
Overview of Meiosis
•Meiosis I▫Reduces the
number of chromosomes from one diploid cell to two haploid cells
•Meiosis II▫Two haploid
daughter cells become four haploid daughter cellshttp://highered.mcgraw-hill.com/sites/983409233
9/student_view0/chapter11/how_meiosis_works.html
Figure 13.7
Interphase
Homologous pairof chromosomesin diploid parent cell
Chromosomesreplicate
Homologous pair of replicated chromosomes
Sisterchromatids Diploid cell with
replicatedchromosomes
1
2
Homologous chromosomes separate
Haploid cells withreplicated chromosomes
Sister chromatids separate
Haploid cells with unreplicated chromosomes
Meiosis I
Meiosis II
DNA replication
homologous chromosomes separate
Sister chromatids separate
Overview of meiosis
interphase 1 prophase 1 metaphase 1 anaphase 1
telophase 1
prophase 2 metaphase 2 anaphase 2 telophase 2
2n = 4
n = 2n = 2
n = 2
A Comparison of Mitosis and Meiosis
•Meiosis and mitosis can be distinguished from mitosis▫By three events in Meiosis l that are unique
to meiosis
http://highered.mcgraw-hill.com/sites/9834092339/student_view0/chapter11/unique_features_of_meiosis.html
•Synapsis and crossing over▫Crossing Over: Homologous chromosomes physically
connect and exchange genetic information (sections of DNA) Synapsis: nonsister chromatids connection preceding
crossing over▫Occurs at the end of Prophase I
Nonsister chromatids remain connected via chiasmata Chiasmata: location on nonsister chromatids where DNA
exchange occurred
synapsis
•Tetrads on the metaphase plate▫At metaphase I of meiosis, paired
homologous chromosomes (tetrads) are positioned on the metaphase plates
tetrad
synapsis
•Separation of homologues▫At anaphase I of meiosis, cohesins between
nonsister chromatids are cleaved and homologous pairs move toward opposite poles of the cell Sister chromatids remain attached
▫In anaphase II of meiosis, cohesins between sister chromatids are cleaved and the sister chromatids separate
http://highered.mcgraw-hill.com/sites/9834092339/student_view0/chapter11/the_function_of_cohesin.html
Figure 13.9
MITOSIS MEIOSIS
Prophase
Duplicated chromosome(two sister chromatids)
Chromosomereplication
Chromosomereplication
Parent cell(before chromosome replication)
Chiasma (site ofcrossing over)
MEIOSIS I
Prophase I
Tetrad formed bysynapsis of homologouschromosomes
Metaphase
Chromosomespositioned at themetaphase plate
Tetradspositioned at themetaphase plate
Metaphase I
Anaphase ITelophase I
Haploidn = 3
MEIOSIS II
Daughtercells of
meiosis I
Homologuesseparateduringanaphase I;sisterchromatidsremain together
Daughter cells of meiosis II
n n n n
Sister chromatids separate during anaphase II
AnaphaseTelophase
Sister chromatidsseparate duringanaphase
2n 2nDaughter cells
of mitosis
2n = 6
•A comparison of mitosis and meiosis
http://highered.mcgraw-hill.com/sites/9834092339/student_view0/chapter11/comparison_of_meiosis_and_mitosis.html
Origins of Genetic Variation Among Offspring•Three mechanisms that contribute to
genetic variation from sexual reproduction:
1.Independent assortment of chromosomes2.Crossing over3.Random Fertilization
Independent Assortment of Chromosomes
•Homologous pairs of chromosomes▫Orient randomly at metaphase I of meiosis▫Maternal and paternal sister chromatids can be closer to either pole▫This results in 2n possibilities
In humans, this means there are 223 or 8.4 million possible combinations of maternal and paternal chromosomes
metaphase1
• Independent assortment
Figure 13.10
Key
Maternal set ofchromosomesPaternal set ofchromosomes
Possibility 1
Two equally probable arrangements ofchromosomes at
metaphase I
Possibility 2
Metaphase II
Daughtercells
Combination 1 Combination 2 Combination 3 Combination 4
• Homologous pair DNA is not exclusively maternal or paternal though; due to crossing over
• Crossing over▫ Produces recombinant chromosomes that carry genes
derived from two different parents
1. Prophase I: synapsis and crossing over occur, homologs separate slightly
2. Chiasmata and cohesin between nonsister chromatids hold homologs together; they move to the metaphase plate
3. Break down of proteins holding between nonsister chromatid arms together allow homologs with recombinant chromosomes to separate
prophase 1
Random Fertilization•The fusion of gametes
▫Will produce a zygote with any of about 64 trillion diploid combinations
▫This does not even account for variations produced by crossing over
Evolutionary Significance of Genetic Variation Within Populations• Genetic variation
▫Is the raw material for evolution by natural selection
• Darwin: a population evolves through the differential reproductive success of its variant members▫ Individuals best suited to their environment
reproduce more – leaving offspring behind to continue transmitting beneficial genes
•Mutations▫Are the original source of genetic variation