meiosis ap biology. hereditary similarity and variation heredity is the transmission of traits from...
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Meiosis
AP Biology
Hereditary Similarity and Variation
• Heredity is the transmission of traits from one generation to the next
• Variation shows that offspring differ in appearance from parents and siblings
• Genetics is the scientific study of heredity and variation
Genes
• Genes are the units of heredity• Genes are segments of DNA• Each gene has a specific locus on a certain
chromosome• One set of chromosomes is inherited from
each parent
Sexual/Asexual
• Sexual reproduction- two parents give rise to offspring that have unique combinations of genes inherited from the two parents
• Asexual reproduction- one parent produces genetically identical offspring by mitosis
Karyotype
• karyotype -ordered display of the pairs of chromosomes from a cell
• homologous chromosomes (homologues)• Both chromosomes in a pair. They carry
genes controlling the same inherited characteristics– Alleles-
LE 13-3
5 µmPair of homologouschromosomes
Sisterchromatids
Centromere
LE 13-4
Key
Maternal set ofchromosomes (n = 3)
2n = 6Paternal set ofchromosomes (n = 3)
Two sister chromatidsof one replicatedchromosomes
Two nonsister chromatids in a homologous pair
Pair of homologouschromosomes(one from each set)
Centromere
“Hey, that’s my chromosome!”
Homologous Chromosomes
• Each pair of homologous chromosomes includes one chromosome from each parent
• For humans, the diploid number is 46 (2n = 46) 23 Pairs.
• Gametes are haploid cells, containing only one set of chromosomes
Meiosis: twice as nice
Meiosis I
• Synapsis
• Crossing over
• Tetrad formation
Meiosis II
same as mitosissister chromatids split
4 unique cells result
LE 13-7
Homologous pairof chromosomesin diploid parent cell
Interphase
Homologous pair of replicated chromosomes
Chromosomesreplicate
Meiosis I
Diploid cell withreplicatedchromosomes
Sisterchromatids
Meiosis II
Homologouschromosomesseparate
Sister chromatidsseparate
Haploid cells withreplicated chromosomes
Haploid cells with unreplicated chromosomes
LE 13-8ab
Sisterchromatids
Chiasmata
Spindle
Centromere(with kinetochore)
Metaphaseplate
Homologouschromosomesseparate
Sister chromatidsremain attached
Microtubuleattached tokinetochore
Tetrad
MEIOSIS I: Separates homologous chromosomes
PROPHASE I METAPHASE I ANAPHASE I
Homologous chromosomes (red and blue) pair andexchange segments; 2n = 6in this example
Pairs of homologouschromosomes split up
Tetrads line up
• Beginning of telophase I- each half of the cell has a haploid set of chromosomes– each chromosome still consists of two sister
chromatids
• Cytokinesis usually occurs simultaneously, forming two haploid daughter cells
LE 13-8b
Cleavagefurrow
MEIOSIS II: Separates sister chromatids
PROPHASE II METAPHASE II ANAPHASE IITELOPHASE I ANDCYTOKINESIS
TELOPHASE II ANDCYTOKINESIS
Sister chromatidsseparate
Haploid daughter cellsforming
Two haploid cellsform; chromosomesare still double
During another round of cell division, the sister chromatids finally separate;four haploid daughter cells result, containing single chromosomes
Crossing Over
• Because of crossing over in Prophase I, the two sister chromatids of each chromosome are no longer genetically identical
• Anaphase I: Homologous chromosomes separate. sister chromatids stay together
• Anaphase II: sister chromatids separate• The sister chromatids of each chromosome
are now two newly individual chromosomes.
Four for the Price of One
• At the end of meiosis, there are four daughter cells, each with a haploid set of unreplicated chromosomes
• Each daughter cell is genetically distinct from the others and from the parent cell
LE 13-9
Propase
Duplicated chromosome(two sister chromatids)
Chromosomereplication
2n = 6
Parent cell(before chromosome replication)
Chromosomereplication
MITOSIS MEIOSIS
Chiasma (site ofcrossing over) MEIOSIS I
Prophase I
Tetrad formed bysynapsis of homologouschromosomes
Tetradspositioned at themetaphase plate
Metaphase IChromosomes positioned at themetaphase plate
Metaphase
AnaphaseTelophase
Homologuesseparateduringanaphase I;sisterchromatidsremain together
Sister chromatidsseparate duringanaphase
Daughtercells of
meiosis I
Haploidn = 3
Anaphase ITelophase I
MEIOSIS IIDaughter cells
of mitosis
2n2n
n
Sister chromatids separate during anaphase II
n n nDaughter cells of meiosis II
Genetic variation contributes to Evolution
• Mutations create different versions of genes• Reshuffling of different versions of genes
during sexual reproduction produces genetic variation
• Three mechanisms for variation in Sexual:– Independent assortment of chromosomes– Crossing over (prophase I)– Random fertilization
Independent Assortment
• Independent assortment-each pair of chromosomes sorts maternal and paternal homologues into daughter cells independently of the other pairs (not all dad’s chroms to 1 daughter! And mom’s to the other)
• For humans (n = 23), there are more than 8 million (223) possible combinations of chromosomes
Crossing over to the other side
• Crossing over produces recombinant chromosomes, which combine genes inherited from each parent
• In crossing over, homologous portions of two nonsister chromatids trade places
• Prophase I
LE 13-11
Prophase Iof meiosis
Tetrad
Nonsisterchromatids
Chiasma,site of crossingover
Recombinantchromosomes
Metaphase I
Metaphase II
Daughtercells
• Random fertilization adds to genetic variation because any sperm can fuse with any ovum
• The fusion of gametes produces a zygote with any of about 64 trillion diploid combinations
• Crossing over adds even more variation
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