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AP BiologyChapter 13Meiosis and Sexual Life
Cycles
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Introduction to Heredity

• Heredity Inheritance transmission of traits
  from one generation to the next
• Variation offspring differ in appearance from
  siblings and parents
• Genetics Study of heredity
• Genes coded information in hereditary units,
  given to offspring by parents
• Segments of DNA
• Make up the genome
• Have a locus or specific location within the
  genome or chromosome

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Reproduction

• Asexual like begets like. Single parent
  passes all its genes to the offspring
• clone
• Mitosis
• Only differences care caused by mutations
• Single-celled eukaryotes
• Sexual Two parents pass a unique combination of
  genes to the offspring
• Results in greater variation
• Meiosis and Fertilization
• Multicellular organisms

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Life Cycle

• Generation-to-generation sequence of stages in
  the reproductive history of an organism from
  conception to production of its own offspring

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Human Life Cycle

• Somatic Cells or body cells have 46 chromosomes
• Diploid have two sets of chromosomes (2n)
• Sex Chromosomes X and Y determine sex of the
  individual
• Autosomes all other 44 chromosomes
• Gametes or sex cells have 23 chromosomes
• Haploid have a single set of chromosomes (n)
• Fertilization Syngamy joining of gametes
• Zygote fertilized egg? restores 2n chromosome
• Meiosis production of gametes, halves the
  chromosome number.

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Karyotypes

• Photomicrograph of chromosomes from a cell
• Chromosomes are paired by length, centromere
  position, and staining pattern into the
  homologous pairs
• Used to determine sex or identify abnormal
  disorders

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Variety of Sexual Life Cycles

• All sexually reproducing organisms alternate
  between meiosis and fertilization however the
  timing of the cycles can differ
• 3 major types

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Human Life Cycle

• Used by most animals
• Gametes are the only haploid cells
• Meiosis produces gametes
• Fertilization produces a diploid zygote

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Fungi, Protists, Algae Life Cycle

• Gametes fuse to form a diploid zygote
• Meiosis occurs before development
• Organism is haploid
• ONLY diploid stage is zygote

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Plant Life Cycle Alternation of Generations

• Includes both diploid and haploid multicellular
  stages
• Sporophyte Diploid stage. Meiosis produces
  haploid cells called spores
• Spores (unlike gametes) produce multicellular
  organisms without another cell.
• Gametophyte Multicellular haploid organism that
  comes from a spore. Makes gametes by mitosis
• Fertilization results in diploid zygotes

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Meiosis

• Chromosomes replicate once
• Diploid cell divides twice Meiosis I and Meiosis
  II
• Divisions result in 4 daughter cells with half
  the chromosome of the parent, each genetically
  different from each other and the parent

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Interphase

• Chromosomes replicate
• Each chromosome is made of identical sister
  chromatids attached at the centromere
• Centrosomes replicate

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Phases of Meiosis IProphase I

• Chromosomes condense
• Homologous chromosomes (each made of 2 sister
  chromatids) pair up to form tetrads in a process
  called synapsis.
• Legs of chromatids can crisscross forming
  chiasmata. This can lead to crossing over or an
  exchange of pieces of DNA between chromosomes
• Cell prepares for division
• Nuclear membrane Nucleoli disappear
• Centrosomes move to poles
• Spindle forms

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Phases of Meiosis IMetaphase I

• Tetrads line up on metaphase plate
• Kinetochore fibers attach to each of the
  homologous chromosomes

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Phases of Meiosis IAnaphase I

• Spindle fibers guide the separation of the
  homologous chromosomes and pulls them towards the
  poles
• Sister chromatids stay attached

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Phases of Meiosis ITelophase I Cytokinesis

• Each homologous pair moves apart until they reach
  the poles
• Each pole now has a haploid set of chromosomes
• Cytokinesis forms a cleavage furrow and separates
  the daughter cells
• Nuclear membrane can reappear and chromosomes
  decondense
• NO further replication of DNA before second
  division

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Phases of Meiosis II

• During a second round of cell division, sister
  chromatids separate
• Forms 4 haploid, genetically different daughter
  cells

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Comparing Mitosis Meiosis
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Comparing Mitosis Meiosis

• 3 key events is meiosis
• Synapsis formation of tetrads and chiasmata and
  crossing over in Prophase I
• Homologous pairs rather than individual
  chromosomes line up at Metaphase I
• Homologous pairs rather than sister chromatids
  separate at Anaphase I

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How do we account for genetic variation?

• Independent Assortment of Chromosomes
  positioning of homologous chromosomes at
  metaphase I is a matter of chance and
  determines which chromosomes go into each
  haploid daughter cell

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How do we account for genetic variation? Cont.

• Crossing Over produces recombinant chromosomes
  which combine genes from parents.
• Homologous portions of non-sister chromatids
  trade places
• On average, two or three crossover events occur
  per chromosome pair

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How do we account for genetic variation? Cont.

• Random Fertilization
• A human ovum (egg) representing one of
  approximately 8 million possible chromosome
  combos is fertilized by a single sperm
  representing one of approximately 8 million
  different possible chromosome combos without
  crossing over or mutations
• Chance of any 2 parents producing a specific
  zygote is 1 in 64 trillion!