Transcript Meiosis and Sexual Life Cycles

Meiosis
Chapter 13
Living Things Reproduce
 Results in similarities and differences between offspring and
parents
 Facilitated by heredity or inheritance
 Passing on of characteristics from one generation to the next
 Genetics is the study
Reproduction
 Asexual produces
offspring that are exact
copies of parents
 Single parent
 Produces clones, groups
of genetically identical
individuals
 Variation from mutation
 Sexual produces offspring
with unique combinations
of genes
 Two parents
 Vary genetically
Maternal and Paternal Chromosomes
 23 different types
 Size, banding, and
centromere
 Karyotype
 Homologs
 Genes for same
characters
 Autosomes and sex
chromosomes
Identifying Cells
 n = number of chromosomes in a single set
 Cells with 2 sets are diploid (2n)
 In cells where synthesis has occurred, each appears as sister
chromatids
 Somatic cells
 Cells with 1 set are haploid (n)
 Gametes
 22 autosomes and 1 sex chromosome
 Egg is X and sperm is Y
 Assign information to cell at right
Human Life Cycle
 Egg and sperm fuse =
fertilization = zygote
 2 chromosomes sets
 Mitosis throughout life
 All cells but gametes
 Conserves chromosome
numbers
 Meiosis forms gametes
 Reduce chromosome sets by
how much?
Sexual Life Cycles
 Alternation of fertilization and meiosis is common
 Timing varies between species
Alternation of generations
Cellular Division
 Mitosis
 n or 2n are capable
 Depends on life cycle type
 Meiosis
 2n only
 Chromosomes can’t be further reduced
Meiosis (Overview)
 Similar to mitosis
 2 cell divisions
 Produces 4 daughter cells with ½
number of chromosomes
 Only time homologs associated
with each other
 Each with different alleles or
different versions of genes at same
loci, or location
• Homologs pair
• Synapsis and
crossing over
of nonsister
chromatids
• Resembles mitosis
• Sister chromatids split
Comparing Cell Division
Mitosis
Meiosis
 Conserves number of
 Reduce chromosome set
chromosomes
 Daughter cells genetically
identical
from diploid to haploid
 Daughter cells genetically
different
 Meiosis I
 Synapsis and crossing over
 Homologs on metaphse
plate
 Separation of homologs
Genetic Variation
 Mutation is ultimate source of genetic diversity
 Creates alleles, which are what?
 Sexual reproduction reshuffles alleles = unique combos
 3 mechanisms contribute
 Random fertilization
 Fusion of any 2 gametes with previous variation = zygote with more
 (223)2 = 70 trillion
 Independent assortment
 Random orientation of homologs
 223 = 8.4 million
 Crossing over
 Produces recombinant chromosomes, chromosomes with genes from 2
different parents
 1-3 times per chromosome is normal
Genetic Variation Mechanisms
Independent Assortment
Crossing Over