Download AP Bio Ch 10

Chapter 10
Meiosis and Sexual Life Cycles
heredity
- continuity of biological traits from 1 generation to the next
- results from the transmission of genes from parents to offspring
- offspring more closely resemble parents and relatives than other members of
the same species
variation
- inherited differences among individuals of the same species
genetics
- study of heredity and variation
Offspring acquire genes from parents by inheriting chromosomes.
DNA - a nucleic acid
- polymer of 4 different kinds of nucleotides
genes - units of hereditary information
- made of DNA; located on chromosomes
- have specific sequences of nucleotides
- most program cells to make specific proteins
traits
- result from the actions of proteins coded for by genes
inheritance is possible because:
DNA is replicated producing copies of genes to be passed to offspring
sperm & ova carrying each parent’s genes combine
chromosome - unit of hereditary material in nucleus of eukaryotic organisms
- consists of a single long DNA molecule with proteins
- DNA (double helix) is highly folded and coiled
- contains genetic information arranged in a linear sequence
- each contains 100s to 1000s of genes
- each species has a characteristic number of chromosomes
locus - specific location of a chromosome that contains a gene
Like begets like, more or less: a comparison of asexual versus sexual reproduction.
Asexual Reproduction
1 parent
all parent’s genes passed to offspring
offspring genetically identical to parent
results in a clone (genetically identical
individual)
rarely, genetic difference occur due to
mutations (change in DNA)
Sexual Reproduction
2 parents
each parent passes half its genes to
offspring
offspring have unique combination of
genes inherited from both parents
results in greater genetic variation;
offspring vary genetically from siblings
and parents
Role of Meiosis in Sexual Life Cycles
Fertilization and meiosis alternate in sexual life cycles.
life cycle
- sequence of stages in an organism’s reproductive history, from
conception to production of its own offspring
human life cycle
somatic cell
- meiosis & fertilization result in alternation between haploid and
diploid condition
- same basic pattern in all sexually reproducing organisms
- any cell other than a sperm or egg cell
- contain 46 chromosomes in humans
chromosomes - differ in size, position of centromere, and staining/banding pattern
- can be matched into homologous pairs to produce a karyotype
autosomes
sex chromosome
karyotype
- chromosomes that aren’t sex chromosomes (human cells = 22 pairs)
- determine individual’s sex (human cells = 1 pair)
- a display or pictomicrograph of an individual’s somatic cell
chromosomes arranged in a standard sequence
- can be used to screen for chromosomal abnormalities
homologous chromosomes
- homologues
- pair of chromosomes with the same size, centromere
position, and banding pattern
- they have same genetic loci (except 1 pair)
homologous autosomes
sex chromosomes
- carry same genetic loci on each
- carry different loci
- X chromosome and Y chromosome
females
males
- cells carry homologous pair of X chromosomes
- cells carry one X and one Y chromosome
chromosomal pairs in human karyotype:
1 homologue is inherited from each parent
the 46 somatic cell chromosomes are actually 2 sets of 23 (1 set from mom, 1 from dad)
diploid - cells contain 2 sets of chromosomes (2n)
- chromosome number in somatic cells
haploid- cells contain 1 set of chromosomes (n)
- chromosome number in gametes
gamete - haploid reproductive cell
- sperm cells and ova
- human gametes have 22 autosomes and 1 sex chromosome (X or Y)
fertilization
- union of 2 gametes to form a zygote
- diploid number is restored
zygote - diploid cell that results from the union of 2 haploid gametes
- contains maternal and paternal haploid sets of chromosomes
- as humans develop from the zygote, the genetic information is passed to all
somatic cells by mitosis
gametes
- only cells in body not produced by mitosis
- produced in ovaries and testes by meiosis
meiosis
- special type of cell division
- produces haploid cells
- produces sperm cells and ova in humans
3 basic patterns of sexual life cycles:
animal - gametes are only haploid cells
- meiosis results in gamete production
- gametes don’t divide further before fertilization
- fertilization produces a diploid zygote that divides by mitosis to produce
a diploid multicellular animal
fungi and some protists
- zygote is only diploid stage
- meiosis occurs immediately after zygote forms
- resulting haploid cells divide by mitosis to produce
a haploid multicellular organism
- gametes produced by mitosis from already haploid cells
plants and some algae - alternate between multicellular haploid and diploid generations
- called alternation of generations
- sporophyte - multicellular diploid stage
- spore producing plant
- meiosis produces haploid cells called spores
- gametophyte - multicellular haploid stage
- gamete producing plant
- produced by division of haploid spores by mitosis
- produce gametes by mitosis
- fertilization produces a diploid zygote which
develops into next sporophyte generation
Meiosis reduces chromosome number from diploid to haploid: a closer look
meiosis
- contributes to genetic variation among offspring
- steps resemble steps in mitosis
some differences between mitosis and meiosis:
MITOSIS
preceded by replication of chromosomes
chromosome replication followed by 1 cell
divisions in mitosis
2 daughter cells produced
daughter cells have same number of
chromosomes as parent cell
2 daughter cells genetically identical to
parent cell and each other
1 nuclear division
MEIOSIS
preceded by replication of chromosomes
chromosome replication followed by 2 cell
divisions in meiosis
4 daughter cells produced
daughter cells have half the number of
chromosomes as parent cell
creates genetic variation; 4 daughter cells
genetically different from parent cell and
each other
2 successive nuclear divisions
stages of meiosis:
Interphase I - precedes meiosis
- chromosomes replicate (like in mitosis)
- each duplicated chromosomes consists of 2 identical sister chromatids
attached at centromeres
- in animals, centrioles pairs also replicate
Meiosis I
- segregates the 2 chromosomes of each homologous pair
- reduces chromosome number by half
- includes 4 phases:
Prophase I
- 90% of time needed for meiosis
- chromosomes condense
- synapsis occurs  homologous chromosomes pair up
forming a tetrad (complex of 4 chromatids)
- chromatids of homologous chromosomes criss-cross at
numerous places (called chiasmata)
- centrioles pairs move apart; spindle microtubules form
between them
- nuclear envelope and nucleoli disperse
- chromosomes begin moving to metaphase plate
Metaphase I - tetrads aligned on metaphase plate
- each homologue attached to kinetochore microtubules
Anaphase I - homologues separate and moved toward opposite poles by
spindle
- sister chromatids remain attached at centromeres; move
together toward same pole
Telophase I / Cytokinesis
- chromosomes reach poles
- each pole has a haploid set of chromosomes
- usually cytokinesis occurs with Telophase I
- 2 haploid daughter cells formed (cleavage furrow in
animals, cell plate in plants)
- some cells enter interkinesis (nuclear membranes and
nucleoli reappear)
- other cells immediately prepare for meiosis II
Meiosis II
- separates sister chromatids of each chromosome
- includes 4 phases:
Prophase II - nuclear membrane & nucleoli disperse (if cell entered
interkinesis)
- spindle apparatus forms
- chromosomes move toward metaphase II plate
Metaphase II - chromosomes align singly on metaphase plate
- kinetochores of sister chromatids point towards opposite
poles
Anaphase II - centromeres of sister chromatids separate
- sister chromatids of each pair move towards opposite poles
Telophase II / Cytokinesis - nuclei form at opposite poles
- cytokinesis produces 4 haploid daughter cells
comparison of meiosis I and mitosis:
MEIOSIS I
Prophase
Metaphase
Anaphase
synapsis forms tetrads;
chiasmata appear (results in
crossing over)
homologous pairs align on
metaphase plate
chromosome pairs separated
centromeres don’t divide;
sister chromatids stay
together; sister chromatids
move to same pole
MITOSIS
no synapsis;
no crossing over
individual chromosomes
align on metaphase plate
sister chromatids separated
centromere divide; sister
chromatids move to opposite
poles
Sexual life cycles produce genetic variation among offspring.
meiosis and fertilization
- primary sources of genetic variation among sexually reproducing
organisms
sources of genetic variation in offspring due to sexual reproduction:
independent assortment
crossing over (prophase I)
random fertilization
independent assortment of chromosomes
- random distribution of maternal and paternal
homologues to the gametes
- there is a 50-50 chance a daughter cell will receive the
maternal homologue or the paternal homologue after
meiosis I (orientation of the pair is random)
- each homologous pair assorts independently from all
the others (2n possible combinations of maternal and
paternal chromosomes in gametes) (n = haploid number)
- in humans, there are 223 possible combinations (about
8 million)  each human gamete contains 1 of
8 million possible assortments of chromosomes
inherited from the parents
crossing over - exchange of genetic material (genes) between homologous chromosomes
- occurs during Prophase I (synapsis)
- homologous portions of 2 nonsister chromatids trade places
- X-shaped chiasmata appear at places where this exchange occurs
- results in chromosomes with genes from both parents (recombinant chromosomes)
- an average of 2 – 3 crossovers occurs per chromosome pair in humans
random fertilization - in humans:
each ovum has 1 in 8 million possible chromosome combinations
each sperm has 1 in 8 million possible chromosome combinations
so each resulting zygote has 1 in 64 trillion possible combinations of maternal and
paternal chromosomes (without even considering crossing over)
Evolutionary adaptation depends on a population’s genetic variation.
heritable variation
- basis for Darwin’s theory that natural selection is the mechanism for
evolutionary change
natural selection
- increases frequency of heritable variations that favor the reproductive
success of some individuals over other
- results in adaptation (accumulation of heritable variations that are
favored by the environment)
- genetic variation increases the likelihood that some individuals in a
population will have heritable variations that help them cope with
environmental changes
2 sources of genetic variation:
sexual reproduction (independent assortment, crossing over, random fertilization)
mutation
- random and relatively rare structural changes in a gene made during DNA
replication