Download Chapter 13 - Warren County Schools

13.1
Offspring acquire genes from parents by
inheriting chromosomes
Genes
Segments of DNA that code for heredity
Transmitted from generation to generation
Gametes are the reproductive cells that do this
A locus (plural loci) is the location
of a gene on a chromosome
Asexual
A single parent is the sole parent
Passes copies of all its genes to its offspring
New offspring arise by MITOSIS & are virtually
exact copies of the parent’s genome
Offspring is called a clone (genetically identical)
Sexual Reproduction
2 parents contribute genes
Results in greater genetic variation than asexual
13.2
Fertilization and meiosis alternate in sexual life
cycles
Life Cycle
Generation-to-generation sequence of stages in
the reproductive history of an organism
From conception to production of its own offspring
Somatic Cells
Any cells that are NOT gametes
Each somatic cell in humans has 46
chromosomes
Karyotype
A picture of its complete set of chromosomes
Arranged in pairs of homologous chromosomes
from the largest pair to the smallest pair
The 46 chromosomes will be paired to make 23
homologous chromosomes
Homologous
Chromosomes
Both chromosomes of each pair carry genes
that control the same inherited characteristic
Similar in length & centromere position
Have same staining pattern
One from each parent
Sex Chromosomes vs.
Autosomes
Sex chromosomes are exceptions to the
homologous pair rule
In humans, X & Y
Females = XX
Males = XY
Nonsex chromosomes are called Autosomes
Gametes
Sperm & Ova (eggs) are HAPLOID cells
Contain ½ the number of chromosomes of
somatic cells
Humans  gametes contain 22 autosomes plus
a single sex chromosome
A haploid of 23 – symbolized by n
Meiosis & Fertilization are the key events in
sexually reproducing life cycles
Fertilization = combination of sperm + egg
One haploid from each parent fuse to form a
ZYGOTE
The zygote is DIPLOID & represented by 2n
Meiosis
Type of cell division that reduces the number of
sets of chromosomes from 2 to 1
Fertilization restores the diploid number as the
gametes are combined
F & M alternate in the life cycles of sexually
reproducing organisms
• Plants and some algae exhibit an alternation of
generations
• This life cycle includes both a diploid and haploid
multicellular stage
• The diploid organism, called the sporophyte,
makes haploid spores by meiosis
• Each spore grows by mitosis into a haploid
organism called a gametophyte
• A gametophyte makes haploid gametes by
mitosis
• Fertilization of gametes results in a diploid
sporophyte
13.3
Meiosis reduces diploid to haploid
Looks similar to mitosis:
Both are preceded by the replication of DNA
Difference:
In meiosis it is followed by 2 stages of cell division
Meiosis I & II
Final result = 4 daughter cells (1/2 the chromosomes as
original cell)
Meiosis
Interphase
Chromosomes make a copy of themselves
Replicates DNA (doubles amount of DNA in cell)
Centrosome divides
Meiosis I
Prophase I (critical to understand meiosis)
Chromosome condense – results 2 sister
chromatids attached at centromere
Synapsis occurs
Joining of homologous chromosomes along their
length
Called a tetrad & aligns the HC gene by gene
Prophase I cont.
Crossing over
DNA from one homologue is cut & exchanged with
an exact portion of DNA from the other homologue
A small part of DNA from one parent is exchanged
with the DNA from another
INCREASES GENETIC VARIABILITY
Happens 2-3 times per pair – forms criss-crossed
regions termed CHIASMATA
Prophase I cont.
After crossing over
Centrioles move away from each other
Nuclear envelope disappears
Spindle microtubules attach to the kinetochores
forming on the chromosomes that begin to move to
the metaphase plate
Organize thoughts
1) Synapsis occurs, forming tetrads
2) Tetrads undergo crossing over
3) Crossing over increases genetic variation
4) Areas of crossing over form chiasmata
5) Nuclear envelope disappears, allowing
spindle to attach
Metaphase I
Homologous pairs are lined up on the metaphase
plate
Microtubules from each pole attach to each
member of the pairs
Preparing to pull to opposite poles
Anaphase I
Spindle apparatus helps to move chromosomes to
opposite poles
Sister chromatids stay connected & move together
Telophase I
HC move until they reach opposite poles
Each pole contains a haploid set of chromosomes
Still consisting of 2 sister chromatids
Cytokinesis
Division of cytoplasm & occurs during telophase
A cleavage furrow in animals & cell plates in plants
Will form 2 haploid cells
Meiosis II
2 cells move together
Prophase II
Spindle apparatus forms
Sister chromatids move toward metaphase plate
Metaphase II
Chromosomes are lined up on the plate
Kinetochores of each sister chromatid prepare to
move to opposite poles
Anaphase II
Centromere of the sister chromatids separate
Individual chromosomes move to opposite ends
Telophase II & Cytokinesis
Nuclei reappear & cytokinesis occurs
4 daughter cells have a haploid of the original parent
cell
All 4 are genetically different from one another
Mitosis
vs
Meiosis
The following occur in Meiosis, NOT in mitosis
1) Synapsis & crossing over
2) At metaphase I, paired HC (tetrads) align on
the metaphase plate
3) At anaphase I, the centromere stays intact
13.4
Genetic variation produce in sexual life cycles
contributes to evolution
1) Crossing over
2) Independent assortment of chromosomes
3) Random fertilization
1) Crossing over
Exchange of genetic material in Prophase I
Results in all 4 chromatids of the tetrad being
different
In metaphase II, sister chromatids separate
increasing variation
2) Independent assortment
Metaphase I allows the HC to pair up in any
combination
There is a 50-50 chance that a daughter cell will
get a maternal chromosome or paternal
chromosome from the pair
3) Random fertilization
Each egg & sperm is different
Each combination of the 2 is unique due to #1 & #2