Cell Cycle-Mitosis,Sexual Reproduction-Meiosis
& Inheritance-Genetics
CHROM…words
• Chromatin – uncoiled DNA + proteins
• Chromosome – coiled DNA + proteins (Looks like an X)
• Chromatid – only half of a chromosome
• Sister chromatids – Two chromatids joined together, by a centromere, to form a chromosome
Chromatin
Cell Cycle
• 4 distinct periods
What does the cell spend most of its life doing?
Do you think DNA synthesis is an “expensive” process? Why or why not?
Mitosis is a continuous process described in 5 phases:
p. 128
p. 128
Cell Cycle
• Four stages to the cell cycle– Growth period -
Interphase includes:• G1• S Stage • G2
– Division period - Includes:
• Mitosis
Interphase:
• Known as the growth period
• Majority of cells life
• Three stages within Interphase– G1– S Stage– G2
G1 Stage #1
• Chromosomes are not visible under a microscope - because they are uncoiled, therefore called chromatin
• Proteins are quickly made
S Stage Stage #2
• Chromatin is replicated in the nucleus
• Chromatin divides to form sister chromatids which are connected by centromeres
G2 Stage #3
• Chromatin shortens and coils
• Organelles are made
• Most proteins made are for mitosis
• Animals - centriole pair replicates and prepares to form spindle fibers.
Interphase Information• Busiest phase of cell cycle • What are the three parts?• When are the
chromosomes replicated?• When is the most protein
production?• When are organelles
made?• When are cell parts made?• Which is the longest stage
of interphase?• Which is the shortest
stage of interphase?
Prophase
Metaphase
Anaphase
Mitosisthe process of organizing and distributing nuclear DNA
Martini pgs 97-98
• Early Prophase– the chromatin begins to condense into
chromosomes
Mitosis
Martini pgs 97-98
• Late Prophase• one of the centriole pairs moves to the opposite side of the cell.
• microtubules begin to grow from the centrioles building the spindle apparatus.
• the nuclear envelope begins to dissolve.
Aster
Mitosis
Martini pgs 97-98
• Transition to Metaphase
– the spindle apparatus forms completely and
the chromosomes attach
Mitosis
Martini pgs 97-98
• Metaphase
– the chromosomes line up along the equator of the
cell
Mitosis
Martini pgs 97-98
• Anaphase
– the sister chromatids are taken to opposite
poles of the
Mitosis
• Telephase
• The chromosomes decondense back into chromatin
• Nuclear membranes form around each set of unduplicated chromosomes
Martini pgs 97-98
Cytokinesis
• The actual division of the cytoplasm usually occurs toward the end of telephase.
Somatic cell division results in two identical cells
Martini pgs 97-98
Mitosis is regulated by growth factors
Mitosis is inhibited by suppressor genes
• For example: p53
Cancer
• When the rate of cell division (mitotic rate) is greater than that of cell death in a tissue
Martini pgs 99-100
Screening for Cell Division Cycle (cdc) Mutants
cdc mutants 1) continue cell growth2) arrest with a single cell morphology i.e. at a defined cell cycle stage
Temperature Sensitive Yeast cdc Mutant
PermissiveTemperature
RestrictiveTemperature
Cell Division
• Mitosis (used during somatic cell division)– Diploid to Diploid– creates 1 new somatic daughter cell– parent and daughter cell are genetically identical
• Meiosis (used during production of sex cells)– Diploid to Haploid (1 copy of chromosomes)– creates 4 reproductive cells (eggs or sperm)– new combination of chromosomes (mix of mom and dad)
Sexual Reproduction:creating genetic diversity
Sexual Reproduction:creating genetic diversity
As opposed to asexual reproduction which makes genetic clones.
An overview:from germ cell to babies
• Germ Cells – diploid cells of the reproductive organs.
• Gametes – haploid cells (sperm/egg = 23 chromosomes) made from germ cells by a process called meiosis.
• babies – conceived when the nuclei of sperm and egg join to make 46 total chromosomes (23 homologous pair)
Germ Cells:homologous chromosomes
• All somatic cells have 46 chromosomes (23 homologous pairs), one copy of each pair is inherited from the mother and the other from the father.
Because of homologous chromosomes there are 2 copies of each gene.
From Egg From Sperm
One gene can come in different varieties.
• Allele: variant forms of the same gene. • Can you think of an example of a gene that has more
than 1 allele?
Sexual Reproduction Shuffles Alleles
• Through sexual reproduction, offspring inherit new combinations of alleles, which lead to variations in traits
Gamete Formation• Gametes are sex cells (sperm, eggs)• Gametes are formed when germ cells in reproductive organs undergo meiosis.
testes
ovaries
Two important things happen during meiosis
1. The number of chromosomes is cut in half (46 to 23)
2. The alleles are rearranged so that any offspring produced are genetically different from the parents.
chromosome number in gametes
• n is equal to the total number of chromosomes in a cell
• Germ cells (like somatic cells) are diploid (2n)
• Gametes are haploid (1n)
How does 1 germ cell (2n) become 4 gametes (1n)?
• Two consecutive cell divisions, but only 1 replication of the DNA 1. Meiosis I 2. Meiosis II
DNA replication:
cell division w/o replication
Meiosis I• Prophase I
– Each duplicated chromosome pairs with homologue (mom’s copy with dad’s copy)
– Homologues form tetrads during synapsis and swap segments (cross over) to increase genetic variation
– Each chromosome becomes attached to spindle
Crossing Over
• The maternal and paternal chromosomes swap a segments while they are paired.
Outcome of Crossing Over
• After crossing over, a chromosome will contain both maternal and paternal segments
• Creates new allele combinations in offspring
Meiosis I• Metaphase I
– The spindle apparatus is fully formed
– the homologous chromosomes (tetrads) line up randomly along the equator of the cell
Random Alignment
• In Meiosis I the chromosomes line up at the equator randomly
• This means that the genetic contributions from mom and dad can be mixed up in the gametes.
or
or
or
1 2 3mom’s chromosome
dad’s chromosome
Meiosis I
• Anaphase I
– homologous chromosomes segregate
– the sister chromatids remain attached
Meiosis I
• Telophase I
– chromosomes arrive at opposite ends of the cell and cytokinesis separates the cytoplasm
Meiosis I results in:
• 2 genetically different diploid (2n) cells
Prophase II
• Microtubules attach to the kinetochores of the duplicated chromosomes
Metaphase II
• Duplicated chromosomes line up at the spindle equator, midway between the poles
Anaphase II
• Sister chromatids separate to become independent chromosomes
Telophase II• The chromosomes arrive at
opposite ends of the cell
• A nuclear envelope forms around each set of chromosomes
• The result is four haploid cells (gametes)
Oogenesis
GrowthMeiosis I,
Cytoplasmic DivisionMeiosis II,
Cytoplasmic Division
ovum (haploid)
primary oocyte (diploid)
oogonium (diploid)
secondary oocyte
first polar body
three polar bodies
(haploid)
Spermatogenesis
Growth Meiosis I Meiosis II
cell differentiation, sperm formation
spermatids (haploid)
secondary spermatocytes
primary spermatocyte
(diploid)
spermato-gonium (diploid )
sperm (mature, haploid male
gametes)
Fertilization
• Male and female gametes uniteand their nuclei fuse together
combining the chromosomes.
• Fusion of two haploid nuclei
produces a diploid nucleus in the
zygote
Factors that contribute to variation among Offspring
1. Crossing over during prophase I
2. Random alignment of chromosomes at metaphase I
3. Random combination of gametes at fertilization
Genetic variation in offspring is important. It protects the species from environmental changes (like a new disease).
Mitosis vs. Meiosis