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:

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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

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• Early Prophase– the chromatin begins to condense into

chromosomes

Mitosis

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• 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

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• Transition to Metaphase

– the spindle apparatus forms completely and

the chromosomes attach

Mitosis

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• Metaphase

– the chromosomes line up along the equator of the

cell

Mitosis

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• 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

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Cytokinesis

• The actual division of the cytoplasm usually occurs toward the end of telephase.

Somatic cell division results in two identical cells

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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

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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 unite

and 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