CHAPTER 9 & 10

The Cell Cycle, Meiosis & Sexual Life Cycles Sexual

reproduction

mitosis

asexual reproduction

meiosis

Chromosomes

• Consists of a long DNA molecule (represents thousands of genes)

• Also consists of proteins (structure, helps control gene activity)

Chromosomes (cont.)

• This DNA-protein complex is chromatin

• Chromatin is organized into a long, thin fiber

• Fiber is coiled & folded to form the chromosome

• Gene – a unit of information on a chromosome

- consists of DNA

- can be passed along to offspring

Chromosomes (cont.)

• All cells in the body (somatic) have the same # of chromosomes (46)

• Except for sperm & eggs (sex cells – 23)

Chromosomes (cont.)

• When chromosomes replicate, they form 2 identical structures called sister chromatids (joined by a centromere)

• When the sister chromatids separate during mitosis, each goes into a daughter cell

Cell Cycle (summary)

• Cell doubles its amount of cytoplasm including organelles

• DNA is duplicated

• Nucleus & its contents divide by mitosis

• Cytoplasm is divided between 2 daughter cells

Cell Cycle (Interphase) • Lasts 90% of total

time for cell cycle

• Divided into 3 phases:

- G1 Phase (1st gap) period of interphase before DNA synthesis begins (growth in size, proteins, organelles)

- S Phase (synthesis) when DNA synthesized

- G2 Phase (second gap) period after DNA synthesis begins but before mitosis begins (proteins synthesized essential to cell division)

Cell Cycle (M Phase)

• 10% of cell life

• Mitosis – nucleus & contents divide

• Cytokinesis – cytoplasm divides which generates 2 daughter cells

Cell Cycle (Phases of Mitosis)

• Prophase

• Prometaphase

• Metaphase

• Anaphase

• Telophase/Cytokinesis

p. 178-179

Prophase • Chromatin tightens

& thickens (chromosomes now visible)

• Each duplicated chromosome is seen as 2 identical sister chromatids joined by a centromere

•Nucleus disappears

•Microtubules form a radial array called an aster around each centriole

•Mitotic spindle forms

Prometaphase • Nuclear envelope disappears & spindle enters nucleus

• Bundles of microtubules extend from pole to equator

• Centromeres have kinetochores to which kinetochore fibers are attached

Metaphase

• Centriole pairs are now at opposite poles

• Chromosomes line up on the metaphase plate (a plane between the 2 poles)

Anaphase

• Paired centromere of each chromosome moves apart

-sister chromatids are now free of each other

-each chromatid is now a chromosome

Anaphase (cont.)

• Spindle moves the chromosomes to opposite poles

• Poles move farther apart

• Each pole now has a complete set of chromosomes

Telophase

• Polar fibers elongate

• Nuclear envelopes are formed from earlier fragments from the parent cell

•Nucleolus reappears •Mitosis is complete

Cytokinesis (animals – cleavage)

• Shallow groove in cell surface near old metaphase plate (called cleavage furrow)

• Contractile ring of microfilaments is found on cytoplasm side of furrow

-ring contracts & shrinks in diameter

-Cleavage furrow deepens until parent cell pinches in two

Cytokinesis (plants – no cleavage furrow)

• Cell plate forms across midline of parent cell

• Double membrane is formed

• Fusion of each of these 2 membranes w/plasma membrane results in the formation of 2 daughter cells

p. 181 & 182

Prokaryotic vs. Eukaryotic • Binary fission

• Cell divides in half

• Chromosome replicates & attaches to plasma membrane

• 2 copies will be separated by growth that occurs between them

• 2 cells form

• Genes grouped into chromosomes (long thread-like structures)

• Cell duplicates chromosomes as well as genes

• Mitosis occurs

• Duplicated chromosomes are evenly distributed into 2 daughter nuclei

• cytokinesis

Cell-Cycle Control Systems

•It is a cyclically operating set of proteins (mostly growth factors) in the cell that triggers & coordinates major events in the cell cycle.

•Major checkpoints in G1, G2, and M:

- G1 checkpoint seems to be most critical in

many cells (gives go-ahead to start cell-cycle & divide, non-dividing cells are stuck at G1 checkpoint called Go)

-G2 checkpoint gives go-ahead to start M phase

-M checkpoint go-ahead signals that the proper attachment of the spindle fibers to the chromosomes & prompts the separation of the sister chromatids

-What is signal transduction?

* Protein that receives signals for specific activities

What affects mitosis?

Normal Cell Division – 3 factors:

Anchorage Dependent – cells must be “anchored” to a solid surface to divide; keeps cells separated from their normal surroundings from dividing inappropriately

Density-Dependent Inhibition – rate of cell division decreases as cell population becomes denser

Growth Factors – proteins secreted by certain body (somatic) cells that stimulate other cells to divide

p. 186

2 Types of Regulatory Molecules

1. Protein Kinases – activate or inactivate other proteins by phosphorylating them

- constant [ ]

2. Cyclin – attaches to a protein kinase to activate it

- fluctuating [ ]

Uncontrolled Cell Growth- can produce excessive growth resulting in an abnormal mass called a tumor • Benign = abnormal

mass of normal cells that normally remain at their original site

• can usually be removed by surgery

• Malignant = mass of cancer cells that can spread beyond their original site to neighboring cells (growth) or other sites of the body (metastasis – circulatory system)

Cancer cells do not have a properly functioning cell-cycle.

• Carcinomas – cancers of the external & internal linings of body (skin, linings of intestines)

• Sarcomas – cancers of tissues that support body (bone, muscle)

• Leukemias & Lymphomas – cancer of blood-forming tissue (bone marrow, spleen, lymph nodes)

Treatment for Cancer

• In radiation therapy, cancer cells are most likely dividing at any given time & will destroy cells by disrupting cell division w/out injuring normal cells

• Chemotherapy uses the same strategy as radiation therapy but uses chemicals instead

Chapter 10 Sexual & Asexual Reproduction

• When offspring results from 2 parents

• Offspring has a unique combination of genes inherited from BOTH parents

• When an individual inherits all its genes from a SINGLE parent

• Consists of simple division or budding

Sexual Life Cycle (human)

• Each somatic cell = 46 chromosomes

• Chromosomes differ from each other in size, position of centromere, staining pattern

• Matched in pairs (this creates a karyotype)

•Chromosomes that make up a pair are called homologous chromosomes

- same size & have same centromere position

- each carries genes controlling same

inherited trait but possibly different versions

-genes for a certain trait are located in a particular place - locus

blue green

•23rd pair of chromosomes is the sex chromosomes

- XY for males

- XX for females

•All other chromosomes are called autosomes

Chromosome Numbers

• Humans inherit 2 sets

- maternal = 23

- paternal = 23

• Cells containing 2 sets are diploid or 2N

• Cells containing 1 set are haploid or 1N

•Gametes are haploid cells

- they are reproductive cells, either sperm or egg

•Each sperm or egg has single set of 22 autosomes plus 1 sex chromosome

•Fertilization occurs when egg & sperm unite & create a zygote

•Production of gametes requires a special type of division

- use reduction division called meiosis

- reduces chromosome number by half

Meiotic Cell Division

Meiosis I

- Prophase I

- Metaphase I

- Anaphase I

- Telophase I/ Cytokinesis

Meiosis II

- Prophase II

- Metaphase II

- Anaphase II

- Telophase II/ Cytokinesis

p. 198-199

Meiosis I • Produces haploid gametes in diploid

organisms

(Prophase I)

•Homologous chromosomes come together as pairs

- each is made of 4 chromatids

- each forms a tetrad

- 90% of meiotic cell division

- crossing over of genetic material can occur (segments of non-sister chromatids can exchange places)

- spindle forms

- nuclear envelope & nucleolus disappear

(Metaphase I)

•Tetrads line up on the metaphase plate

(Anaphase I)

•Homologous pairs separate & go to opposite poles (only tetrad splits)

(Telophase I/ Cytokinesis) •Cytoplasm divides making 2 daughter cells •Interkinesis occurs (rest time)

Meiosis II

(Metaphase II)

•Chromosomes line up on the metaphase plate

(Prophase II) Spindle moves chromosomes toward the equator

(Anaphase II)

•Centromeres of sister chromatids separate

•Individual chromosomes move to poles

(Telophase II/ Cytokinesis) •Nuclear envelope reforms •4 daughter cells are made (each has ½ the number of chromosomes of parent)

Comparing Mitosis & Meiosis p. 200

• Behavior of chromosomes is same

• 2 daughter cells that are each 2N

• Behavior of chromosomes is same

• 4 daughter cells that are each N

• Very different than mitosis because of tetrad

Genetic Variation

• Independent Assortment

• Random Fertilization

• Crossing Over

Independent Assortment

• Orientation of homologous pairs relative to the 2 poles is random (there are 2 possibilities)

• The first meiotic division results in independent assortment of maternal and paternal chromosomes into daughter cells

•In humans, the number of combinations of maternal or paternal chromosomes is 223 = 8.4 million

•There are 2N possible combinations where N is the haploid number

Random Fertilization

• An egg cell represents 8.4 million different possibilities

• A sperm cell represents 8.4 million different possibilities

• A zygote will have 70 trillion diploid combinations

Crossing Over

• Identical regions of chromatids are exchanged

- results in a change in genetic material

- this is known as genetic recombination

• Produces combinations different from those originally inherited

“Omnis cellula e cellula”

Every cell from a cell

Rudolph Virchow