Chapter 9: The Cell Cycle and Cellular Reproduction (Outline) The Cell Cycle

Interphase

Mitotic Stage

Control of the Cell Cycle

Apoptosis

Mitosis and Cytokinesis Phases of Mitosis Cytokinesis in Animal and Plant Cells Prokaryotic Cell Division

The Cell Cycle An orderly set of stages and substages

between one division and the next

Just prior to the next division: The cell grows larger

The number of organelles doubles

The amount of DNA is doubled as DNA is replicated

The two major stages of the cell cycle: Interphase

Mitotic stage

Interphase Most of the cell cycle is spent in interphase Cells perform normal functions, depending on

body location Interphase time varies widely

Nerve and muscle cells are permanently arrested (i.e. cell reaches end stage of development and no longer divide); G0 stage

Embryonic cells complete entire cycle in few hours

In adult mammalian cells, interphase lasts for 20 hours (i.e. 90% of cell cycle)

Consists of three phases; G1, S and G2

Interphase (cont.)

G1 Phase: Between the end of mitosis and beginning of S phase

Cell increases in size and doubles its organelles

Accumulates raw materials for DNA synthesis

S (Synthesis) Phase: Growth and DNA synthesis or replication occurs

Chromosomes are duplicated with 2 identical chromatids

G2 Phase: Between DNA replication and the onset of mitosis

Protein synthesis increases in preparation for division

M (Mitotic) Stage

Involves two main processes:

Mitosis (karyokinesis) Nuclear division

Daughter chromosomes are identical to parental nuclei and distributed to two daughter nuclei

Cytokinesis Division of the cell cytoplasm

Results in two genetically identical daughter cells

The Cell Cycle

Control of the Cell Cycle

Signal – an agent that influences the activities of a cell

Growth factors

Signaling proteins received at the plasma membrane

Ensure that the stages follow one another in the normal sequence

Cause completion of cell cycle (even cells arrested in G0)

Cell Cycle Checkpoints

Internal signals Family of proteins called cyclins

Cell cycle stops at the G2 checkpoint if DNA replication not completed

Prevents initiation of the M stage before completion of the S stage

Allows time for any DNA damage (i.e. exposure to solar radiation or X-rays) to be repaired

Cell Cycle Checkpoints (cont.)

Signal protein p53 in mammalian cells

Stops cycle at G1 when DNA is damaged

Initiates DNA attempt at repair If successful, cycle continues to mitosis

If not, apoptosis is initiated

RB (retinoblastoma) protein responsible for interpreting growth signals and nutrient availability signals

Cell Cycle Checkpoints (cont.)

There is also a cell cycle checkpoint that occurs during the mitotic stage

The cell cycle stops if the chromosomes are not distributed accurately to the daughter cells

Apoptosis

Often defined as programmed cell death

Cells harbor apoptosis enzymes (caspases)

Ordinarily held in check by inhibitors

Can be unleashed by internal or external signals

The sequence of events during apoptosis Cell rounds up and loses contact with its

neighbors Nucleus fragments and plasma membrane

blisters Cell fragments are engulfed by white blood cells

Apoptosis (cont.)

Apoptosis and Cell Division Mitosis and apoptosis are opposing forces

Cell division (mitosis) increases the number of somatic (body) cells

Apoptosis decreases the number of somatic cells

Example:

Tadpole tail disappears (apoptosis) to become a frog

Webbed fingers and toes of human embryos disappear through apoptosis

Both mitosis and apoptosis are normal parts of growth and development (homeostasis)

Eukaryotic Chromosomes DNA in the chromosomes of eukaryotes is

Associated with histone proteins In very long threads and collectively called

chromatin Before mitosis begins:

Chromatin condenses (coils) into distinctly visible chromosomes

Each species has a characteristic chromosome number Example: humans 46, corn 20, goldfish 94

Chromosome Number

Most familiar organisms are diploid Have two chromosomes of each type

Humans have 23 different types of chromosomes Each type is represented twice in each body cell

(diploid) Only sperm and eggs have one of each type

(haploid)

The n number for humans is n=23 Two representatives of each type Makes a total of 2n=46 in each nucleus

Chromosome Duplication

Dividing cell is called the parent cell; the resulting cells are called the daughter cells

At the end of S phase: Each chromosome is internally

duplicated Consists of two identical DNA chains

Sister chromatids Attached together at a single point

(centromere) Protein complex (kinetochores) form

on both sides of the centromere

Division of the Centrosome

Centrosome – the main microtubule-organizing center of the cell

Consists of a pair of barrel-shaped organelles – centrioles

Present in animal cells only

Organizes the mitotic spindle (composed of microtubules)

Microtubules of the cytoskeleton disassemble when spindle fibers form

Phases of Mitosis

Mitosis is a continuous process that is divided into five stages

Prophase

Prometaphase

Metaphase

Anaphase

Telophase

Phases of Mitosis:Prophase Chromatin condensed and

chromosomes are visible Nucleolus disappear Nuclear envelope fragments Spindle begins to assemble The 2 centrioles migrate in

opposite poles Asters -arrays of microtubules

radiate from centrioles (in animal cells)

Chromosomes have no orientation, why?

Phases of Mitosis:Prometaphase (Late Prophase)

Preparation for separation of sister chromatids

Kinetochores appear on each side of the centromere

Kinetochores attach sister chromatids to the kinetochore spindle fibers → chromatids are pulled toward opposite poles

Chromosomes are still not in allignment

Phases of Mitosis:Metaphase

Centromeres of chromosomes are now in alignment at the center of the cell

This center is called metaphase plate

Polar spindle – non attached spindle fibers

Polar fibers reach beyond metaphase plate and overlap

Phases of Mitosis:Anaphase Kinetochore spindle fibers pull

sister chromatids as they disassemble at the kinetochore

Two sister chromatids separate at centromere to the opposite poles

Poles move further apart due to spindle fibers sliding past one another

Microtuble proteins (kinesin and dynein) are involved

Anaphase is the shortest phase

Phases of Mitosis:Telophase

Spindle disappears New nuclear envelopes form

around the daughter chromosomes

Nucleolus appears in each daughter nucleus

Ruminants of polar spindle fibers are still visible between the 2 nuclei

Chromosomes become chromatin fibers

Cytokinesis Division of the cytoplasm to yield two daughter

cells

Allocates mother cell’s cytoplasm equally to daughter nucleus

Encloses each in it’s own plasma membrane

Often begins in anaphase, continues during telophase

Cytokinesis:Animal Cells During animal cytokinesis:

A cleavage furrow appears between daughter nuclei

Formed by a contractile ring of actin filaments

Like pulling on a draw string

Narrow bridge between 2 cells during telophase

Eventually pinches mother cell in two

Cytokinesis in Animal Cells

Cytokinesis:Plant Cells

In plant cells, cytokinesis begins by forming a cell plate Many small membrane-bounded vesicles

originating in the Golgi complex Eventually fuse into one thin vesicle extending

across the mother cell The membranes of the cell plate become the

plasma membrane between the daughter cells Contents of vesicles become the middle lamella

between the two daughter cells Daughter cells later secrete primary cell walls on

opposite sides of middle lamella

Cytokinesis in Plant Cells

The Functions of Mitosis

Maintenance of tissue

Cut finger or skin

Broken bone

Replacement of damaged cells (i.e. accidents)

Growth of multicellular organisms

We all start out as a single cell…?

Fertilized egg (zygote)

Meristematic tissue (shoot tips) in plants

Stem Cells

Stem cells – adult mammalian cells that retain the ability to divide

Red bone marrow stem cells used for therapeutic cloning; used to produce human tissue

Embryonic stem cells in reproductive cloning; used to produce new individual

Prokaryotic Cell Division Asexual reproduction is the formation of new

individuals identical to the original parent cell Prokaryotic chromosome a ring of DNA

Folded up in an area called the nucleoid 1,000 X length of cell Replicated into two rings prior to division Replicate rings attach to plasma membrane

Binary fission Splitting in two between the two replicate chromosomes No spindle apparatus is formed Example: E. coli living in the intestines have a

generation time of about 20 minutes!

Binary Fission of Prokaryotes

Comparing Prokaryotes and Eukaryotes

Both binary fission & mitosis result in daughter cells that are identical to the parent cell

Cellular division in unicellular organisms produces 2 new individuals – form of asexual reproduction Prokaryotes (bacteria & archea) Protists (algae & protozoans) Yeasts

Cell division in multicellular organisms is part of the growth process and important for renewal & repair (e.g. plants & animals)

Cell Division and Function