Download The cell cycle - U of L Class Index

The cell cycle
(Lecture 22)
The cell cycle is an ordered set of events, culminating in cell growth and division into two daughter
cells
Structure
1. The role of cell division – what is a need to divide?
2. The mitotic cell cycle – how to divide?
1. The role of cell division
• In reproduction – main difference between living organism and nonliving matter
•
In growth – for multicellular organism (including cell renewal and cell repair)
The important definitions
• Cell division is a finely controlled process that results in the distribution of identical hereditary
material – DNA – to two daughter cells.
• A dividing cell:
Precisely replicates its DNA
Allocates the two copies of DNA to opposite ends of the cell
Separates into two daughter cells containing identical hereditary information
•
The DNA is distributed among 1 to more then 50 long linear stretches called chromosomes.
• The number of chromosomes is a characteristic of every eukaryote.
For example our somatic cells – all cells except reproductive – contain 46 chromosomes (2n). Our
gametes (reproductive cells) have only 23 chromosomes (n).
•
The total DNA in the chromosomes of an organism is referred as its genome.
•
Chromosome contain thousands of genes – units of DNA that contains the information to
specify synthesis of single polypeptide chain.
•
The DNA structure is maintained via association with different protein – formation of the DNAprotein complex – chromatin.
Before the cell can divide all DNA must be copied and then separated to daughter cells.
After copying each duplicated chromosome consists of two sister chromatids –identical copies of
chromosome’s DNA molecule.
Characteristics of chromosomes:
They are supercoils of a DNA-proteins complex called chromatin. Each chromosome consists of :
Genes
Proteins
They exist in characteristic number (somatic - 2x, gametes – x)
They exists in different states:
Loosely folded during interphase
Highly folded during mitotic phase
2. The mitotic cell cycle
• The cell cycle is composed from interphase (I) and mitotic (M) phase
•
Interphase consists of several subphases
•
Mitotic phase consists of both mitosis and cytokinesis. The former one is divided to five
subphases with special characteristic for each one
Short summary of interphase
•
•
•
•
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Comprised by three (four) subphases: G1, (G0), S and G2 .
Common feature of all (except G0) subphases: growth of the cell by producing proteins and
cytoplasmic organelles.
G1 – “first gap”
S phase – chromosome duplication
G2 – “second gap”, DNA repair
Interphase (G2)
• The nucleus is well defined
• It contains one or more nucleoli
• Two centrosomes are formed
• Duplicated chromosomes are
still not visible
Prophase
• Chromatin condenses into discrete chromosome
• The nucleoli disappear
• Mitotic spindle begins to form
Prometaphase
• Fragmentation of nuclear envelope
• Formation of kinetochore
• Entension of spindle fibers
• Invasion of microtubules into the nucleus and attachment to kinetochores
Metaphase
• Centrosomes are at opposite poles of the cell
• Chromosomes move to the metaphase plate
• Formation of imaginary metaphase plate: alignment of all chromosome at the centre
• Spindle formation
Anaphase
• Centromeres separate and sister chromatids move toward opposite poles of the cell as their
kinetochore microtubules shorten
•
Nonkinetochore microtubules elongate moving the poles further apart
Telophase
• Forming of nuclear envelope
• Chromatin is less condensed
• Mitosis is complete
• Cytokinesis: formation of cleavage furrow and separation of two daughter cells
Cytokinesis in animal and plant cells
• Cytokinesis begins during telophase of mitosis
• It is different in plants and animals.
• In animal cells it occurs by cleavage furrow, in plants two daughter cells are separated by cell
wall that is build from the center of cytoplasm and expends to the edges.
Summary
1. The role of cell division – reproduction and growth (cell renewal and repair).
2. The mitotic cell cycle – consists of:
- interphase (I=G1+S+G2) – important steps for DNA duplication and initiation of mitosis;
- mitosis (P+PM+M+A+T) – separation of two daughter cells with equal amount of genetic material
(chromosomes).
Reading
Ch. 12 pp. 218-228