UNIT 2: THE CELL CYCLE

Cells need to build proteins (especially

enzymes) to carry out the living processes of

growth, reproduction, tissue repair, etc

The genetic information the cell needs to

make proteins is stored in the DNA.

DNA is located in the nucleus of eukaryotic

cells.

We have aprox. 1.8m of DNA in each of our cells!!

how can it fit inside the nucleus (6µm)?

This is geometrically equivalent to packing 40 km of extremely fine

thread into a tennis ball!

DNA is tightly packaged:- When the cell is not dividing (INTERPHASE), DNA is associated with

proteins (histones) forming a thread-like mass called CHROMATIN.

- Just before cell division starts, the cell duplicates the DNA and

condenses it into more tightly packaged structures called

CHROMOSOMES.

DNA

CHROMATINC

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Chromosomes are rod-shaped structures with a constriction called

CENTROMERE.

The centromere divides the chromosomes in two ARMS that may be of

equal or different length.

After the DNA duplication before cell division, chromosomes consist of

two identical CHROMATIDS joined together by the centromere.

The terminal part of each chromatid is called TELOMERE.

SISTERCHROMATIDS

CENTROMERE

TELOMERE

SHORT ARM

LONG ARM

The number, shape and size of the chromosomes is characteristic of

each species.

The number of chromosomes does not correlate with the complexity

of the organism.

A KARYOTYPE is a picture of the chromosomes of an organism, ordered

in pairs.

The members of each pair are called HOMOLOGOUS CHROMOSOMES.

The human karyotype has 23 pairs of chromosomes. All diploid cells have

this 23 pairs, that is 46 chromosomes, except gametes, which are

haploid and have only one set of chromosomes, that is, 23.

Humans have a chromosomic number of 2n=46, where n

stands for the number of homologous chromosomes.

The chromosomes (X and Y) which determine the sex of an individual

are called SEX CHROMOSOMES.

The rest of the chromosomes are called AUTOSOMES.

The chromosomes (X and Y) which determine the sex of an individual

are called SEX CHROMOSOMES.

The rest of the chromosomes are called AUTOSOMES.

GENES are segments of DNA with the necessary information to build a

protein.

Genes are located linearly in the chromosomes.

All the cells of an individual contain the same genes, BUT not all the genes

are active in all the cells.

The genes in the two sister chromatids in a chromosome are identical (they

come from the replication of the same segment of DNA).

The group of genes of an organism is called GENOME.

Humans have a genome of around 35000 genes.

The Cell Cycle:

- Growth

- Repair of damaged cells and tissues

- Renovation of cells

Why do the cells need to divide?

CELL CYCLE (= period of time that goes from one cell division to the next one)

A) INTERPHASE: the cell is not dividing, but is getting ready to divide

- increase in cell size, number of organelles, cytoplasm volume

- protein synthesis

G0

(e.g.neurons)

DNA duplicates (replicates)

- new organelles are produced- cell prepares to divide

B) CELL DIVISION (PHASE M)

MITOSIS: division of nucleusand distribution of chromosomes

CYTOKINESIS: division of cytoplasm, distribution of organelles in the two daughter cells

VARIATION OF DNA CONTENT DURING CELL CYCLE

Remember: we can only see chromosomes in the cell when it is dividing!

During

INTERPHASE:

DNA is

associated to

proteins called

histones

forming

CHROMATIN

During MITOSIS

DNA condenses into

CHROMOSOMES.

This ensures and

makes easier its

equal distribution

in the two daughter

cells

MITOSIS

- Division of the nucleus and distribution of chromosomes in the two

daughter cells.

- Most animal cells divide (epithelial cells, blood cells, etc) except

neurons and muscle cells.

- In plants, only the meristem cells undergo mitosis. This meristems

originate all the other plant tissues.

- makes sure that one cell originates two IDENTICAL daughter

cells (asexual reproduction)

- keeps the number of chromosomes constant

- allows growth: from zygotes (one cell) to adult (billions of cells)

- originates new cells for tissue repair

IMPORTANCE of MITOSIS

- The movement of the chromosomes during mitosis is controlled by the

centriols and the mitotic spindle, which belong to the cytoskeleton.

The starting point is G2 of interphase: the DNA is

replicated, the nuclear envelope is complete, and the

cell has two centriols.

- Mitosis can be divided in the following stages:

- PROPHASE

- METAPHASE

- ANAPHASE

- TELOPHASE

1) PROPHASE

- Chromatin starts to condense and chromosomes can be seen.

- Nucleolus disappears.

- Centriols start to migrate to opposite sides of the cell, and start to

form the microtubules of the mitotic spindle, which will help the

cromosomes to move.

- The nuclear envelope starts to disappear.

2) METAPHASE

- Nuclear envelope has

disappeared.

- Chromosomes attach to the

microtubules of the mitotic

spindle and align in the centre of

the cell (equatorial plate).

3) ANAPHASE

4) TELOPHASE

- The sister chromatides of each chromosome are pulled

apart to opposite poles of the cell as the spindle

microtubules shorten, and they become new

chromosomes.

- The new chromosomes reach the two poles of the cell

and start to decondense to form chromatin again.

- Mitotic spindle disintegrates.

- Nuclear envelope starts to form around each new

nucleus.

- Cytokinesis starts.

CYTOKINESIS

- Division of cytoplasm and organelles in the two daughter cells.

- Starts during the Telophase.

In animal cells a furrow forms

thanks to a contractile ring that

splits the cell in two.

Due to the rigid cell wall, there is no

contractile ring in plant cells. Instead, a cell

plate is formed in the middle of the cell by

vesicles coming from the Golgi Apparatus.

This finally separates the two cells and

originates the new cell wall

MEIOSIS- Organisms that reproduce sexually have two types of cells:

● Somatic cells, are diploid (=with 2n chromosomes)

● Reproductive cells or GAMETES are haploid (= with n chromosomes,

so that when they fuse in fertilization the zygote will have again 2n

- Gametes are produced in a special

type of cell division called MEIOSIS, in

which the number of chromosomes is

reduced to half.

- Meiosis is actually TWO consecutive cell divisions (I and II), similar to

mitosis, with no DNA duplication between them.

- The result of meiosis is 4 daughter cells that are haploid (n).

- In addition to this, the daughter cells are not identical: they contain

different combination of genes (genetic recombination) due to the

exchange of fragments of homologous chromosomes (crossing-over)

during the prophase of the first meiotic division.

MEIOSIS I

1) PROPHASE I: Chromosomes (formed of 2 chromatids) condense

and thicken. Each pair of homologous chromosomes pairs and forms a

tetrad (4 chromatids). The chromatids of homologous chromosomes

exchange fragments (crossing-over). This will result in genetic

recombination.

2) METAPHASE I: Centriols are already in opposite

poles of the cell. Each pair of homologous (tetrad)

attaches the spindle fibers and aligns in the

equatorial plate.

3) ANAPHASE I: When the microtubules of the

spindle shorten, homologous chromosoms

(with 2 chromatids each) separate and

migrate to different poles of the cell.

4) TELOPHASE I: Chromosomes are now in

different poles of the cell. Spindle

disintegrates, a new nuclear envelope starts to

form, and cytokinesis begins

After the first meiotic division, the two daughter cells are now haploid (n).

Each chromosome is made of two chromatids.

The second meiotic division will start without replication of DNA.

1) PROPHASE II: Similar to a normal mitotic prophase.

MEIOSIS II

2) METAPHASE II: Chromosomes align in the middle of the cell and attach

to the microtubules of the spindle.

3) ANAPHASE II: Chromatids separate as the fibers shorten, and

migrate to opposite poles of the cell.

4) TELOPHASE II: The spindle disappears, a new nuclear envelope

forms, cytoplasm divides. The result of the process are 4 haploid cells.

MEIOSIS: GAMETOGENESIS (Spermatogenesis and Oogenesis)

SEXUAL REPRODUCTION AND VARIABILITY

Biological Evolution needs VARIABILITY, that is, genetic diversity, to occur.

Sexual reproduction creates this genetic variability through 3 mechanisms:

- Random distribution of homologous

chromosomes in meiosis I.

- Genetic Recombination due to crossing-over

in Prophase I.

- Random fertilization of gametes

SEXUAL REPRODUCTION versus ASEXUAL REPRODUCTION