2. 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.
3. 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!
4. 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.
6. 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.
TELOMERE
SHORT ARM
CENTROMERE
LONG ARM
SISTER
CHROMATIDS
7. 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.
8. 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.
9. Humans have a chromosomic number of
2n=46, where n
stands for the number of homologous chromosomes.
10. The chromosomes (X and Y) which determine the sex of an individual
are called SEX CHROMOSOMES.
The rest of the chromosomes are called AUTOSOMES.
11. The chromosomes (X and Y) which determine the sex of an individual
are called SEX CHROMOSOMES.
The rest of the chromosomes are called AUTOSOMES.
12. 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.
17. A) INTERPHASE: the cell is not dividing, but is getting ready to divide
- increase in cell size,
number of organelles,
cytoplasm volume
- new organelles
are produced
- cell prepares to
divide
- protein synthesis
G0
(e.g.neurons)
DNA duplicates
(replicates)
18. B) CELL DIVISION (PHASE M)
MITOSIS: division of nucleus
and distribution of chromosomes
CYTOKINESIS:
division of cytoplasm,
distribution of
organelles in the two
daughter cells
20. 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
21. 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)
IMPORTANCE
of MITOSIS
- keeps the number of chromosomes constant
- allows growth: from zygotes (one cell) to adult (billions of cells)
- originates new cells for tissue repair
22. - 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
23. 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).
24. 3) ANAPHASE
- 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.
4) TELOPHASE
- 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.
25. CYTOKINESIS
- Division of cytoplasm and organelles in the two daughter cells.
- Starts during the Telophase.
In animal cells a furrow forms
Due to the rigid cell wall, there is no
thanks to a contractile ring that
contractile ring in plant cells. Instead, a cell
splits the cell in two.
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
26.
27. 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.
28. - 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.
29. 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.
30. 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.
31. 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.
32. MEIOSIS II
1) PROPHASE II: Similar to a normal mitotic prophase.
2) METAPHASE II: Chromosomes align in the middle of the cell and attach
to the microtubules of the spindle.
33. 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.
38. 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
