chromosomesmitoticcycle-160401133710.pdf

What is chromosome

1. What is a chromosome?What is a chromosome?
Chromosomes are thread-like structures located
inside the nucleus of animal and plant cells. Each
chromosome is made of protein and a single
molecule of deoxyribonucleic acid (DNA).
Passed from parents to offspring, DNA contains
the specific instructions that make each type of
living creature unique.
The term chromosome comes from the Greek
words for color (chroma) and body (soma).
Scientists gave this name to chromosomes
because they are cell structures, or bodies, that
are strongly stained by some colorful dyes used
in research.

2. Chromosomes vary in number and shape
among living things.
Prokaryotic cells (Bacteria) have one or two
circular chromosomes.
Eukaryotic organisms have linear
chromosomes that are arranged in pairs
within the nucleus of the cell.
Somatic cells (nonreproductive cells) have
two sets of chromosomes
Gametes (reproductive cells) have half as
many chromosomes as somatic cells; they
carry just one copy of each chromosome.
WHY???????

3. Chromosome structureChromosome structure

4. • Each chromosome is formed by two
chromatids, held together by a narrow
region called centromere
• Each chromatid contains one molecule of
DNA (and NOT only one gene but a series
of them!!!)

5. Chromatin is a complex of DNA and protein
that condenses during cell division
 There are two kinds of chromatin:
EUCHROMATIN:
Loosely coiled, where active genes are located
HETEROCHROMATIN:
Tightly coiled, where inactive genes are located
Telomeres: located at the ends of
chromosomes

10. Terms to rememberTerms to remember
Chromosome/s
Chromatid/s
Chromatin
Centromere
Telomeres
Nucleosome
Euchromatin
Heterochromatin

11. Human karyotypeHuman karyotype
Every eukaryotic species has a
characteristic number of chromosomes in
each cell nucleus
To study them, geneticists organice them
in karyotypes (very useful to study
genetic disorders, for instance)
Karyotype: a chromosome chart that
displays chromosomes arranged by shape
and size

12. Example of human karyotypeExample of human karyotype

14. Mitosis and cell cycleMitosis and cell cycle
Mitosis is a process of nuclear division in eukaryotic
cells that occurs when a parent cell divides to
produce two identical daughter cells.
During cell division, mitosis refers specifically to the
separation of the duplicated genetic material carried
in the nucleus.
To study mitosis it is important to understand that
this process takes place in the context of the cell
cycle.
The cell cycle is the regular sequence of events that
takes place between one cell division and the next.

15. In order to divide, a cell must complete several important
tasks: it must grow, copy its genetic material (DNA), and
physically split into two new daughter cells.
Cells perform these tasks in an organized and predictable
series of steps that collectively make up the cell cycle.
In eukaryotic cells, cells with a nucleus, the stages of the cell
cycle are divided into two major phases: interphase and
the mitotic (M) phase.
During interphase, the cell grows and makes a copy of its
DNA.
During the mitotic (M) phase, the cell separates its DNA
into two sets and divides its cytoplasm, forming two new
cells.

16. Phases of cell cyclePhases of cell cycle
http://www.cellsalive.com/cell_cycle_js.htm

17. What happens in each phase?What happens in each phase?
Interphase
G1.  During this phase, also called the first gap phase, the cell grows larger
and makes more of its ribosomes, organelles, and proteins. This phase ensures
that division will produce functional daughter cells, ones that are the right
size and have all the parts they need.
S phase. In this phase, the cell synthesizes a complete copy of the DNA in
its nucleus, which it needs in order to divide allowing it to give one full set to
each of its two daughter cells. During S phase, the cell also duplicates a
microtubule-organizing structure called the centrosome.
G2. Once DNA synthesis is complete, the cell enters a second gap phase.
During this period, the cell grows more, makes additional proteins and
organelles, and begins to reorganize its contents in preparation for mitosis,
the separation of the copied DNA into two equal sets. This phase ends when
mitosis begins.
The G1, S, and G2 together are known as interphase.

18. What happens in each phase?What happens in each phase?
During M phase, the cell divides its copied nuclear DNA and
cytoplasm to form two new cells. M phase is further divided into two
phases: mitosis and cytokinesis.
In mitosis, the nuclear DNA of the cell condenses into visible
chromosomes and is pulled apart by the mitotic spindle, a specialized
structure made out of microtubules.
In cytokinesis, the cytoplasm of the cell is split in two, making two
new cells. Cytokinesis usually begins just as mitosis is ending, with a
little overlap.
In animal cells, cytokinesis involves constriction of the cytoplasm
between the two new nuclei.
In plant cells, I it involves the formation of a new cell wall.

19. MitosisMitosis
Mitosis takes place in four stages: prophase
(sometimes divided into early prophase and
prometaphase), metaphase, anaphase, and
telophase.

21. Mitosis phase by phaseMitosis phase by phase
Early prophase:
•Centrosomes replicate just before prophase.
•Chromosomes start to appear as the chromatin coils
up, becoming shorter and thicker.
Late prophase:
•Nuclear envelope ´disappears´ (breaks up into small
vesicles which can´t be seen with a microscope).
•Nucleolus ´disappears´.
•Chromosomes are seen to consist of two identical
chromatids.
•Centrosomes start moving to opposite directions and
form de poles of the mitotic sprindle. The sprindle is
completed by the end of the prophase.

22. Mitosis phase by phaseMitosis phase by phase
Metaphase:
•Each centrosome reaches a pole.
•Sprindle made from protein microtubules is
completed.
•Chromosomes line up across the equator of
the sprindle; they are attached by their
centromeres to the sprindle.

23. Mitosis phase by phaseMitosis phase by phase
Anaphase:
•Each chromosome splits at
the centromere.
•The chromatids start to be
pulled apart by
microtubules.
•Chromatids move to
opposite poles, centromeres
first, pulled by the
microtubules.

24. Mitosis phase by phaseMitosis phase by phase
Telophase:
•Nucleolus and nuclear envelope reform.
•Chromatids have reached the poles of
the sprindle; they will now uncoil again.
•Each new cell will have only one
chromatid of each chromosome, but the
complete genetic set, HOW COME?
•Once the telophase is reaching the end,
cytokinesis will take place.

26. Centromeres, centrosomes, and Centromeres, centrosomes, and
centriolescentrioles
•Centromere is needed for the separation of the chromosomes
during mitosis.
•Each metaphase chromosome has two kinetochores at its
centromere, one on each chromatid.
•Kinetochores are made of proteins that bind specifically to the
DNA in the centromere and also to the microtubules.
•Shortening of the microtubules during anaphase: allows the
chromatids to migrate to each pole; as the microtubule shortens, it
pulls the kinetochore, dragging the rest of the chromatid behind.
•The poles of the spindle are where the centrosomes are located, one
at each pole.

27. Chromosomes and the mitotic Chromosomes and the mitotic
spindle during mitosisspindle during mitosis

28. Biological significance of mitosisBiological significance of mitosis
•Growth: the two daughter cells are identical; same
number of chromosomes and genetically identical.
•Replacement of cells and repair of tissues
•Asexual reproduction: Mitosis is the basis of
asexual reproduction. Example: budding in plants.
•Inmune response

30. TelomeresTelomeres
Telomeres are repetitive stretches of DNA located at the ends of
linear chromosomes, rich in guanine and cytosine. They protect the
ends of chromosomes in a manner similar to the way the tips of
shoelaces keep them from unraveling.
Every time a cell carries out DNA replication the chromosomes are
shortened by about 25-200 bases (A, C, G, or T) per replication.
However, because the ends are protected by telomeres, the only part
of the chromosome that is lost, is the telomere, and the DNA is left
undamaged.
Without telomeres, important DNA would be lost every time a cell
divides, which would eventually lead to the loss of entire genes.

31. What happens to What happens to
telomeres as we age?telomeres as we age?
Each time a cell divides, 25-200 bases are lost from the ends of the
telomeres on each chromosome. Two main factors contribute to
telomere shortening during cell division:
1.The “end replication problem” during DNA replication: the copying
enzyme cannot run to the end of the DNA and complete the
replication. This accounts for the loss of about 20 base pairs
?
 per cell
division.
2.Oxidative stress: The amount of this stress in the body is thought to
be affected by lifestyle factors such as diet, smoking and stress.
Accounts for the loss of between 50-100 base pairs per cell division.  
When the telomere becomes too short, the chromosome reaches a
‘critical length’ and can no longer be replicated. Then a process called
´apoptosis
´
(also known as programmed cell death) is triggered.

32. How is telomere length How is telomere length
maintained?maintained?
Telomerase

is an enzyme that adds the TTAGGG telomere sequence
to the ends of chromosomes.
Telomerase is only found in very low concentrations in our somatic
cells. Because these cells do not regularly use telomerase they age
leading to a reduction in normal function. The result of ageing cells, is
an ageing body.
Telomerase is found in high levels in germline

cells (egg and sperm)
and stem cells. In these cells telomere length is maintained after DNA
replication and the cells do not show signs of ageing. 
Telomerase is also found in high levels in cancer cells. This enables
cancer cells to be immortal and continue replicating themselves.
The action of telomerase allows cells to keep multiplying and avoid
ageing. 

33. Stem cellsStem cells
•Stem cells are unspecialized (undifferentiated) cells that are
characteristically of the same family type (lineage).
•They retain the ability to divide throughout life.
•When a stem cell divides, each new cell has the potential to remain a
stem cell or to develop and give rise to cells that can become highly
specialized.
•Stem cells contribute to the body's ability to renew and repair its
tissues. Unlike mature cells, which are permanently committed to
their fate, stem cells can both renew themselves and create new cells
of whatever tissue they belong to (and other tissues).

34. Potency: extent of the power of a stem cell to
produce different cell types.
•Totipotent: cells that can produce any type of cell
•Pluripotent: embryonic cstem cells.
•Multipotent: cells that can only produce a few types
of cells. For example, stem cells in the bone marrow.
•Stem cell therapy:
Introduction of new adult stem cells into damaged
tissue or to treat an injury.

35. CancerCancer
Definition:
Cancer is an abnormal growth of cells caused by
multiple changes in gene expression leading to
dysregulated balance of cell proliferation and cell
death and ultimately evolving into a population
of cells that can invade tissues and metastasize to
distant sites, causing significant
morbidity and, if untreated, death of the host.

36. •Cancer shows us the importance of controlling cell
division properly, because cancers are the result of
uncontrolled mitosis.
•Cancer starts when certain cells undergo changes in
the genes that control cell division (mutation).
•Oncogenes: particular term for a mutated gene
that causes cancer.
•Cancerous cells start dividing ´out of control´, and
end up forming a tumour.

37. •Benign tumours: do not spread! Not cancerous, but
may become cancerous.
•Malignant tumours: interfere with the normal
functioning of the area where they are located.
Metastasis: Cells of malignant tumours can break off
and spread through the blood and lymph to other
parts of the body.
Carcinogens: any agent that causes cancer. Examples:
UV light, tar in tobaco smoke, X-rays.

38. Steps in the development of Steps in the development of
cancercancer
1.Oncogenes transformed by
carcinogens.
2.Cancerous cell does not respond
to signals: continues to divide´out
of control´.
3.Tumour is formed. It gets bigger.
Tumour cells look ´different´
under the microscope.
4.Tumour is supplied with blood
and lymph vessels. Tumour cells
spread to other parts of the
body.
5.Metastasis. Tumour cells invade
other tissues.

40. Some links =)Some links =)
Genetic facts:
http://learn.genetics.utah.edu/content/chromosomes/
http://www.yourgenome.org/facts/what-is-a-telomere
https://es.khanacademy.org/science/biology/cellular-molecular-
biology/mitosis/a/cell-cycle-phases
Chromosome structure:
http://www.macroevolution.net/diagram-of-chromosome.html
Mitosis:
https://www.youtube.com/watch?v=5uPC-HMFNMo
https://www.youtube.com/watch?v=IHSs7HQs3d4
https://www.youtube.com/watch?v=aDAw2Zg4IgE
https://youtu.be/2WwIKdyBN_s