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.

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???????

Chromosome structureChromosome structure

• 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!!!)

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

locatedHETEROCHROMATIN:Tightly coiled, where inactive genes are

located

Telomeres: located at the ends of chromosomes

Terms to rememberTerms to rememberChromosome/sChromatid/sChromatinCentromereTelomeresNucleosomeEuchromatinHeterochromatin

Human karyotypeHuman karyotypeEvery 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

Example of human Example of human karyotypekaryotype

Mitosis and cell cycleMitosis and cell cycleMitosis 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.

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.

Phases of cell cyclePhases of cell cycle

http://www.cellsalive.com/cell_cycle_js.htm

What happens in each What happens in each phase?phase?InterphaseG1.  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.

What happens in each What happens in each phase?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.

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

Mitosis phase by phaseMitosis phase by phaseEarly 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.

Mitosis phase by phaseMitosis phase by phaseMetaphase:•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.

Mitosis phase by phaseMitosis phase by phaseAnaphase:•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.

Mitosis phase by phaseMitosis phase by phaseTelophase:•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.

Centromeres, Centromeres, centrosomes, and 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.

Chromosomes and the Chromosomes and the mitotic spindle during mitotic spindle during mitosismitosis

Biological significance of Biological significance of mitosismitosis•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

TelomeresTelomeresTelomeres 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.

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.

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. 

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).

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.

CancerCancerDefinition:Cancer is an abnormal growth of cells caused by multiple changes in gene expression leading todysregulated balance of cell proliferation and cell death and ultimately evolving into a populationof cells that can invade tissues and metastasize to distant sites, causing significantmorbidity and, if untreated, death of the host.

•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.

•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.

Steps in the development Steps in the development of cancerof cancer1. 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.

Some links =)Some links =)Genetic facts:http://learn.genetics.utah.edu/content/chromosomes/http://www.yourgenome.org/facts/what-is-a-telomerehttps://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-HMFNMohttps://www.youtube.com/watch?v=IHSs7HQs3d4https://www.youtube.com/watch?v=aDAw2Zg4IgEhttps://youtu.be/2WwIKdyBN_s