Mitocondris i cloroplasts: orgànuls energètics

• Membranous organelles are energetic
• Energy organelles provide energy to the cell to perform vital functions, these are mitochondria and chloroplasts
• Both have their own genetic information, so they have a degree of autonomy within the cell

Mitochondria: Energy Factories

• Mitochondria are cylindrical or elongated organelles composed of a double membrane: an outer membrane, smooth, and an inner membrane with folds called mitochondrial cristae
• Inside is the mitochondrial matrix, which mainly contains ribosomes, enzymes, and genetic material
• Mitochondria are considered the powerhouses of eukaryotic cells, as their function is to obtain energy through cellular respiration
• They are found in all eukaryotic cells
• Mitochondria are dynamic organelles
• In most eukaryotic cells, mitochondria fuse, divide, and form complex tubular networks that are constantly changing

Chloroplasts: The Site of Photosynthesis

• Chloroplasts are ellipsoid organelles consisting of a double membrane with a series of membranous sacs, thylakoids, in the membrane of which chlorophyll is located, a pigment that gives them their characteristic green color
• Photosynthesis occurs in chloroplasts, that is, the synthesis of organic molecules from other inorganic ones using chemical energy obtained from light energy
• They are only present in the cells of photosynthetic organisms (plants and some chromists)

How Cells Move

• Cilia and flagella are external appendages present in some eukaryotic cells and used for movement
• Cilia and flagella are motile organelles formed from cytoskeletal protein fibers, which constitute external appendages
• The movement of cilia and flagella is directed and coordinated by a structure known as the centriole
• The centriole consists of protein tubules arranged similarly to those that form cilia and flagella
• It also participates in the distribution of chromosomes during cell division
• Some eukaryotic cells are capable of moving and can do so in two ways: through cilia or flagella, or by changes in the viscosity of the cytoplasm
• Changes in the viscosity of the cytoplasm are caused by cytoskeletal proteins, which group together or separate
• This produces changing protrusions, pseudopodia, that modify the cell shape
• Pseudopodia are used to surround and capture certain materials from the external environment (microbes, food particles.)
• This process is called phagocytosis

The Nucleus

• The nucleus contains the genetic material
• Generally, it is located in the central region of the cell, although in some cases, such as in certain secretory cells and many plant cells, it may be in peripheral areas
• The nucleus is the cell's control center, and its structure varies depending on the stage of the cell's life
• There are two different organizations: the nucleus in interphase, when the cell is not dividing, and the nucleus in division
• The nuclear membrane is double and has pores that allow the exchange of substances with the cytoplasm
• Nucleoplasm is an aqueous environment where the nucleolus and chromatin are located, facilitating chemical reactions

Interphase

• The nucleus in interphase has a double membrane with pores surrounding a nucleoplasm similar to the cytoplasm
• Inside is chromatin, a substance formed by DNA bound to proteins called histones, and a spherical organelle, the nucleolus, which participates in ribosome synthesis
• Nucleolus is a spherical structure where ribosomes are formed
• There may be one or two, and they are formed of proteins and RNA
• Chromatin is formed by DNA bound to proteins called histones
• At the beginning of cell division, it condenses, forming chromosomes

Division

• As cell division begins, the nucleus changes completely
• Chromatin condenses, giving rise to chromosomes, structures with a characteristic shape that appear in a specific number in the cells of each species

Chromatin and Chromosomes

• In the nucleus in interphase, the genetic material is in the form of chromatin, which is formed by DNA and proteins, of which histones are the most abundant
• Its function is to package DNA, that is, to organize it so that it fits inside the nucleus
• Therefore, chromatin is the way DNA is structured inside the cell
• As has already been mentioned, when the nucleus begins to divide, the chromatin condenses and forms chromosomes

Chromosomes

• A chromosome is formed by two filaments of chromatin, called chromatids, joined by a centromere
• Each of these chromatids is a copy of the other, so the genetic information is duplicated
• In each chromatid, the centromere defines two regions called arms
• The number of chromosomes in cells is fixed and depends on the species to which they belong
• The number of different chromosomes in a cell is expressed as n and is called the haploid number
• Sex cells are haploid because they have this number of chromosomes
• All other cells, called somatic cells, have pairs of chromosomes (2n chromosomes), that is, a diploid number
• Telomeres are structures at the ends of chromosomes that protect genetic material
• As our cells divide, they get shorter over time
• Therefore, telomere length is considered a key indicator of aging, although it is not the only one

The Animal and Plant Cell

• Most cytoplasmic organelles are common to all eukaryotic cells; however, some differ between animal and plant cells

The Cell Cycle

• The cell cycle is the set of processes by which cells grow and divide to produce two new cells
• During the cell cycle, a cell alternates between interphase and mitosis, that is, it undergoes cycles of growth and division
• Interphase: During this time, the cell grows, doubles its chromosomes, synthesizes organelles, and prepares for cell division
• The interphase stage consists of the G1, G0, S, and G2 phases
• Mitosis: In this phase of the cell cycle, the cell stops growing and divides into two daughter cells, each with the same number of chromosomes
• After completing mitosis, the resulting daughter cells enter their respective interphase stages, thus beginning a new series of cell cycles
• The M phase (mitosis) is the cell duplication phase
• Toward the end of the G1 phase, a cell can enter a resting or non-dividing phase and exit the cell cycle
• This happens, for example, when the cell receives a signal to differentiate, or when resources are insufficient to grow and divide
• In G1 Phase, the cell enters this phase immediately after the previous cell division (or mitosis) has been completed
• During the G1 phase, the cell increases in size and prepares to copy its DNA
• If the DNA is free of damage, and there are sufficient resources for cell division, growth signals will stimulate the cell to proceed to S phase, or DNA synthesis
• In S Phase, the cell synthesizes a copy of all its DNA and duplicates it
• In G2 Phase, the genetic material condenses and prepares for cell division
• Specialized proteins regulate the cell cycle and control the transition from one phase of the cell cycle to the next
• It is a strictly controlled process and generally works without errors
• When errors occur, they can have very negative consequences, such as the development of cancer

How Cells Divide

• Cell division occurs to obtain new individuals, in the case of unicellular organisms, or to replace tissues, in multicellular organisms
• Apart from sex cells, it is essential that the daughter cells obtained from an initial cell are equal to it and to each other
• Therefore, it is essential that the same genetic information is transmitted to each of them exactly
• Since the genetic material is in the nucleus, the most important process is its division, mitosis
• To distribute the same amount of genetic material to two daughter cells, the mother cell must have two copies
• Therefore, before starting mitosis, the genetic information is duplicated
• Mitosis provides a copy to each daughter cell
• The cells obtained by mitosis have the same number of chromosomes as the mother cell
• Mitosis can be divided into four stages: prophase, metaphase, anaphase, and telophase
• The chromatids are copies of each other, so the genetic information is duplicated

Prophase, Metaphase, Anaphase and Telophase

• In this first stage, four fundamental processes occur
  • The chromatin fibers coil into a spiral
  • As a result, chromosomes are formed
  • The nucleolus disappears
  • Protein fibers (spindle fibers) appear between the two cell poles
  • At the end of this first stage, the nuclear membrane disappears, and the chromosomes are free in the cytoplasm
• The chromosomes are joined by the centromere to the spindle fibers
• This union occurs in the middle plane of the cell (equatorial plate)
• The sister chromatids of each chromosome are oriented towards the opposite poles of the cell
• The fibers of the achromatic spindle break in half, that is, at the level of the equatorial plane
• As a result, the centromere of the chromosome attached to the fibril is broken
• As a result, each half-fibril is left with half a chromosome attached to it (that is, a chromatid)
  • The half-fibrils of the spindle contract and drag the chromatids that they carry attached to the poles and that, from that moment on, are considered independent chromosomes
• Once the migration of the chromatids has finished, the remains of the spindle disappear
  • A nuclear membrane appears around each group of chromatids
  • In this way, the daughter nuclei are formed
  • The chromatids progressively decondense and become chromatin
  • The nucleolus appears
• If the whole process has been done correctly, the two daughter nuclei will have identical genetic information and the same number of chromosomes as the mother cell
• This is due to the fact that the two chromatids that constitute the chromosome of the initial cell are two identical copies, and each one has been directed to a daughter nucleus
• Once mitosis is finished, the division of the cytoplasm takes place, by means of which the cellular organelles are distributed
• This process is called cytokinesis and can occur in two different ways
  • By constriction and subsequent separation of the cytoplasm: it occurs in animal cells
  • By formation of a separation wall in the cytoplasm: it takes place in plant cells

Meiosis

• Sometimes, the division of a cell does not give rise to cells equal to it, but to others with half the genetic information
• This division is necessary for the formation of gametes or sex cells
• The gametes unite in fertilization to form a cell called a zygote, which after successive mitotic divisions will give rise to the complete organism
• Half of the zygote's chromosomes are contributed by one parent, and the other half by the other

Maintaining Genetic Information

• To maintain the number of chromosomes, and therefore the genetic information, specific to each species, the gametes must have only half the chromosomes
• This is achieved by meiosis, a special type of nuclear division after which four daughter cells are obtained with half the chromosomal complement of the mother cell
• Meiosis consists of two successive divisions
• It is also known as reductional division, because it originates cells with half the chromosomes
  • It consists of the same phases as mitosis, but with important peculiarities:
    • In prophase I, the chromosomes appear, but associated in homologous pairs (one comes from the father and another from the mother)
    • There is an exchange of genetic material between homologous chromosomes, which is called recombination
    • In metaphase I, homologous chromosome pairs, not individual chromosomes, are attached to the spindle fibers
    • In anaphase I, a complete chromosome, not half chromosomes, is directed to each cell pole
    • In telophase I, the nuclei of the daughter cells are reconstructed, and cytokinesis occurs
• At the end of the first division, two daughter nuclei with n complete chromosomes each are obtained
• The chromosomes split into their chromatids, and each daughter cell receives one
• Therefore, meiosis gives rise to four daughter cells, each with half the chromosomes of the starting cell

Genetic Variability

• Sexual reproduction originates genetic variability through two phenomena that take place in the first meiotic division
  • The distribution of the chromosomes of the progenitors
  • The exchange of genetic information through recombination

Similarities and Differences Between Mitosis and Meiosis

• Both mitosis and meiosis start from the condensation of chromatin and the consequent formation of chromosomes
• Chromosomes separate towards both cell poles, either split into their two chromatids or whole
• At the end of the processes, the chromatin unwinds, and the daughter nuclei are constructed; thus, the cellular reproductive process ends
• However, mitosis and meiosis have major differences
  • In mitosis, a diploid mother cell originates two daughter cells with identical genetic endowment, between them and with their progenitor.
  • In meiosis, four different haploid daughter cells are obtained from a diploid mother cell, genetically
    • In mitosis there is a single process, while in meiosis, two successive divisions occur, the first reductional and the second normal
    • Analyzing the phases of each process, there are the following differences
• Prophase I of meiosis is longer than that of mitosis, and in it, there is an exchange of genetic material between homologous chromosomes (recombination)
• In anaphase of mitosis, the chromatids of each chromosome separate and each one goes to a cell pole
• In anaphase I of meiosis, the complete homologous chromosomes separate and one of each pair goes to a cell pole
• In telophase of mitosis, there are two identical daughter nuclei with the same number of chromosomes as the starting cell
• In telophase I, two daughter nuclei are obtained with half of the chromosomes than the mother cell and genetically different from each other and from the progenitor
• The two cells obtained in the first meiotic division begin the second division and originate four cells
  • Mitosis is a process thanks to which the genetic information of the mother cell is exactly preserved
  • It is linked to asexual reproduction
• It takes place when it is necessary to replace tissues, if they have deteriorated, or to form new ones, when there is growth, as well as to originate cells identical to those already existing
• It is a conservative process
• Any change in the genetic information would have serious consequences
• An example of problems that take place during mitosis is cancer, a serious disease in which the cells of the affected tissues grow in a disordered and harmful way
• Meiosis, on the other hand, is linked to sexual reproduction, and allows obtaining daughter cells with modifications with respect to the genetic information of the progenitors, which combines and enriches
• The exchange of genetic material between homologous chromosomes that takes place in prophase I is an enrichment, as it makes it possible for there to be new genetic combinations