DNA Replication, Meiosis, Endocytosis & Exocytosis

DNA Replication

In Prokaryotic Cells

There is one place of origin of replication, shown in the replication fork, and the advance of the copy is noted. The fork indicates that we are making the separation and replication at a time. The advance is bidirectional, which shortens the time. Where replication begins, proteins are organized in a complex called a replisome. DNA replication in prokaryotes occurs at a rate of 500 nucleotides per second.

In Eukaryotic Cells

The process is essentially the same, but the DNA is much longer and more linear. There are several origins of replication, and it is bidirectional. The progress is slower than in prokaryotes because there are more proteins associated with DNA to be released. DNA replication, which happens only once in each cell generation, needs many “bricks”, enzymes, and a large amount of energy in the form of ATP (remember that after the S phase of the cell cycle, cells pass through a G2 phase to, among other things, recover energy for the next phase of cell division). The replication of DNA in humans is performed at a speed of 50 nucleotides per second. The nucleotides must be assembled and be available to the nucleus, along with the energy to join them.

Meiosis

Meiosis I

Interphase I

Interphase, the first meiotic division, is the most complex process and is divided into five sub-stages, which are:

• Leptotene: The first stage of prophase I is the leptotene stage, during which individual chromosomes begin to condense into long filaments inside the nucleus.
• Zygotene: The homologous chromosomes start to get closer until they are paired throughout their length. This is known as synapsis (union), and the resulting complex is known as a bivalent.
• Pachytene: Once the homologous chromosomes are fully paired, forming bivalent structures, the phenomenon of crossing over occurs, in which non-sister homologous chromatids exchange genetic material.
• Diplotene: Chromosomes continue condensing until the two chromatids of each chromosome can begin to be seen.
• Diakinesis: This stage is difficult to distinguish from diplotene. We can see the chromosomes a little more condensed.

Prophase I

The nuclear membrane disappears.

Metaphase I

The homologous chromosomes are aligned in the equatorial plane.

Anaphase I

Pairs of chromosomes are separated.

Telophase I

Each daughter cell now has half the number of chromosomes, but each chromosome consists of a pair of chromatids.

Meiosis II

Prophase II

The nuclear membrane disappears, and the achromatic spindle is formed.

Metaphase II

Spindle fibers bind to the kinetochores of the chromosomes.

Anaphase II

The chromatids separate at their centromeres, and a set of chromosomes moves toward each pole.

Telophase II

In telophase II, there is a member of each homologous pair at each pole. They duplicate.

Endocytosis and Exocytosis

These processes allow the transport of large molecules, forming membranous vesicles that are lined around filaments of clathrin (a protein). If transport takes place to the inside, we are talking about endocytosis, and if we are talking about transport to the outside, we are talking about exocytosis. Endocytosis has functions of nutrition, and exocytosis usually has a secretory function. Both processes (endocytosis and exocytosis) are based on the ability of self-sealing and self-assembly. The membrane can deform. If a virus enters, an endosome can penetrate a lysosome that destroys it. After this process of exocytosis, the membrane returns to close, atom by atom, individually. Sometimes, the process of exocytosis takes place because of the required presence of species like receptors in the immune system. When this process is observed under a microscope, it shows a vesicle that is like a pocket that is surrounded by filaments of clathrin.