Cell Division, Nucleic Acids, and Gene Expression
Mitosis
Stages:
- Prophase: Chromatin fibers thicken and shorten to form chromosomes. The nucleolus and nuclear membrane disappear, allowing chromosomes to move freely in the cytoplasm. Mitotic spindles (protein fibers) appear at the cell poles.
- Metaphase: Chromosomes connect to mitotic spindles through centromeres along the cell equator and align, forming the equatorial plate, where sister chromatids are pulled to opposite poles.
- Anaphase: Mitotic spindles break at the equatorial plate and contract, pulling attached chromatids to opposite poles. Chromatids become independent chromosomes.
- Telophase: Mitotic spindles disappear, and the nucleolus and nuclear membrane reappear, forming two new nuclei, where chromatids condense, forming chromatin.
After this, nuclei will have identical genetic information and the same chromosome number as the parent cell because the two chromatids that made up the chromosomes were identical.
Cytokinesis is the division of the cytoplasm after mitosis.
Meiosis
Cells from this cell division aren’t identical because cells have half of their genetic information; the division forms gametes or reproductive cells, which join in fertilization, forming a zygote. Half of the chromosomes come from the mother, and the other half from the father.
1st Meiotic Division (Reduction Division):
- Prophase I: Homologous pairs of chromosomes appear, and there is an exchange of genetic material (genetic recombination).
- Metaphase I: Pairs of homologous chromosomes bind to mitotic spindles.
- Anaphase I: Whole chromosomes move to opposite poles.
- Telophase I: Reconstruction of daughter cell nuclei and cytokinesis happens. The result is two daughter nuclei with a full ‘n’ set of chromosomes.
During the 2nd meiotic division, four daughter cells are produced, and the stages and process are the same as in mitosis.
Mitosis vs. Meiosis
Similarities: Both start with chromosome condensation, then they separate into opposite poles.
Differences:
- Mitosis (2n=2n, same genetic information, 1 process, creates 2 individuals).
- Meiosis (2n=n, different genetic information, 2 processes, creates 4 individuals), larger prophase, and recombination occurs.
- In meiosis, during anaphase, whole homologous chromosomes separate, while in mitosis, the chromatids of each chromosome separate.
Importance:
- Mitosis: Asexual reproduction. Provides new cells, substituting dead ones. A malfunction in the process can lead to problems like cancer.
- Meiosis: Sexual reproduction. Genetic material exchange.
Nucleic Acids
Discovered by Miescher by isolating a substance in the interior of white blood cells and calling it nuclein.
They are macromolecules formed by the union of nucleotides, which are:
- Orthophosphoric acid remains (H3PO4)
- Monosaccharide with 5 carbon atoms (pentose) that can be ribose or deoxyribose.
- A nitrogenous base (Cytosine, Guanine, Adenine, Thymine, Uracil)
Nucleotides join, forming polynucleotides.
Nucleic acids can be DNA (deoxyribonucleic acid) or RNA (ribonucleic acid):
- DNA contains phosphates, deoxyribose, and ATGC. Its molecular structure is based on two polynucleotides connected with nitrogenous bases, double-stranded (double helix). They can be Nuclear (forms chromatin), Mitochondrial, and Plastid (found in mitochondria and chloroplasts) DNA. Its function is to contain genetic information.
- RNA contains phosphates, ribose, and AUGC. Its molecular structure is based on one polynucleotide, single-stranded. It can be messenger (mRNA), transfer (tRNA), or ribosomal (rRNA). It can be found in the cytoplasm, free or associated with ribosomes, and synthesizes proteins.
DNA
A gene carries biological information, and DNA and genes are inside the cells. Evolution:
- Mendel described the factors in peas’ inheritance characteristics.
- The gene is described as a unit of biological inheritance. Morgan discovered genes were in chromosomes.
- Avery, McLeod, and McCarthy showed that genes are made of DNA.
- Start of molecular biology dogma (gene, RNA, protein) and genetic code is decoded.
- The gene is a physical and functional unit of inheritance that could code for protein synthesis.
A=T; G≡C
Replication or self-replication is the replication of DNA by itself for cell division and occurs in the nucleus. New molecules are made of one original strand from DNA and another complementary one that is identical to the original. This replication is called semiconservative replication:
- The DNA double helix opens, and its chains separate.
- Free nucleotides join original strands according to their corresponding bases.
- Nucleotides connect to each other.
- Production of two molecules identical to the original DNA.
Gene Expression
Process where organisms transform nucleic acid information into proteins.
In replication, DNA copies itself when a cell is going to divide. When certain proteins are reproduced, the DNA gene transcribes the information into RNA, which leaves the nucleus into the cytoplasm for protein translation and synthesis.
This was the central dogma of nuclear biology described by Crick, which was later modified due to:
- Dogma (unquestioned belief) is no longer used due to its incompatibility with science.
- Some viruses (HIV) were discovered and used reverse transcription to synthesize DNA from RNA.
This was the central dogma of nuclear biology described by Crick that later modified due to:
- Prions discovery (proteins capable of reproducing without DNA help)
- Ribozymes discovery (self-replicating RNA molecules).
Transcription Process: Genetic information is copied from original DNA to mRNA. Occurs in the nucleus.
- The double helix opens.
- Complementary nucleotides are placed in front of one of the strands (template strand).
- Only one DNA strand is copied.
- Creation of an mRNA strand complementary to the DNA one.
- New mRNA molecules will exist in the cell nucleus.
Translation Process: Protein synthesis with information contained in the form of an mRNA molecule formed during transcription. Occurs in ribosomes. Elements involved:
- mRNA molecules from DNA fragment transcription.
- Ribosomes can read the mRNA molecule.
- tRNA are RNA molecules that carry amino acids to the ribosomes.
- Amino acids are free in the cytoplasm.
Translation occurs when a cell needs specific proteins:
- mRNA leaves the nucleus with the copied DNA information.
- mRNA joins ribosomes.
- tRNA carries amino acids to ribosomes according to the genetic message of mRNA.
- Ribosomes read the mRNA and join amino acids in the correct order to form a protein.
- The finished protein separates from the ribosome.