Meiosis, DNA Replication, Transcription, and Translation

Posted on Jan 28, 2025 in Biology

Meiosis: Maintaining Chromosome Number

This division is necessary for the formation of gametes during sexual reproduction in order to keep the number of chromosomes of each species, generation after generation. Meiosis gives rise to 4 daughter cells, each with half the chromosomes as the parent cell.

Haploid and Diploid Cells

Haploid cells have half the number of chromosomes and contain one copy of genetic information. Diploid cells have the full number of chromosomes (n) and contain two complete copies of genetic information, i.e., paired chromosomes (2n).

1st Reductional Meiotic Division (Similar to Mitosis)

This division results in two cells with half the number of chromosomes as the parent cell. These cells are haploid, but their chromosomes are duplicated, so a normal mitosis follows.

• Prophase: Chromosomes are associated by pairs of homologues. Recombination occurs between them, an exchange of genetic material.
• Metaphase: Spindle fibers bind a complete chromosome of each pair of homologues to each pole of the cell.
• Anaphase: An entire chromosome is pulled to each pole, not just half.
• Telophase: No special phenomena occur with the chromosomes.

2nd Meiotic Division (Similar to Mitosis)

No interphase occurs. The whole chromosomes, in their chromatids, splinter and go to each cell. Four daughter cells originate, each with a single haploid chromosome number. The stages are set in the two cells in a normal manner until the completion of telophase and cytokinesis, resulting in 4 haploid cells.

DNA and the Genetic Message

DNA Replication

1. Helicases break the hydrogen bonds between DNA strands, separating them to serve as templates.
2. Topoisomerases unwind the double helix, preventing supercoiling.
3. Stabilizing proteins (SSB) bind to the single-stranded DNA, maintaining the separation between the two complementary strands.
4. Replication forks form bidirectionally, with a helicase in each channel, each moving in a different direction.
5. DNA polymerase cannot act without a primer. Primase synthesizes an RNA primer.
6. DNA polymerase begins synthesis from the primer in the 5′ to 3′ direction. This is an ongoing process; the helicase does not stop.
7. In the antiparallel strand, RNA polymerase synthesizes RNA about 40 nucleotides in a matter that was about 1,000 nucleotides in the initiation signal. From them, DNA polymerase III synthesizes about 1,000 DNA nucleotides, forming an Okazaki fragment.
8. DNA polymerase I removes the RNA fragments and fills the gaps with DNA.
9. DNA ligase joins the different fragments together. The antiparallel strand, therefore, has discontinuous growth.

Transcription

1. RNA polymerase binds to specific sequences (promoter).
2. Transcription bubbles are opened.
3. Synthesis of the primer.
4. Separation of RNA polymerase.
5. Elimination of introns from exons.

Translation

This is the process by which the message contained in mRNA is translated into the language of proteins.

1. The message is copied into mRNA, which exits the nucleus via nuclear pores.
2. mRNA is located on the ribosomes, organelles responsible for protein synthesis. Ribosomes read the mRNA information in the form of triplets, i.e., three nucleotides.