The main topic of DNA replication is the process of copying the genetic information in a cell. This process occurs during the S phase of the cell cycle and is essential for cell division. The joining of the two DNA strands is catalyzed by an enzyme called DNA polymerase. Membrane-bound organelles called mitochondria also have their own DNA replication machinery. The RNA molecules required for protein synthesis are called messenger RNA (mRNA), transfer RNA (tRNA), and ribosomal RNA (rRNA). mRNA carries the genetic information from DNA to the ribosomes, where protein synthesis occurs. tRNA brings the amino acids to the ribosomes, and rRNA forms the structural components of the ribosomes. The process of transcription involves the synthesis of mRNA from a DNA template. This process is catalyzed by an enzyme called RNA polymerase. The DNA strand that serves as the template for transcription is called the antisense strand, while the newly synthesized mRNA is called the sense strand. Introns are non-coding regions of DNA that are transcribed into mRNA but are removed during the process of RNA splicing. Exons, on the other hand, are the coding regions of DNA that are translated into protein. The genetic code is a set of rules that determines how the nucleotide sequence of a gene is translated into the amino acid sequence of a protein. Each three-nucleotide sequence, called a codon, corresponds to a specific amino acid. There are 20 different amino acids that can be encoded by the genetic code. Sex-linked conditions are genetic disorders that are more frequently observed in males. This is because males have only one X chromosome, so if they inherit a defective gene on their X chromosome, they will manifest the condition. Females, on the other hand, have two X chromosomes, so they have a higher chance of having a normal copy of the gene to compensate for the defective one.