RNA, which contains uracil (U) instead of thymine, carries the code to protein-making sites in the cell. To make RNA, DNA pairs its bases with those of the “free” nucleotides (Figure 2). Messenger RNA (mRNA) then travels to the ribosomes in the cell cytoplasm, where protein synthesis occurs (Figure 3).

As we all know, a protein can shuttle between the nucleus and the cytoplasm. Whereas many proteins are selectively transported from the cytoplasm into the nucleus, most RNAs are exported from the nucleus to the cytoplasm. And snRNAs can shuttle between the nucleus and the cytoplasm.

DNA and Protein Synthesis mRNA is synthesized in the nucleus using the nucleotide sequence of DNA as a template. This process requires nucleotide triphosphates as substrates and is catalyzed by the enzyme RNA polymerase II. The process of making mRNA from DNA is called transcription, and it occurs in the nucleus.

Nuclear proteins are transported actively through nuclear pores by a selective, mediated process. The process is mediated by a nuclear localization signal (NLS), and can be divided into at least two steps, (a) targeting to the pores and (b) translocation through the pores.

Each nuclear pore is a large complex of proteins that allows small molecules and ions to freely pass, or diffuse, into or out of the nucleus. Nuclear pores also allow necessary proteins to enter the nucleus from the cytoplasm if the proteins have special sequences that indicate they belong in the nucleus.

These DNA-associated proteins organize the DNA and help it fit into the nucleus, and they also play a role in determining which genes are active or inactive. The complex formed by DNA and its supporting structural proteins is known as chromatin.

The Golgi apparatus is found close to the nucleus of the cell, where it modifies proteins that have been delivered in transport vesicles from the RER. It is also involved in the transport of lipids around the cell. Pieces of the Golgi membrane pinch off to form vesicles that transport molecules around the cell.

During initiation, the small ribosomal subunit binds to the start of the mRNA sequence. Then a transfer RNA (tRNA) molecule carrying the amino acid methionine binds to what is called the start codon of the mRNA sequence.

tRNA uses (anticodons/codons) to match to the mRNA. (TRNA/MRNA) attaches the amino acids into a chain.

The mRNA (messenger RNA) carries the info regarding what protein is to be made. The tRNA (transport RNA) carries the amino acid to the rRNA. The rRNA (ribosomal RNA) makes up the ribosome. The ribosome builds the protein according to the instructions written in the mRNA with the amino acids ferried in by the tRNA.

During the initiation step of translation, the fMet charged tRNA assembles in which site of the ribosome? How does the ribosome know if the entering charged tRNA is correct? The anticodon on the tRNA base pairs to the codon on the mRNA. Where would one find an uncharged tRNA molecule in a ribosome?

In mRNA, three-nucleotide units called codons dictate a particular amino acid. For example, AUG codes for the amino acid methionine (beige). In mRNA, three-nucleotide units called codons dictate a particular amino acid.