Nucleic Acid Processing, Hemoglobin Function and Collagen Structure

FORM.CAP: Cap Structure and Methylation

FORM.CAP: This form exhibits methylation and sequence features. Cap0 + CH₃ → CH₃ → Cap1 + CaF₂. mG = methylation on the base; Gm = methylation on the ribose. At the carboxyl terminus of the terminal amino acid, there are numerous nucleases that can be phosphorylated.

Cleavage and Polyadenylation

The CSTF (cleavage stimulation factor) and the CPSF (cleavage and polyadenylation specificity factor) are two proteins that are also phosphorylated. The 1st terminal unit links the carboxyl group, transferring the terminal polyA RNA (AAUUAA). The CSTF creates a splicing messenger. The activity known as the "cut" captures another nuclear protein that activates nucleases.

• Recognizes the AAUUAA sequence and makes the cut at 10-30 nucleotides downstream.
• CPSF recognizes the poly-adenylation signal and supports its binding to the recognition sequence.
• Adds the corresponding AMP residues to stabilize it.

When the poly(A) tail reaches its final length, CPSF is released and the polymerase stops. Its overall function is protective: it shields the mature message against 3'→5' exonucleases.

CORE: Nucleotides and Their Components

CORE: Glucosides derived from ribose and 1-deoxyribose form the sugar component. A monosaccharide is joined to a phosphate group.

Nucleotides: Phosphoric esters of nucleosides. They result from the esterification between a mono- or dialcoholic molecule of H₃PO₄ and a sugar group.

DNA Topoisomerases

TOPOISOMERASE 1: Cuts one DNA strand, allowing the other to rotate and relax supercoiling.

TOPOISOMERASE 2: Breaks both DNA strands at the same point.

O₂ and Hemoglobin

O₂ HEMOGLOBIN: Hemoglobin loads O₂ in the lungs. Depending on the O₂ content, it can become fully saturated, showing cooperativity.

As such, the hemoglobin saturation curve as a function of O₂ content is sigmoidal. Initially, binding is slow, but it increases as affinity improves. In total, tissues that require O₂ have a partial pressure below 20 Torr. The capacity for saturation is high when affinity for O₂ is low, which releases it to the tissues. Myoglobin, by contrast, has a higher affinity for O₂, so it captures O₂ in the tissues. This is the reason hemoglobin has 4 subunits: in the lungs it can bind O₂ and become saturated, while in tissues it has lower affinity and releases it.

COLLAGENS: Structure of Collagen Fibers

COLLAGENS: Collagen fibers are formed from repeating units called tropocollagen, the basic structural unit. Within each row, tropocollagen units are staggered by 1/4 of their length relative to the adjacent unit, so that each of the 4 fibers forms a matching pattern of units along the axis.

This staggered arrangement of tropocollagen is what gives collagen fibers their characteristic striated appearance under the electron microscope. Each tropocollagen unit consists of 3 polypeptide chains, each of approximately 1000 residues, forming a triple helical structure about 300 nm long and 1.5 nm in diameter.