Genome complexity: irreducibly complex sperm cells (Introduction)

by David Turell , Monday, July 03, 2023, 14:07 (299 days ago) @ David Turell

They have to be designed:

https://evolutionnews.org/2023/06/on-the-irreducible-complexity-of-sperm-cells/

"The head carries densely coiled chromatin fibers, containing the haploid genome — totaling half of the genetic material that will be inherited by the next generation (the other half will come from the mother’s egg cell). The tight packaging of the DNA serves to minimize its volume for transport.

"On the tip of the sperm head is a membranous organelle, called the acrosome, that contains various hydrolytic enzymes. When these are secreted, they digest the egg cell membrane, thereby facilitating penetration of the ovum. Without the acrosome, the sperm cell will be unable to penetrate the egg cell membrane to fertilize the ovum.

"When a sperm reaches the vicinity of the egg, it undergoes a series of molecular interactions with the zona pellucida, which is a specialized extracellular matrix surrounding the egg. Specific receptors on the sperm’s plasma membrane, such as spermadhesins or integrins, recognize and bind to corresponding ligands on the zona pellucida. This binding triggers the activation of signaling pathways in the sperm. Binding of the sperm receptors to the zona pellucida ligands leads to an influx of calcium ions (Ca2+) into the sperm cell. This calcium influx is typically mediated by ion channels or receptors on the sperm’s plasma membrane, which are activated upon ligand-receptor binding. The increase in intracellular calcium levels initiates a signaling cascade within the sperm cell. Calcium ions act as second messengers and trigger the activation of various downstream signaling molecules and enzymes, including protein kinases. As a result of the calcium-mediated signaling cascade, the acrosome undergoes exocytosis. The membrane surrounding the acrosome fuses with the sperm’s plasma membrane, causing the release of the acrosomal contents, including enzymes such as hyaluronidase and acrosin. The enzymes released from the acrosome help degrade the glycoprotein matrix of the zona pellucida, allowing the sperm to penetrate and reach the egg’s plasma membrane. The acrosomal contents aid in the breakdown of the protective layers surrounding the egg, facilitating the fusion of the sperm and egg membranes.

"The formation of the acrosome itself is divided into four stages. The first stage, the “Golgi phase,” is dependent upon the Golgi apparatus, which produces and packages the proteins and enzymes needed for acrosome formation. These proteins are then transported into the developing acrosome vesicle. In the second phase, the “cap phase,” the Golgi-derived vesicle (known as the proacrosomal vesicle) fuses with the anterior portion of the nucleus, forming a cap-like structure over the nucleus. The fusion of the vesicle with the nucleus is mediated by membrane trafficking processes... In the final phase, the “maturation phase,” the acrosome undergoes further modifications and maturation. Enzymes within the acrosome become fully activated and the acrosomal matrix undergoes changes, becoming more condensed. The acrosomal granule, which is the central region of the acrosome, becomes highly electron-dense due to the accumulation of enzymes and proteins. The mature acrosome is now ready for its role in fertilization.

***

"The middle piece consists of a central filamentous core, around which are many strategically placed mitochondria that synthesize the energy molecule adenosine triphosphate (ATP). The complexity and design of energy generation within the mitochondria — including the processes of glycolysis, the citric acid (or, Krebs) cycle, the electron transport chain, and oxidative phosphorylation — could be its own series of articles.

"The ATP generated by the mitochondria energizes the power strokes of the flagellum, driving its journey through the female cervix, uterus, and uterine tubes. As such, the middle piece of the sperm cell is absolutely essential to its function of swimming through the female uterus and fallopian tube to fertilize her egg. Without the middle piece and its mitochondria, the sperm cells are completely immobile.

"Unlike a bacterial flagellum (which rotates like a motor), a sperm flagellum beats with a whip-like motion to produce motility. How does the flagellum work? In 2018, Jianfeng Lin and Daniela Nicastro elucidated the mechanism of flagellar motility.3 Their data indicated that “bending was generated by the asymmetric distribution of dynein activity on opposite sides of the flagellum”4 (dyneins are ATP-powered molecular motors that “walk” along microtubules towards their minus end). Their results also revealed that alternating flagellar bending occurs due to “a ‘switch-inhibition’ mechanism in which force imbalance is generated by inhibiting…dyneins on alternating sides of the flagellum. In other words, regulatory signals lead to the inhibition of dynein motors on one side of the flagellum. Meanwhile, on the other side, the dyneins walk along the microtubules. The flagellum bends in one direction due to molecular linkers that resist this sliding. The flagellar bending alternates by repeatedly switching the side of dynein inhibition."

Comment: the design of sexual reproduction is irreducibly complex. Much more so than simple cell splitting. Only a designing mind can create this.