Answer:
Two groups of proteins, called cyclins and cyclin-dependent kinases (Cdks), are responsible for the progress of the cell through the various checkpoints. Cyclins regulate the cell cycle only when they are tightly bound to Cdks.
Explanation:
The endosymbiotic theory states that eukaryotic cells might have evolved from prokaryotic cells through a close realtionship.
What we know about shark and ray fossils comes from bones in rocks from the Cretaceous period in Mexico that left behind, their only hard parts.
<h3>Shark fossils</h3>
The idea of a stingray-like shark may seem like something out of a B-movie. But paleontologists have just reported the discovery of such a creature in Cretaceous rocks in Mexico. This strange shark combines a streamlined body with large, wing-shaped fins, an ancient animal unlike any seen in the fossil record.
<h3>Whose are the fossils?</h3>
Fossils are the remains or traces of living beings that inhabited the Earth in previous geological periods. Fossil remains are parts of animals or plants, such as a bone or some limb or organic remains that have been preserved in some way.
With this information, we can conclude that Fossils are remains or traces of animals and plants preserved in rocks.
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B) Circulatory.
The Circulatory system spreads the blood all around the heart. It makes sure blood is able to reach every part in the body.
<span>Having more lung capacity is advantageous because the lungs can expand more. More air in the lungs will fill all the alveoli (air sacs in the lungs) and allow more oxygen to enter the bloodstream via the capillaries that surround the air sacs. Excess waste (such as carbon dioxide) is also transferred from the blood to the air to be exhaled. As lungs fill, more gas exchange can occur.</span>