Early cleavage divisions in most embryos are also rapid. One reason for this is that cleaving cells have a modified cell cycle, in which the two gap phases, G1 and G2 are completely omitted, and the cells cycle rapidly between M and S phases. In addition, because the major cytoskeletal systems of the cells are largely devoted to mitosis (microtubules) and cytokinesis (microfilaments) during rapid cleavage division, there is typically an antagonism between periods of rapid cell division and cell movement. Thus as the embryo approaches gastrulation, there is a tremendous slowing in the rate of division (note the slope of the curve at the right).
Answer:
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Explanation:
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Answer:
Yes, two hydrogen bonds could form between thymine and cytosine.
Explanation:
Cytosine is a pyrimidine base found in both DNA and RNA, and Thymine is a pyrimidine base found in only DNA. Electronegative Oxygen and Nitrogen atoms with free lone pairs are potential hydrogen bond acceptors. Hydrogen atoms attached to very electronegative atoms like Oxygen and Nitrogen have strong partial positive charge and are potential hydrogen bond donors.
One hydrogen bond could form between the C4 carbonyl group on thymine which is a hydrogen bond acceptor and the C4 amino group on cytosine which is a hydrogen bond donor. Also, another hydrogen bond could be formed between N3 of thymine which is a hydrogen bond donor and the N3 of cytosine that is a hydrogen bond acceptor.
It is important to note that hydrogen bond cannot be formed between them because the C2 carbonyl groups found on both bases are both hydrogen bond acceptors.
Answer:
Explanation:
Australopithecina or Hominina is a subtribe in the tribe Hominini. The members of the subtribe are generally Australopithecus (cladistically including the genera Homo, Paranthropus,[2] and Kenyanthropus), and it typically includes the earlier Ardipithecus, Orrorin, Sahelanthropus, and Graecopithecus. All these related species are now sometimes collectively termed australopithecines or homininians.[3][4] They are the extinct, close relatives of humans and, with the extant genus Homo, comprise the human clade. Members of the human clade, i.e. the Hominini after the split from the chimpanzees, are now called Hominina[5] (see Hominidae; terms "hominids" and hominins).
While none of the groups normally directly assigned to this group survived, the australopithecines do not appear to be literally extinct (in the sense of having no living descendants) as the genera Kenyanthropus, Paranthropus and Homo probably emerged as sister of a late Australopithecus species such as A. africanus and/or A. sediba.
The terms australopithecine, et al., come from a former classification as members of a distinct subfamily, the Australopithecinae.[6] Members of Australopithecus are sometimes referred to as the "gracile australopithecines", while Paranthropus are called the "robust australopithecines".[7][8]
The australopithecines occurred in the Plio-Pleistocene era and were bipedal, and they were dentally similar to humans, but with a brain size not much larger than that of modern apes, with lesser encephalization than in the genus Homo.[9] Humans (genus Homo) may have descended from australopithecine ancestors and the genera Ardipithecus, Orrorin, Sahelanthropus, and Graecopithecus are the possible ancestors of the australopithecines.[8]
