Tài liệu Khoa Học Tự Nhiên

(5) The transformation of a zygote to an animal of specific form depends on the controlled expression in the developing embryo of special regulatory genes called Hox genes. These genes regulate the expression of other genes. Many of these Hox genes contain common “modules” of DNA sequences, called homeoboxes. Only animals possess genes that are ...

Chia sẻ: huyhoang44 | Ngày: 18/03/2020 | Lượt xem: 655 | Lượt tải: 0

Parasitic fungi usually have some hyphae modified as haustoria, nutrient-absorbing hyphal tips that penetrate the tissues of their host. Some fungi even have hyphae adapted for preying on animals.

Chia sẻ: huyhoang44 | Ngày: 18/03/2020 | Lượt xem: 587 | Lượt tải: 0

In seeds plants, the delicate female gametophyte and young embryos are protected from many environmental stresses because they are retained within the moist sporangia of the parental sporophyte. The gametophytes of seed plants obtain nutrients from their parents, while those of seedless vascular plants are free-living and fend for themselves.

Chia sẻ: huyhoang44 | Ngày: 18/03/2020 | Lượt xem: 638 | Lượt tải: 0

Ferns and other pteridiophytes are sometimes called seedless plants because there is no seed stage in their life cycles. The evolution of the seed in an ancestor common to gymnosperms and angiosperms facilitated reproduction on land. A seed consists of a plant embryo packaged along with a food supply within a protective coat. The first seed plan...

Chia sẻ: huyhoang44 | Ngày: 18/03/2020 | Lượt xem: 581 | Lượt tải: 0

Reproduction and life cycles are highly varied among protists. Mitosis occurs in almost all protists, but there are many variations in the process. Some protists are exclusively asexual or at least employ meiosis and syngamy (the union of two gametes), thereby shuffling genes between two individuals. Others are primarily asexual but can also rep...

Chia sẻ: huyhoang44 | Ngày: 18/03/2020 | Lượt xem: 573 | Lượt tải: 0

Molecular evidence accumulated over the last two decades has led to the conclusion that there are two major branches of prokaryote evolution, not a single kingdom as in the five-kingdom system. These two branches are the bacteria and the archaea. The archaea inhabit extreme environments and differ from bacteria in many key structural, biochemical...

Chia sẻ: huyhoang44 | Ngày: 18/03/2020 | Lượt xem: 605 | Lượt tải: 0

For the first three-quarters of evolutionary history, Earth’s only organisms were microscopic and mostly unicellular. The Earth formed about 4.5 billion years ago, but rock bodies left over from the origin of the solar system bombarded the surface for the first few hundred million years, making it unlikely that life could survive. No clear fossi...

Chia sẻ: huyhoang44 | Ngày: 18/03/2020 | Lượt xem: 562 | Lượt tải: 0

Rarer than mineralized fossils are those that retain organic material. These are sometimes discovered as thin films between layers of sandstone or shale. As an example, plant leaves millions of years old have been discovered that are still green with chlorophyll. The most commonfossilized material ispollen, which has ahard organic casethat r...

Chia sẻ: huyhoang44 | Ngày: 18/03/2020 | Lượt xem: 598 | Lượt tải: 0

No single barrier may be completely impenetrable to genetic exchange, but many species are genetically sequestered by multiple barriers. Typically, these barriers are intrinsic to the organisms, not simple geographic separation. Reproductive isolation prevents populations belonging to different species from interbreeding, even if their ranges ove...

Chia sẻ: huyhoang44 | Ngày: 18/03/2020 | Lượt xem: 571 | Lượt tải: 0

The Origin of Species convinced most biologists that species are the products of evolution, but acceptance of natural selection as the main mechanism of evolution was more difficult. What was missing in Darwin’s explanation was an understanding of inheritance that could explain how chance variations arise in a population while also accounting for ...

Chia sẻ: huyhoang44 | Ngày: 18/03/2020 | Lượt xem: 577 | Lượt tải: 0