Answer:
Watson and Crick proposed the double helical model of DNA.
Explanation:
- This model says that DNA exist as double helix. It has two polynucelotide strands running in opposite direction i.e the strands are anti-parallel.
- Each polynucleotide strand is made up of several nucleotide liknked together by phophodiester bonds.
- The two strands are not coiled upon each other but the double strand is coiled upon itself around a common axis like the spiral stair case. Here base pairs forms the stairs and backbone of two strands form railing. Back bone contains sugar and phosphate.
- The base pairing is specific, adenine always pairs with thymine and guanine always pairs with cytosine. Thus if we see any base pair, it consists of one purine and one pyrimidine.
- The two strands of DNA thus show complementary base pairing.
- The strands are held together because of hydrogen bonding between the bases in opposite strand.
- One full helical turn involves ten base pairs and the base pairs are stacked 3.4 A apart.
- This form is called as B-form of DNA.
It would cause the cell membrane to "pop" like a balloon. b the cell would not be able to get enough of the gases and nutrients it needed. c the cell could not be part of a small organism, such as a mouse. d there would not be enough resources in the cell's environment to feed it.
Answer:
Earth's surface didn't look much different. With few exceptions.
The global climate is cooler and drier, and seasonal, similar to modern climates. As the climate began to cool down grasslands continued to expand and forests started to dwindle in extent. In the seas, kelp forests made their first appearance and soon became one of Earth's most productive ecosystems. Water from the Atlantic Ocean poured in through the Strait of Gibraltar to deluge the Mediterranean Basin. The event is called the Zanclean flood. Parts of of southern Norway and southern Sweden that had been near sea level rose to form the Hardangervidda plateau and the South Swedish highlands.
Shorty after, the glaciations, uplift of the Rocky mountains and Panama seaway closure began to reshape the Earth.
Explanation:
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(A nebula)
is a cloud of gas (hydrogen) and dust in space. Nebulae are the birthplace biths. There are different types of nebula. An Emission Nebubla such as Orion nebula, glows brightly because the gas in it is energised by the stars that have already formed within it.
(A star)
is a luminous globe of gas producing its own heat and light by nuclear reactions (nuclear fusion). They are born from nebulae and consist mostly of hydrogen and helium gas.
(red giant)
This is a large bright star with a cool surface. It is formed during the later stages of the evolution of a star like the Sun, as it runs
out of hydrogen fuel at its centre.
(red dwarf)
These are very cool, faint and small stars, approximately one tenth the mass and diameter of the Sun. They burn very slowly and have estimated lifetimes of 100 billion years.
(white dwarf)
This is very small, hot star, the last stage in the life cycle of a star like the Sun. White dwarfs have a mass similar to that of the Sun, but only 1% of the Sun's diameter; approximately the diameter of the Earth.
(Black holes)
are believed to form from massive stars at the end of their life times. The gravitational pull in a black hole is so great that nothing can escape from it, not even light. The density of matter in a black hole cannot be measured.
