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  • In this animation well see the remarkable way our DNA is tightly packed up so that 6

  • feet of this long molecule fits into the microscopic nucleus of every cell.

  • The process starts when DNA is wrapped around special protein molecules called histones.

  • The combined loop of DNA and protein is called a nucleosome. Next the nucleosomes are packaged

  • into a thread. The end result is fiber know as chromatin.

  • The fiber is then looped and cooled again, leading finally to the familiar shapes known

  • as chromosomes, which can be seen in the nucleus of dividing cells.

  • Chromosomes are not always present. They form around the time cells divide when the two

  • copies of the cells need to be separated.

  • Using computer animation based on molecular research we are now able to see how DNA is

  • actually copied in living cells. Youre looking at an assembly line of amazing miniature

  • bio-chemical machines that pulling apart the DNA double helix and cranking out a copy of

  • each strand.

  • The DNA to be copied enters the production line from the bottom left. The whirling blue

  • molecular machine is called helicase. It spins the DNA as fast as a jet engine as it unwinds

  • the double helix into two strands. One strand is copied continuously and can be seen spooling

  • off to the right.

  • Things are not so simple for the other strand because it must be copied backwards. It is

  • drawn out repeatedly in loops and copied one section at a time. The end result is two new

  • DNA molecules.

  • What you are about to see is DNA’s most extraordinary secret; how a simple code is

  • turned into flesh and blood. It begins with a bundle of factors assembling at the start

  • of a gene. A gene is simply a length of DNA instruction stretching away to the left. The

  • assembled factors trigger the first phase of the process; reading off the information

  • that will be needed to make the protein.

  • Everything is ready to go. 3-2-1-Go! The blue molecule racing along the DNA is reading the

  • gene it’s unzipping the double helix and copying one of the two strands. The yellow

  • chain snaking out of the top is a copy of the genetic message and it’s made of a close

  • cousin of DNA called RNA.

  • The building blocks to make the RNA enter through an intake hole. They are matched to

  • the DNA letter by letter to copy the A’s, C’s, T’s, and G’s of the gene. The only

  • difference is that in the RNA copy the letter T is replaced is replaced with a closely related

  • building block known as U.

  • You are watching this transcription process in real time. It’s happening right now in

  • almost every cell in your body.

  • When the RNA copy is complete it snakes out into the outer part of the cell. Then, in

  • a dazzling display of choreography all the components of a molecular machine lock together

  • around the RNA to form a miniature factory called a ribosome. It translates the genetic

  • information in the RNA into a string of amino acids that will become a protein.

  • Special transfer molecules, the green triangles, bring each amino acid to the ribosome. The

  • amino acids are the small red tips attached to the transfer molecules.

  • There are different transfer molecules for each of the twenty amino acid. Each transfer

  • molecule carries a three letter code which is matched with the RNA in the machine.

  • Now we come to the heart of the process. Inside the ribosome the RNA is pulled through like

  • a tape. The code for each amino acid is read off, three letters at a time and matched to

  • three corresponding letters on the transfer molecules. When the right transfer molecule

  • plugs in, the amino acid it carries is added to the protein chain.

  • Again, you are watching this in real time.

  • And after a few seconds the assembled protein starts to emerge from the ribosome. Ribosomes

  • can make any kind of protein. It just depends on what genetic message you feed in on the

  • RNA. In this case the end product is hemoglobin.

  • The cells in our bone marrow churn out a hundred trillion molecules of it per second, and as

  • a result our muscles, brain, and all the vital organs in our body receive the oxygen they

  • need.

In this animation well see the remarkable way our DNA is tightly packed up so that 6

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B2 中上級

DNA複製 (DNA Replication)

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    Leo に公開 2021 年 01 月 14 日
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