133206
Description
Mind Map by harry_bygraves, updated more than 1 year ago
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Created by harry_bygraves
over 11 years ago
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Transcription
- DNA carries the inherited instructions for making polpeptides. DNA however, needs to be kept intact in the
nucleus as the permenant store of gentic information. Therefore it is not used directly for protein synthesis
- The section of DNA that holds the information for one polypeptide chain is called a cistrone or gene.
Before transcription can take place, the double helix in the cistron has to unwind and the two
polynucleotides chains have to seperate in order to expose the nucleotide bases. This is done with the
help of an enzyme. Only one of the strands is used as a template for the synthesis of mRNA
- The basic mechanics of trnascription is similar in both eukaroytes and prokaryotes; The
two strands that make up the double helix of DNA are prized apart by an enzyme called
RNA polymerase. hoydigen bonds are broken, exposing the bases in the transcribing
DNA strand. The polymerase attaches to the strand at aparticular base sequence, the
promotor,intiating transcription. During transcription, the polymerase moves along the
cistron in the 5'-3' direction. It passes over the nucleotides inthe transcribing DNA strand
one at a time and builds up mRNA by adding complementary nucleotides as it goes.
When the enzyme moves on to another region of the transcribing DNA strands, the
double helix of DNA reforms behind it. On reaching a special 'stop' sequence called a
terminator, the enzyme detaches and the mRNA molecule peels away from the DNA
- In eukaryotes, the nuclear envelope isolates DNA from the ribosomes. This means
that transcription and translation have to take place at different times and in different
locations. Transcription occurs in the nucleus and translation in the cytoplasm
- The mRNA that leaves the nucleus is much shorter that the length of the DNA that is initally transcribed.
This length difference is becuase eukaryotic genes contain nucleotide base sequences called introns that
do not code for polypeptides, as well as base sequences called exons that are expressed
- The eukaryotic RNA intially transcribed from DNA is called pre-mRNA. It
includes both exons and introns. The introns are interspersed between the
coding segments. To make mRNA, the introns are removed and the exons
are spliced together by splicosomes, intracellular structures composed of
RNA and protein molecules. Splicosomes are almost as large as ribosomes
- In RNA spicing, a splicosome interacts with the ends of an intron, cutting it
at specific points and then immediately joining the exposed ends of
adjacent exons. It is only after this processing that mRNA leaves the
nucleus, moves onto ribosomes in the cytoplasm, and is used in translation
- In addition to RNA splicing, before pre-mRNA is converted
to mRNA, its ends are modified. the 5' end, made first during
transcription is capped with a modified guanine nuleotide.
This 5' cap helps to protect the mRNA from hydrolysis, and
signals the point of attachment when mRNA reaches a
ribosome. The 3' end is modified by having 50 to 250
adenine nucleotide incorporated into it. This poly(A)tail
inhibits degradation facilities movement of mRNA into the
cytoplanm and helps ribosomes attach to the mRNA.
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