7.2 : Transcription & Gene Expression Flashcards
where does transcription occur and what is it?
- occurs in the nucleus.
- a complementary copy of the genetic information in a part of a DNA molecule (a gene) that is made by the building of a molecule of mRNA.
- when DNA triplet codes are transcribed into codons in mRNA.
enzyme in transcription & what is the differing nucleotide in RNA than DNA?
- RNA polymerase
- RNA has Uridine (U) in RNA synthesis (not Thymine)
what is the coding strand?
one strand of the DNA double helix that serves as a template for the synthesis of mRNA.
how does transcription occur?
DNA double helix unwinds –> hydrogen bonds are weakened and broken where the gene is being transcribed –> RNA polymerase recognises this & binds to promoter region (‘start signal’ for transcription - located before the gene) –> RNA polymerase draws & selects free nucleotides from the surrounding pool of nucleotides (as nucleotide triphosphates) & matches complementary nucleotides –> working in a 5’-3’ direction–> RNA synthesis occurs (nucleotides joined onto growing mRNA strand = CONDENSATION REACTION).
–> transcription initiated –> RNA polymerase moves along DNA –> unwinding helix –> exposing DNA nucleotides –> RNA nucleotides can pair & mRNA strand is formed & peels away
how do the newly formed complementary bases stay intact?
due to hydrogen bonds that form between complementary bases ( A & U, C & G)
where does the condensation reaction occur in a growing mRNA strand?
when a growing mRNA strand is forming, the sugar and phosphate groups of adjacent nucleotides condense together by RNA polymerase (enzyme).
when does the transcription process stop?
until a base sequence referred to as the ‘transcription termination region’ is reached –> at this signal, RNA polymerase & the new completed mRNA strand –> freed from the site of the gene –> DNA double strand reforms into a compact helix at the site of transcription.
–> mRNA leaves the nucleus through pores in nuclear membrane –> passes to ribosomes (cytoplasm) –> where genetic info is read and used.
what do many genes in eukaryotes have within their boundaries & what are these?
non-coding DNA sequences - Introns : non - coding regions of an RNA transcript (= RNA complementary copy of a gene - after transcription) or the DNA encoding it, that are eliminated by splicing before translation.
what are exons?
the part of a gene that codes for amino acids (genetic information)
why does a gene consisting of exons and introns have to go through post transcriptional modification?
when a gene that consists of exons and introns is transcribed into mRNA (mRNA formed) –> it will contain a sequence of introns & exons exactly as they occur in DNA –> would cause problems when being transcribed & read in ribosomes (protein synthesis) –> as it is unmodified.
what is post-transcriptional modification?
a enzyme-catalysed reaction in which the introns (non-coding) are removed as soon as the mRNA strand is formed. = part of RNA chain that is not meant to be translated into proteins
what is RNA splicing?
a process that removes introns (non-coding) from pre-mRNA and joins exons together to enable translation.
(resulting in shortened mRNA lengths = mature –> passes to cytoplasm –> involved in protein synthesis in ribosomes)
what do genes of prokaryotes not have?
do not have introns –> can’t undergo splicing due to absence of enzymes required.
what do all cells in an organism have in relation to genome?
they all have the same genome –> expression of genes is related to when & where the proteins they code for are needed.
example of genes that are expressed all the time
genes that are responsible for routines & metabolic functions –> seen in respiration.
