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.
example of genes that are expressed at a selected stage in a cell / tissue development
seen in cells from stem cells that develop to muscle fibres / neurons.
example of genes that are only expressed in mature cells
seen in genes responsible for antibody production in mature plasma cell.
example of genes that are expressed in receipt of an internal/external signal.
seen when a particular hormone/metabolic signal or nerve impulse is received by cell (activated gene)
–> gene for insulin production in B cells in islets
what is a chromatin?
chromatin consists of complexes of histones and DNA , which coils up to make a chromosome inside the nucleus.
what are chromatids & where are they found?
- two strands of replicated chromosome.
- found in a chromatin which has two sister chromatids.
what are histones?
proteins that aid in condensing the DNA in the nucleus into chromatin.
how is DNA structured within a chromatin?
In a chromatin –> DNA double helix is wrapped around histones –> forming histone protein beads / nucleosomes (stable protein-DNA complexes)
what role does acetylation play in histone tails?
histone tails can be chemically modified by enzymes by acetylation –> addition or removal of an acetyl group.
what is histone acetylation?
first step in the initiation of transcription of a gene = the addition of an acetyl group to a particular amino acid (lysine) at the end of histone tails –> loosens the tight binding of nucleosomes.
