2.3 gene expression

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what is gene expression

using the information from a gene to synthesize a functional product (protein)

what does ‘transcription’ mean in terms of the mRNA synthesis

to produce a written copy

where does transcription occur

• in the nucleus, where DNA is stored

• the information in the DNA is ‘copied’ into messenger RNA (mRNA)

what is the process of transcription

1. RNA polymerase → key transcription enzyme

2. RNA polymerase binds to promoter sequence and unwinds DNA double helix

3. one DNA strand is used as a template to synthesize mRNA via complementary base pairing

4. mRNA transcript is synthesized in a 5’ to 3’ direction

5. RNA polymerase encounters termination sequence and stops mRNA synthesis

what is the characteristics of the resulting mRNA transcript

• complementary to the template strand

• identical to the non-template strand

  • all the ‘T’s are changed to ‘U’s

what is the process of RNA processing

1. pre-mRNA → newly synthesised mRNA transcript

2. pre-mRNA is processed into mature mRNA before export from the nucleus into cytoplasm

3. mature mRNA is translated by ribosomes in the cytoplasm

why does a newly transcribed mRNA have to be spliced

because the newly-transcribed mRNA contains additional non-coding sequences (introns) that interrupt protein-coding sequences (exons) that has to be removed

what does ‘translation’ mean when it comes to protein synthesis

to change into another language

what occurs in the cytoplasm during translation

• mature mRNA is exported from nucleus through nucleus pores

• ribosomes bind to the mRNA to initiate protein synthesis

what is a codon

a triplet of nucleotides on the mRNA transcript that is non-overlapping, reading from 5’ to 3’ on mRNA

what is used to translate mRNA codons into amino acids

genetic code

what can codons translate to

• specific amino acid

• translation start/stop signal

  • AUG → start codon

  • UAA/UAG/UGA → end codon

what is the role of ribosomes

to bind the mRNA and tRNA and catalyzes the synthesis of polypeptide chain

what are the characteristics of a ribosome

• molecular machines made from protein and RNA

• can be free in cytoplasm or membrane-bound on ER (endoplasmic reticulum)

• comprises 2 subunits

  • large or small

what generally determines the gene expression

transcription

• only genes that are transcribed are active within the cell

what would be some consequences of what would happen by varying the levels of transcription

• some genes may be permanently switched on or off within a cell → leading to cellular differentiation

• other genes may be switched on or off in response to internal or external signals → regulating cellular activity

what are the mechanisms of regulating transcription

• epigenetic modification

• transcription factors

what does epigenetic modification do

the epigenetic modification of chromatin makes the DNA more/less accessible to the transcription machinery

what does transcription factors do

transcription factors (regulatory proteins) bind to control elements (regulatory sites on DNA) to initiate and control transcription