Higher Human Biology Unit 1 Human Cells KEY AREA 3: Gene Expression

Human Cells Learning Intentions KEY AREA 3 – Gene Expression 1.Gene Expression 2.RNA Structure & Function 3.Transcription 4.Translation 5.Post-translation

3a) Gene Expression The genetic composition of a cell is called the cell genotype The cell genotype is determined by the sequence of DNA bases in its genes The cell phenotype is determined by the proteins that are made when the genes are expressed Gene expression is how the cell genotype is shown in the cell phenotype (proteins produced) Only a fraction of the genes in a cell are expressed

3b) Gene Expression Gene expression is controlled by the regulation of Transcription and Translation Gene expression is influenced by intra and extra-cellular environmental factors The order of bases on DNA determines the sequence of amino acids that are made The sequence of amino acids joined together in polypeptide chains determine the structure, shape, function of the protein produced

3c) RNA Structure & Function of RNA RNA is a single strand of RNA nucleotides RNA nucleotides contain a ribose sugar, a phosphate and a base RNA nucleotides contain the bases Adenine, Uracil, Cytosine, Guanine mRNA (messengerRNA) carries a copy of a section of the DNA code for a specific protein from the nucleus to the ribosome rRNA (ribosomalRNA) and proteins form a ribosome tRNA (transferRNA) carries a specific amino acid

3d) Transcription  Transcription is the copying of the DNA code for a specific protein into mRNA  Transcription of DNA into primary and mature RNA transcripts occurs in the nucleus  Stages in Transcription 1. A promotor region of DNA initiates transcription 2. RNA polymerase enzyme moves to the specific section of DNA and unwinds and unzips the DNA double helix at that point 3. RNA nucleotides pair with complementary DNA base pairs (A-U, G-C) forming mRNA 4. RNA polymerase can only add nucleotides to the 3’ end of mRNA 5. RNA polymerase joins the nucleotides together to form a new sugar- phosphate backbone 6. The mRNA becomes separated from the DNA template, and is called the primary transcript of mRNA

3d) Transcription

3e) Transcription  Not ALL nucleotides in a gene play a role in the coding for the amino acids sequence  Introns are non-coding regions of genes  Exons are coding regions of genes  Introns are found between the Exons  The primary transcript of mRNA which codes for the polypeptide chain of amino acids is fragmented  Introns are cut out and removed from the primary transcript  Exons are spliced together (RNA splicing) to form mRNA with a continuous sequence of nucleotides (this is called the mature transcript of mRNA)  The mature transcript of mRNA moves from the nucleus through the cytoplasm to a ribosome Transcription Animation (1:52)

Splicing (1:37)

3f) Translation  mRNA is linear and carries the DNA code in the form of mRNA codons (3 bases=a codon)  A mRNA codon is complementary to the triplet of bases on the original DNA strand  tRNA is found in the cytoplasm  tRNA is folded on itself due to hydrogen bonds forming between bases  tRNA exposes triplets of bases called tRNA anticodons  tRNA anticodons are complementary to a mRNA codon and have attachments sites which bind to a specific amino acid  tRNA pick up specific amino acids in the cytoplasm and carry them to the ribosome  mRNA codons and tRNA anticodons translate the genetic code into a sequence of amino acids

3f) Translation

3f) Stages of Translation 1. mRNA binds to the ribosome 2.Each tRNA picks up specific amino acids in the cytoplasm and carries it to the ribosome 3.Each tRNA anticodon binds to a complementary codon on the mRNA lining up the amino acids in a specific order 4.Peptide bonds form between the amino acids to form a growing polypeptide chain. 5.When the chain is complete it detaches from the ribosome and will be modified 6.When the tRNA detaches from its amino acid, it then collects another

3f) Translation

3g) Translation  mRNA codons AUG complementary to tRNA anticodon UAC codes for the amino acid methionine (met) AND acts as the START CODON  mRNA codons UAA, UAG & UGA do not code for amino acids, but they act as STOP CODONS  Ribosomes have one binding site for mRNA and three binding sites for tRNA  Site 2 holds the tRNA carrying the amino acids and growing polypeptide chain  Site 3 holds the tRNA carrying the next amino acid  Site 1 discharges the tRNA from the ribosome once the amino acid has become part of the polypeptide chain  A string of ribosomes carrying out multiple translation on the same mRNA strand is called a polyribosome Translation animation (2:04)

3h) Post-Translation  Once translation is complete, further modifications (in addition to folding & coiling) may be required to enable a protein to perform its specific function  One gene can create many proteins as a result of RNA splicing and post translational modification  Different mRNA molecules are produced from the same primary transcript depending on which RNA segments are treated as exons and introns  Post-translation protein structure can be modified by:-  Cutting (cleaving) the polypeptide chain to make it active e.g. insulin must have its central section cut by protease enzymes to make the protein active  Combining polypeptide chains  Adding a carbohydrate e.g. mucus is a glycoprotein containing protein & a carbohydrate  Adding a phosphate e.g. regulatory proteins need phosphate added to make them function

Human Cells Questions KEY AREA 3 – Gene Expression 1.Testing Your Knowledge 1Page 37 Q’s Testing Your Knowledge 2Page 43Q’s What you Should KnowPage 43Q’s Quick Quiz