Key Area 1.3: Gene Expression

Transcription

1 Key Area 1.3: Gene Expression

2 RNA There is a second type of nucleic acid in the cell, called RNA. RNA plays a vital role in the production of protein from the code in the DNA.

3 What is gene expression? Gene expression involves the transcription and translation of DNA sequences. Only a fraction of the genes in a cell are expressed. Transcription and translation involves three types of RNA (mrna, trna and rrna).

4 Differences between DNA and RNA 5 end RNA nucleotides are similar in structure to DNA, except Ribose sugar replaces Deoxyribose sugar Phosphate 3 end Ribose Sugar and Uracil replaces thymine 5 pronounced 5 prime

5 The next slide shows the differences between DNA & RNA

6 DNA RNA Double stranded Single stranded Deoxyribose sugar Ribose sugar Bases: A, T, C and G Bases: A, U*, C and G *Uracil replaces thymine

7 Types of RNA Transcription and translation involves 3 types of RNA: 1. Messenger RNA (mrna) 2. Transfer RNA (trna) 3. Ribosomal RNA (rrna)

8 mrna mrna is formed in the nucleus from free nucleotides and carries a copy of the DNA code from the nucleus to the ribosomes (where protein synthesis occurs). Phosphate Ribose Sugar RNA nucleotide Organic Base

9 trna molecules collect amino acids and bring them to the ribosome to build proteins. A trna molecule has an anticodon (an exposed triplet of bases) at one end and an attachment site for a specific amino acid at the other end. trna

10 rrna molecules combine with proteins to create the ribosome the organelle responsible for assembling proteins following the DNA code. rrna The Ribosome Blue = Proteins Orange = rrna subunit 1 Yellow = rrna subunit 2

11 Transcription

12 Protein synthesis We will look at the process of protein synthesis in detail

13 DNA Nucleus A G A G G T T G A C G A A T C T C C A A C T G C T T Overview of gene expression Transcription mrna U C U C C A A C U G C U U codon Ribosome ser pro thr ala Translation Protein

14 Transcription Transcription is the synthesis of mrna from a section of DNA. Transcription of a gene starts from a region of DNA known as the promoter.

15 Promoter: Start of a gene Terminator: End of a gene

16 What is RNA polymerase? This enzyme is responsible for transcription. RNA polymerase binds at the promoter and unwinds the DNA. It breaks the hydrogen bonds between the bases. Responsible for the Synthesis of a primary transcript of mrna.

17 How does it do this? What happens next? 1. RNA polymerase adds nucleotides onto the 3 end of the growing mrna molecule. 2. Due to the base-pairing rules the mrna produced will be complementary to the DNA. 3. The molecule elongates until it reaches the terminator sequence. 4. The molecule produced is called the primary transcript.

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19 Modification of the primary transcript to a mature transcript. Not all the regions in a eukaryotic gene are required to produce the final protein. These non-coding regions are called introns. The coding regions are called exons. The introns of the primary transcript are non-coding regions, and are removed. The exons of the primary transcript are coding regions and are joined together to form the mature transcript.

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21 Translation

22 Genetic code Translation is the synthesis of protein following the code with in the mature mrna transcript. The mrna is made of sequences of three nucleotides (a triplet of bases) called codons. Each codon is code for one amino acid.

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24 trna A further type of RNA is found in the cell s cytoplasm. This is called trna (transfer RNA) and is made of a single chain of nucleotides. It is folded into a 3D structure, held together by hydrogen bonds. Each trna has an attachment site for a specific amino acid and a triplet of bases known as an anticodon. Many different types of trna are present in cell, one or more for each type of amino acid. The trna picks up its appropriate amino acid and takes it to the ribosome to be matched with the mrna.

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27 The Ribosome Ribosomes are small, roughly spherical structures found in all cells. They contain the enzymes essential for protein synthesis. The ribosome s function is to bring the trna molecules bearing amino acids in contact with the mrna.

28 Ribosomes

29 Site P holds the trna carrying the growing polypeptide chain. Site A holds the trna carrying the next amino acid to be joined to the chain. Site E releases the empty trna once it has dropped off its amino acid.

30 The translation process 1. The ribosome binds to the 5 end of the mrna so that the start codon (AUG) is in site P. 2. Next a trna carrying an amino acid becomes attached to site P. 3. The mrna codon at site A bonds complementary anticodon on the appropriate trna bearing the correct amino acid. 4. A peptide bond then forms between these two amino acids.

31 5.. The ribosome then moves along one codon. 6. The trna from Site P moves to Site E and released. 7. Steps 3-6 then repeat until it reaches a stop codon.

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33 Translation Summary trna is involved in the translation of mrna into a polypeptide at a ribosome. Translation begins at a start codon and ends at a stop codon. Anticodons bond to codons by complementary base pairing, translating the genetic code into a sequence of amino acids. Peptide bonds join the amino acids together. Each trna then leaves the ribosome as the polypeptide is formed.

34 One gene many proteins The same gene can be used to make several different proteins by a process called Alternative RNA splicing Different mature mrna transcripts are produced from the same primary transcript depending on which exons are retained.

35 Alternative RNA Splicing Previously we learned that the primary transcript is separated into exons and introns and the exons are spliced together to make the mature transcript. Under certain conditions alternative segments of RNA may be treated as exons and introns. In other words one gene can produce several different mature mrna transcripts. And therefore, several different proteins.

36 Primary transcript Different proteins are formed Alternatively spliced mature mrna transcripts Different mrna molecules are produced from the same primary transcript depending on which segments are treated as exons and which are treated as introns

37 Protein Structure A protein is composed of a chain of amino acids held together by peptide bond, to form a polypeptide chain This chain is then folded to produce a 3D structure stabilised by hydrogen bonds and a variety of bonds dependent on the amino acid.

38 Types of Protein FIBROUS GLOBULAR CONJUGATED Collagen Elastin Keratin Enzymes Antibodies Hormones Chlorophyll Haemoglobin

39 Gene Expression Your PHENOTYPE ( physical characteristics) are determined by the proteins you produce. The proteins you produce are controlled by the gene being expressed (switched on) Only a fraction of genes in a cell are expressed. GENE EXPRESSED PROTEIN PHENOTYPE

40 Gene Expression Gene expression can be influenced by intra cellular and extra cellular environmental factors. It is controlled by the regulation of transcription and translation.