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Chapter 11 Gene Function1 Genes Function: genes in action Genes in Action Chapter 11 Pp 385 - 417.

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Presentation on theme: "Chapter 11 Gene Function1 Genes Function: genes in action Genes in Action Chapter 11 Pp 385 - 417."— Presentation transcript:

1 Chapter 11 Gene Function1 Genes Function: genes in action Genes in Action Chapter 11 Pp

2 TOPIC The ‘role’ of genes: Case Study: beta Thalassaemia 11.1 Pgs: 385 – 388 then Pgs:

3 Genes ‘n’ action Case Study: beta Thalassaemia various kinds of haemoglobin are found in red blood cells each kind consists of four protein chains each with an iron-containing heam molecule the gene that controls the production of beta chains of haemoglobin a (one of the four chains!) is the hbb gene on chromosome 11 inheritance of the t gene is autosomal t – normal, t - thalassaemia absence of beta chains is an inherited disorder known as beta thalassaemia Task: Complete Quick Check 1-3 3

4 HBB gene (Chromosome 11) has 1600bp consisting of Exons (coding for the proteins) and Introns. In all, 146 amino acids are produced in a normal beta chain Affected thalassaemia sufferers have a base substitution in which T is replaced by A Base substituion affects the seventh codon in mRNA Instead of AAG, UAG is produced. UAG codes for STOP Quck Check Q’s: 14 & 15 4

5 TOPIC – PROTEIN SYNTHESIS Genes in Action: Transcription & Translation 11.2 Pgs: 388 – 402 GENE ACTION When a gene becomes active (to produce proteins!), it first makes a mobile copy of the coded instructions that it contains. This occurs by a process known as transcription. This mobile copy of a genetic instruction can leave the nucleus and move to the cytoplasm where the instruction is decoded. This occurs by a process known as translation. So gene action involves two processes: transcription and translation Biozone 202 5

6 6 TRANSCRIPTION Defn: Transcription is the process by which the code contained in the DNA molecule is transcribed (rewritten) into a mRNA molecule transcription involves making a copy of the DNA template. A special molecule called messenger RNA (mRNA) is produced. Consider a DNA template with the base sequence … (3’) ATGCCTGAAT (5’).. This DNA acts as a template to guide the formation of a mRNA molecule with the complimentary base sequence as follows: (5’) UACGGACUUA (3’).. See Biozone pg 205

7 Chapter 11 Gene Function7

8 8 Steps in TRANSCRIPTION (5’) TACGGACTTA (3’).. (3’) ATGCCTGAAT (5’).. Purpose of Transcription! to make a complimentary strand of mRNA! At the start … DNA is made of two complimentary strands.  one is the DNA coding strand (5 – 3) (5’) TACGGACTTA (3’).. (3’) ATGCCTGAAT (5’)..  the other is the DNA Template strand (3 – 5) At the start of Transcription the double stranded DNA molecule has to unzip and the template copied! (5’) UACGGACUUA (3’).. 1: Unzipping DNA… enzyme known as RNA polymerase attaches to a region of DNA in the upstream region of the template strand The double stranded DNA of the gene unwinds and exposes the bases of the template strand.

9 9 3'3' 3' 5' Nascent RNA Template Strand 5'5' 5'5' 3' Co din g Str and Elongat ion Site RNA Polymerase Unwinding Rewinding Movement of polymerase 2. The base sequence of the DNA Template strand guides the building of a complimentary copy of mRNA. The RNA polymerase enzyme moves along the DNA template and as it moves (RNA) nucleotides are brought into place one by one to form a RNA chain 3. The single stranded RNA molecule called pre-messenger RNA (pre-mRNA) is fully complimentary to the original DNA Template molecule. the enzyme transcribes only a gene length of DNA at a time only RNA polymerase is involved in mRNA synthesis It is common to find more than one enzyme transcribing on the same length of gene (but, we are not finished with Transcription!) R

10 Pre-mRNA is modified after transcription 4. The regions of the pre-mRNA that corresponds to the introns in the coding region of the gene are cut out, producing a shorter mRNA molecule. (Splicing of pre mRNA is carried out by a complex known as splicosome, a complex consisting of RNA and protein) 5. The final mRNA molecule is chemically capped with a a methyl “cap” (AUG) Finally, a long tail of ‘A’ (Adenine’s) called a poly-A tail is added to produce a fully functional mRNA strand that is ready for transport out of the nucleus. Once mRNA is formed, it leaves as a small single strand through the nuclear pores. Once in the Cytoplasm the mRNA will engage ribosomes to begin the next stage in protein synthesis

11 11 Quick Check 4 – 8 page 391

12 12 TRANSLATION Defn: The decoding of the genetic instructions occurs through the process of translation which takes place in the cytoplasm. by the end of this process the mRNA have been decoded and translated into a protein chain of amino acids. Key Parts of Translation (see Table 11.3) mRNA Nuclear pore cytoplasms ribosomes tRNA (with anti codon) Amino acids – (forming polypeptides)

13 Chapter 11 Gene Function13 tRNA A key player in Translation tRNA = Transfer RNA Each tRNA consist of a single strand of 76 coiled RNA nucleotides At one end of the tRNA molecule are three bases that make up an anti-codon (complimentary to a codon) At the other end of the tRNA molecule is a region that attached to one specific amino acid An enzyme, amino acyl tRNA synthetase catalyses the linking of each amino acid to a tRNA

14 14 TRANSLATION – The mRNA leaves the nucleus through the pore and attaches to sub-microscopic particles known as ribosomes Each ribosome consists of two sub units Inside the ribosome, the mRNA codon lines up with the tRNA’s anticodon (they are complimentary Each loaded tRNA has an amino acid attached As the mRNA moves through the ribosome, each codon is ‘read’ (translated) As read, the mRNA moves one codon at a time and with each, a tRNA attaches its amino acid to the growing chain on amino acids!

15 Chapter 11 Gene Function15

16 16 TRANSLATION – DECODING INSTRUCTIONS Biozone 206

17 Task: Translating Code 17 Using the table on the previous page…. 1.Write the amino acid sequence from the following DNA template strand tac gga gag gca cct act _____ _____ _____ 2.Write the amino acid sequence if a spontaneous mutation occurred at the sixth nucleotide and cystosine was transcribed onto the pre mRNA strand instead of uracil _____ _____ _____ _____ _____ _____ What kind of mutation is the above?: Substitution or frameshift? Silent, Nonsense or Missense 3.List the amino acid sequence that would result if an insertion of G occurred between the 11 th and 12 th base _____ _____ _____

18 18 Protein Synthesis Summary Biozone: – Review of Protein Synthesis

19 genes but more than characteristics? ‘Alternative Splicing Theory’ How might one gene produce different proteins at different stages of development and in different tissues? Alternative splicing: involves the retention of some introns and or juggling of exons in the prioduction of mRNA resulting in the production of different proteins. 1.Intron retention: 2.Exon Juggling Quick Check

20 COMPARING PROKARYOTES & EUKARYOTES Chapter 11 Gene Function20 Similar with some differences: CharacteristicProkaryotes I.E. Bacteria Eukaryotes Location of genesCytoplasmnucleus age of mRNASurvives only a few minutes Can survive for days Post- transcriptional modification Doesn’t occurOccurs (introns cut out, cap added, poly A tail added Effect of tetracycline on transcription Very effective. Prevents transcription No effect (Nucleus protects!)

21 21 T T halassaemia Biozone– Inherited Metabolic Disorders Case Study p Quick Check

22 22 rRNA A key player in Ribosomes rRNA = Ribosomal RNA Key component of ribosome structure Stored in nucleus as a nucleolus Need lots of it Genes on short arms of chromosomes 13, 14, 15, 21 and 22 code for the production of rRNA The loci of the above are called nucelar organiser regions (NORs) A secondary constriction or narrowing, marks the position of NOR on each of these chromosomes (see Figure 11.20

23 23 Structural and Regulator genes - - Page Genes vary in function: - - Production of RNA All genes produce RNA of some kind (mRNA, rRNA, tRNA!) Structural Genes: produce proteins that become part of the structure and function of organisms Regulator Genes Produce proteins that control the action of other genes Example: Homeotic Genes in insects; HOX genes in Mammals. Actions of Regulator Genes Some produce DNA – Binding Proteins (directly switch genes on or off) Some produce proteins that bind to receptor sites on membranes and function as ‘signalliny proteins

24 TOPIC DNA REPLICATION 11.3 Pgs: then Pgs:

25 25 DNA Replication Defn: The process by which DNA makes exact copies of itself is called DNA Replication When: must occur prior to Interphase Where: for Eukaryotes – nucleus What Cells: Somatic cells (prior to Mitosis) Germ line cells (prior to meiosis) Outcome: Two Double stranded DNA molecules from each one Each ‘new’ molecule consists on one strand of original DNA and one copy! This is known as the semi-conservative model

26 26 PROCESS OF DNA REPLICATION Step One: Unwinding DNA Unwinds to form a region of single stranded DNA Unwinding controlled by enzyme called DNA helicase Occurs at many points along the DNA Step Two: Copying Short RNA Primers attach to the open strands and initiate replication Each ‘open’ strand acts as a template for a new DNA strand Leading strand is formed one nucleotide at a time; Individual DNA nucleotides are added at the 3’ end according to base pairing rules DNA polymerase is responsible for catalysing this reaction Lagging strand formedby fragments (chuncks) aided by enzyme DNA Ligase Result   Each new copy of DNA is an exact copy of the original   Each newdouble stranded molecule contains one of the original DNA strands and one new DNA strand (semi conservative) Biozone: Quick Check 18 & 19

27 Chapter 11 Gene Function27

28 ACTIVE GENES When are genes active? Many genes remain active throughout the life of a person, eg genes controlling enzymes for cellular respiration. Some genes are not expressed on a phenotype until a person is well into adulthood. Eg. Huntington disease. Identifying active genes Defn: Microarrays (DNA Arrays): technique used to Recognise ‘active’ genes using single stranded DNA

29 29 Switching genes off! Taken From Page 410 RNA interference (RNAi) provides a means of selectively targeting and silencing genes. Small interfering RNAs (siRNAs) produced in cells are the active molecules in gene silencing. How Does RNA Interference Work? p410 Case Study – Blue Roses at last! p411 Quick Check Questions p412 Review Biozone – Gene Expression

30 Chapter 11 Gene Function30 Genes in Action Review Biochallenge Questions 1-5 p413 Chapter Review Questions Add Key Words to Glossary List Complete Questions 2-14: p


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