Human Cells Gene Expression Higher Human Biology Human Cells Gene Expression

How do genes control protein synthesis? In this topic you will be Revising your knowledge on protein synthesis Learning more details on protein synthesis Revising your knowledge on RNA Learning more about the structure and function of RNA Learning more about how genes control phenotype Learning more about what controls gene

N5 Revision What can you remember from N5 about how proteins are synthesised? Draw a diagram to detail all you know about protein synthesis and RNA

How does DNA code for life? There are approximately 3 billion base pairs in our DNA The DNA is organised into genes Genes contain a sequence of bases that code for specific proteins The sequence of bases are unique Humans have 20,000 to 25,000 essential genes This only makes up a small part (1 - 2%) of all the DNA Scientists are still working on what the other DNA does

What is all the DNA doing? When RNA is produced not all is coding for protein production. The parts for the RNA coding to produce proteins are called Exons. The parts of the RNA are not coding for proteins are called Introns. When RNA is produced sections of the code must be removed (spliced out) as they are not needed.

Cell DNA All somatic cells contain a full complement of chromosomes These chromosomes are the cells genotype These chromosomes determine the phenotype of the cell This is because the DNA is a sequence of bases that code to make protein if they are expressed by a cell Only a fraction of genes are expressed in cells

Gene expression Gene expression is affected by many factors acting both inside and outside the cell These are intra and extra cellular environmental factors They include chemicals such as hormones and enzymes as well as time and space Gene expression includes 2 processes Transcription Translation

Another type of nucleic acid DNA is a type of nucleic acid and forms the genetic code Gene expression involves another type of nucleic acid RNA (ribonucleic acid) RNA is different to DNA Feature DNA RNA Number of nucleotide strands 2 1 Base paring with adenine thymine uracil Sugar present deoxyribose ribose

Types of RNA There are 3 types of RNA to consider mRNA – the single strand copy produced from the DNA tRNA – the single strand molecule that transfers a specific amino acid and aligns it with the mRNA rRNA – RNA associated with the ribosomes

Transcription At gene expression only a small portion of DNA will be copied to produce mRNA The section to be copied has a promoter and a terminator segment This ensures copying starts and finishes at the correct points Transcription is controlled by an enzyme RNA polymerase

RNA polymerase This enzyme is responsible for controlling unwinding, unzipping and the construction of the nucleotide strand of mRNA This enzyme can only join nucleotides to the strand using the free 3C end of the next nucleotide Complementary base pair rules apply except RNA has Uracil (U) instead of thymine (T) This initial process produces the primary transcript of mRNA

Primary transcript

Transcription – modifying the initial mRNA The primary transcript contains the essential sequence of code to produce a specific protein (exons) These are interspersed with non coding regions called introns In the nucleus the mRNA has these non-coding regions (introns) removed (cut out) The exons are spliced together to produce the continuous code for the protein This changed mRNA is called the mature transcript This process is called RNA splicing

RNA splicing

Overview transcription

Consolidation Draw your own diagram to show this process Answer questions on page 37 in the text book Look at scholar

Translation This is the process of converting the message on the mRNA into a protein molecule It takes place at ribosomes in the cytoplasm The mRNA is a series of base triplets or codons A codon will be complimentary to a sequence of 3 bases on a tRNA molecule (anticodon) carrying a specific amino acid

Ribosomes Ribosomes contain rRNA and enzymes essential for protein synthesis

Ribosomes Ribosomes have 1 binding site for mRNA and 3 binding sites for tRNA (E, P, A) mRNA binding site ensures mRNA is in correct position A allows a tRNA molecule carrying an amino acid to position correctly P holds the growing polypeptide chain as the peptide bond forms between adjacent amino acids E releases the tRNA to be re-used as necessary

tRNA A short section of folded RNA H bonds between nucleotides ensure only 3 nucleotides are exposed These 3 nucleotides form the anticodon and will have a complimentary sequence to a codon The amino acid specific to this anticodon is carried by the tRNA

Start and stop codons tRNA will have start and stop codons so that the process of translation has a beginning and end The start codon must be at position P to enable the polypeptide chain to start forming tRNA enters at A proceeds through P and onto E Amino acids are joined together with peptide bonds Proteins are called polypeptides Further folding and coiling would then happen to the polypeptide chain

One gene – many proteins

One gene – many proteins Because the exons can be spliced and joined in a number of combinations then one gene could code for more than one protein Exons and introns on the mRNA can be treated differently to give variety n the code sequence Proteins can under transformations after forming by the addition of other molecules such as carbohydrates or phosphate groups e.g. mucus is a glycoprotein, haemoglobin has iron added Polypeptide strands can be cleaved to become active such as some digestive enzymes

mRNA codon table

Consolidation Chapters 1-3 homework booklet Draw out the process of translation and label it Text book questions on page 43 Multiple choice book chapter 3 questions Scholar Review PPT from school website and make own notes Chapters 1-3 homework booklet