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Lecture #3 Transcription Unit 4: Molecular Genetics.

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Presentation on theme: "Lecture #3 Transcription Unit 4: Molecular Genetics."— Presentation transcript:

1 Lecture #3 Transcription Unit 4: Molecular Genetics

2 Gene expression - the process by which proteins are made based on the information encoded in DNA. There are two steps to gene expression 1.Transcription 2.Translation

3 The steps of gene expression Transcription - the process in which instructions for making a protein are transferred from a gene to an RNA molecule. Also known as ‘RNA synthesis’ This takes place in the nucleus in eukaryotes & in the cytoplasm in prokaryotes Translation - The process in which the instructions stored in an RNA molecule are used to build a protein. The instructions are ‘read’ and followed as amino acids are linked together to build the protein. This happens at a ribosome in both prokaryotes & eukaryotes!

4 GENES CODE FOR PROTEIN Proteins - Large complex molecules that are built from smaller units called Amino Acids There are 20 amino acids. Their sequence determines the size and shape of a protein The sequence of the amino acids is determined by the sequence of nucleotides that make up the DNA molecule. Remember, GENES are like ‘recipes’ that code for protein.

5 A logistical problem….. Genes are found on Chromosomes. Chromosomes are in the nucleus of eukaryotic cells. Our genetic Information (DNA) never leaves the nucleus. Ribosome The structure within a cell where protein synthesis takes place. This is like the warehouse that receives shipping orders from the genes Since DNA is in the nucleus (and cannot leave) with the blueprint of what proteins are to be made, this creates a problem.

6 SOLUTION: Proteins are not built directly From DNA. RNA molecules are involved! The roles played by DNA and RNA are similar to the master plans and blueprints used by builders.

7 Explanation of analogy A master plan (DNA) has all the information needed to construct a building. Builders never bring a valuable master plan to the building site, where it might be damaged or lost. Instead, they prepare inexpensive, disposable copies of the master plan called blueprints (RNA).

8 RNA (Ribonucleic acid) The molecule that links the chromosomes in the nucleus to the ribosome’s in the cytoplasm of the cell It is very similar to DNA in that it also consists of chains of nucleotides. This makes it a type of nucleic acid.

9 Differences between DNA and RNA Characteristics DNA RNA Name of the sugarDeoxyriboseRibose Base pair rules Adenine – Thymine Guanine – Cytosine Adenine – Uracil ** Guanine – Cytosine Number of strands21

10 How we go from GENE to RNA molecule?

11 Step 1 Transcription begins when the enzyme RNA polymerase binds to the gene’s promoter. Promoter - a specific sequence of DNA that acts as a “start” signal for transcription. RNA polymerase - an enzyme that adds and links complementary RNA nucleotides during transcription.

12 Label the diagram below

13 Step 2: RNA polymerase unwinds the two strands of the double helix. Purpose: This exposes the DNA nucleotides. Step 3: RNA polymerase adds and then links complementary RNA nucleotides as it “reads” the gene. The RNA polymerase moves along the nucleotides of the DNA strand that has the gene. Remember, Uracil is added in the place of Thymine on the newly formed chain. Transcription continues until the RNA polymerase reaches a “stop” signal on the DNA. Stop Signal - a sequence of bases that signals the end of transcription.

14 When RNA nucleotides are added during transcription, they are linked together. As RNA polymerase moves down the DNA strand, a single strand of RNA grows. Behind the RNA polymerase, the two strands of DNA close up, forming new hydrogen bonds to reform the double helix. Finished Product: The newly formed strand is RNA

15 There are 3 types of RNA During transcription, different types RNA are made, depending on the gene that is being expressed. 1.mRNA: Messenger RNA. This type of RNA Is formed when a cell needs a particular protein. JOB: carries instructions for making a protein from a gene and delivers it to the site of translation (ribosome), where the protein will be assembled.

16 2.tRNA - Transfer RNA This type of RNA plays a big role in translation, the second part of gene expression that we’ll talk about next time. JOB: to transfer the amino acids that the gene is ‘ordering’ from the cytoplasm to the ribosome. 3.rRNA – Ribosomal RNA rRNA plays a role in the structure of the ribosome


18 Take 2 minutes to summarize how the code stored in DNA copied during transcription. Also summarize a difference between mRNA, tRNA and rRNA.

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