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Context Cell nucleus chromosome gene double helix

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Presentation on theme: "Context Cell nucleus chromosome gene double helix"— Presentation transcript:

1 Context Cell nucleus chromosome gene double helix

2 DNA vs RNA : Structure Double helix Deoxyribose A - T pair
Single helix Ribose sugar A - U pair

3 DNA vs RNA : Function segments of the sequence
- genes - code for polypeptides rewrites and translates the code into polypeptides (proteins)

4 Types of RNA Ribsomal, rRNA, combines with protein in the cytoplasm to form a ribosome. Si RNA is double stranded

5 Types of RNA Messenger, mRNA, is synthesized from a gene and carries the code into the cytoplasm for protein synthesis Si RNA is double stranded

6 Types of RNA amino acid Transfer (tRNA) a loop of nucleotides three of which form an anticodon. Also contains the amino acid coded for by the mRNA codon complement. The polypeptide is built by linking the amino acids of adjacent tRNA. Si RNA is double stranded anticodon

7 The Ribosome • The ribosome serves as the site for protein synthesis. Made of rRNA and protein, ribsomes are synthesized in two sub units. • The ribosome attaches itself to m-RNA and provides the stabilizing structure to hold all substances in position as the protein is synthesized. tRNA sites ribosome mRNA site

8 The Central Dogma gene expression
Reading & expressing genes Genes in the DNA are transcribed into RNA which is translated into a sequence of amino acids Fundamental & universal to life on Earth

9 mRNA carries the code from the nucleus; tRNA base pair
with mRNA at ribosomes to string the amino acids together

10 Transcription is initiated when RNA polymerase binds at DNA ‘upstream’ of the gene, called the promoter The first step in transcription is initiation, when the RNA polymerase binds to the DNA upstream (5′) of the gene at a specialized sequence called a promoter. In bacteria, promoters are usually composed of three sequence elements, whereas in eukaryotes, there are as many as seven elements. In eukaryotes, the "core" promoter for a gene transcribed by pol II is most often found immediately upstream (5′) of the start site of the gene. Most pol II genes have a TATA box (consensus sequence TATTAA) 25 to 35 bases upstream of the initiation site, which affects the transcription rate and determines location of the start site. The terms "strong" and "weak" are often used to describe promoters and enhancers, according to their effects on transcription rates and thereby on gene expression. Alteration of promoter strength can have deleterious effects upon a cell, often resulting in disease. For example, some tumor-promoting viruses transform healthy cells by inserting strong promoters in the vicinity of growth-stimulating genes, while translocations in some cancer cells place genes that should be "turned off" in the proximity of strong promoters or enhancers. Terminator sequences are found close to the ends of coding sequences. Elongation – polymerase adds nucleotides to transcribe the gene. Termination sequences near the end of the gene communicate a stop-transcription message

11 Template & Direction coding strand anti-sense strand
mRNA is ‘written’ 5’-3 as it can only be built by adding a 5’ C to a 3’C in the chain (mnemonic: write has 5 letters) coding strand The template strand is ALWAYS read in the 3' to 5' direction (that is, starting from the 3' end of the template and reading the nucleotides in order toward the 5' end of the template). The new DNA strand (since it is complementary) MUST BE SYNTHESIZED in the 5' to 3' direction (remember that both strands of a DNA molecule are described as being antiparallel). DNA polymerase catalyzes the formation of the hydrogen bonds between each arriving nucleotide and the nucleotides on the template strand. In addition to catalyzing the formation of Hydrogen bonds between complementary bases on the template and newly synthesized strands, DNA polymerase also catalyzes the reaction between the 5' phosphate on an incoming nucleotide and the free 3' OH on the growing polynucleotide (what we know is called a phosphodiester bond!). As a result, the new DNA strands can grow only in the 5' to 3' direction, and strand growth must begin at the 3' end of the template, anti-sense strand Since DNA is antiparallel, the template is always the 3’-5’ helix

12 The code is redundant but never ambiguous 6 codons for serine!
CCC = proline, only

13 Eukaryote genes are modified before translation
Introns are excised Exons are spliced together Splicing can occur in multiple ways, creating many mRNA from one gene Ex: 30,000 genes, +100,000 proteins

14 Caps and tails are added to the mRNA, slowing destruction by enzymes
STOP Codon START codon CAP at 5’ end Poly–A TAIL

15 Summary of Differences
Replication Transcription & Translation Copies both DNA helices Uses template strand of DNA to create RNA Leads to more cells Leads to proteins (gene expression) Entire molecule copied Single gene transcribed, exons spliced together DNA polymerase adds nucleotide RNA polymerase adds nucleotides RNA primer initiates sequencing Promotor on DNA initiates sequencing

16 Mutations change the code and sometimes the protein
Point mutation/substitution Potentially changes one amino acid - missense

17 Frameshift mutations result from insertion or deletion of nucleotides
Alters the codons from that point down

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