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So a part of the DNA instructions have been transcribed and sent outside the nucleus… PowerPoint PPT Presentation


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mRNA. So a part of the DNA instructions have been transcribed and sent outside the nucleus…. nucleus. mRNA. cytoplasm. Now what? TRANSLATION. How is information in DNA converted into action?. Transcription – enzymes make an RNA copy of a DNA strand

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So a part of the DNA instructions have been transcribed and sent outside the nucleus…

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mRNA

So a part of the DNA instructions have been transcribed and sent outside the nucleus…

nucleus

mRNA

cytoplasm

Now what? TRANSLATION


How is information in DNA converted into action?

  • Transcription –enzymes make an RNA copy of a DNA strand

    • A piece of the DNA code (a gene) is converted into a messenger molecule (mRNA).

  • Translation -bases in the mRNA determine the order of amino acids in a protein

    • Messenger (mRNA) directs the linkage of amino acids into a protein.

    • Proteins carry out the work of the cell


Translation

  • mRNA sequence is used to make proteins (build cells, digestion, enzymes, etc.)

  • Let’s take a close look at our mRNA:

  • read 3 nucleotides at a time

  • group of 3 nucleotides = CODON


Each codon codes for a specific amino acid. 20 different amino acids can be used in different combinations to form a protein.

For example:

mRNA codonamino acid

AAUasparagine

CGCarginine

GGGglycine


Each codoncodes for an amino acid or signals the start or stop of a polypeptide chain.


1. Ribosome attaches to mRNA at start codon (AUG). The ribosome encloses two codons.


As the ribosome reads each codon, a tRNAbrings an amino acid in.

amino acid

  • tRNA –

  • has an anticodon sequence that is complementary to the codon (mRNA)

  • Each tRNA brings an amino acid with it.

Remember our codon chart?


Here it is again! (p. 194) The codon sequence tells which amino acid should be added.


  • As each tRNA brings in a new amino acid, the amino acid is added to the existing polypeptide chain.

  • A polypeptide bond forms between the amino acids.

  • This process continues until a stop codon is reached.

Step 1

Step 2

Step 3


Protein Synthesis is FAST!

  • About 20 amino acids added per second

  • 1 gene can make many mRNA molecules

  • Many ribosomescan work on 1 mRNA at once


DNA  Protein


Visualizing translation

  • Powerpoint animation

http://www.youtube.com/watch?v=41_Ne5mS2ls&feature=related


Summary

  • When a cell needs a certain protein:

  • Transcription turns the gene on in the nucleus (DNA) and makes a copy (mRNA)

  • Translation uses ribosomes and tRNA in the cytoplasm to read the copy and make the protein

  • REMEMBER: THE GOAL OF A GENE IS TO MAKE A WORKING PROTEIN


The “central dogma”…

replication


So how can all the cells in an organism have the same DNA but do different things?

only certain parts of the DNA in each cell are transcribed; “switched on” or kept “off”

http://www.youtube.com/watch?v=41_Ne5mS2ls&feature=related

http://video.pbs.org/video/1506740590


What if things go wrong?

Mutation- a change in the DNA nucleotide sequence

  • Mutations can cause subtle or dramatic effects on observed traits in individuals


3 types of mutations

Substitution

Insertion

Deletion

T

T A C C G A G A T T C A

Substituting one nucleotide for another.

T A C C G A

T

G A T T C A

Inserting one or more nucleotides

T A C C G A

G A T T C A

Deleting one or more nucleotides


What is the observed effect of mutations?

  • No Effect

    • Why?

      • Some mutations code for the same amino acid

      • Most mutations are in sequences of DNA between genes.

  • Variation – there are a variety of traits in a population.

  • Genetic diseases


Tay-Sachs disease

  • An enzyme that breaks down fatty tissue in the brain doesn’t work

  • Gradual loss of brain function

  • Can be caused by one base (letter) difference in the DNA code


Possible effect –

one letter’s difference

DNA sequence: T A C C G A G A T T C A

mRNA sequence: A U GG C UC U AA G U

amino acid sequence: Met -- Ala -- Leu -- Ser

DNA sequence: T A C C G A G A T T C A

mRNA sequence: A U GG C UA U AA G U

amino acid sequence: Met -- Ala -- Iso -- Ser

T


Observed trait

DNA

mRNA

protein

translation

protein function

(enzyme activity)

transcription

Remember:

  • Traitsare determined by the functions of proteins

  • Protein function is determined by protein shape

  • Protein shape is determined by amino acid sequence


How does this mutation change the amino acid sequence?

(Original)

DNA sequence: A A T G C A T A T G C A

mRNA sequence: U U AC G UA U AC G U

amino acid sequence: Leu -- Arg -- Ile -- Arg

(Mutated)

DNA sequence: A A T T C A T A T G C A

mRNA sequence: U U AA G UA U AC G U

amino acid sequence: Leu -- Ser -- Ile -- Arg


Consequences of a mutation (change) over time…

  • If it is in sperm or egg DNA and is not corrected, the new sequence of DNA is passed on to offspring.

  • Over generations, more mutations accumulate.

  • As a result, differences occur between people’s DNA sequences!


How much variation in DNA exists between 2 people?

  • About 1 in every 1,000 nucleotides is different between 2 people

  • (0.1% difference means 99.9% identical)

  • 3 billion total nucleotides -->3 million nucleotide differences between 2 people


Practice identifying mutations

  • Worksheet - we’ll do the first one together

  • Then please work by yourself, but feel free to use notes.

  • You WILL turn this in!

  • If you finish early, try a transcription challenge problem; AP bio text up front


Plan for today

  • Practice – DNA -> RNA -> protein

  • Where we’ve been this week

  • Cell division to make gametes – meiosis

  • Looking at chromosomes - karyotypes


Practice identifying mutations

  • Worksheet - we’ll do the first one together

  • Then please work by yourself, but feel free to use notes.

  • You WILL turn this in!

  • If you finish early, try a transcription challenge problem; AP bio text up front


What makes us who we are? What determines what we look like, etc?

Made of nucleotides (phosphate, sugar, base)

Copies itself through replication

DNA

Organized into chromosomes

mRNA

  • Cells divide by:

  • Mitosis (growth & repair)

  • Meiosis (reproduction)

Proteins


How do cells turn instructions (DNA) into action?

  • Review your notes from yesterday, and answer:

    • What part(s) of the process of turning DNA -> RNA seemed most clear to you (made the most sense)?

    • What part(s) of transcription (turning DNA -> RNA) seemed most fuzzy, unclear, or confusing?

    • Be prepared to share with the class


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