1. Chapter 13 packet: DNA and Protein Synthesis Part II

2. DNA (Deoxyribonucleic acid)
RNA
(Ribonucleic acid)
Sugar
Deoxyribose
Ribose
Bases
Adenine, thymine, guanine, cytosine
Adenine, uracil, guanine, cytosine
Strands
Double-stranded
Single-stranded
Helix
Yes No
Location
Nucleus
Nucleus, cytoplasm (outside nucleus)
Types
XXXXXXXXX
Messenger, transfer, ribosomal

3. RNA vs DNA

4. Messenger RNA (mRNA) – carries instructions for making a protein from the nucleus to the ribosomes
Ribosomes - a part of the cell where proteins are made
Ribosomal RNA (rRNA) - found in the ribosomes
Transfer RNA (tRNA) - transfers amino acids to the ribosomes and “reads the mRNA sequence, translating it into a sequence of amino acids

5. Making Proteins
DNA is found in the nucleus of a cell
Proteins are made outside the nucleus at the ribosomes.

6. Overview of gene expression

7. Two processes are involved in the synthesis of proteins in the cell:
Transcription – making mRNA from the information in DNA, which will take a copy of the DNA code to the ribosome to direct the making of protein; occurs in nucleus
Translation – uses the information in mRNA to make a specific protein, the sequence of bases of mRNA is “translated” into a sequence of amino acids; occurs in ribosome
These processes are the same in all organisms

8. The Genetic Code
DNA holds instructions to make a protein
Instructions are copied into mRNA, which will be used to make a protein
Codon - each three-nucleotide sequence of an mRNA molecule
Each codon represents 1 amino acid
There are 64 possible codons, and only 20 amino acids, so most amino acids have more than one codon

9. Messenger RNA codons

10. Transcription
Purpose – Makes a copy of the DNA code that can leave the nucleus and travel to the ribosome to direct protein synthesis – mRNA
Occurs in the nucleus
Occurs at only 1 gene at a time
Adenine in DNA pairs with uracil in RNA, not thymine
Thymine in DNA pairs with adenine in RNA

11. Animation Steps: RNA polymerase attaches to DNA at the start of a gene
DNA unwinds and unzips
Complementary bases are added along DNA (½ of DNA will serve as a template)
Once “stop” signal is reached, process ends, DNA closes back up, and mRNA is released
Animation

12. Transcription and mRNA synthesis

13. Translation Protein constructed during this process
Occurs at the ribosomes
Key players in translation:
mRNA (messenger RNA)
Made during transcription, has codons
Travels from nucleus to ribosome
Contains copy of DNA code to make protein
tRNA (transfer RNA)
rRNA (ribosomal RNA)

14. Transfer RNA (tRNA) tRNA molecules bring amino acids to the ribosomes
Free-floating in the cytoplasm of the cell
Anticodon – sequence of 3 nucleotides on tRNA (complementary to mRNA)– it is this sequence that determines which amino acid each tRNA has

15. Complementary base pairing occurs between anticodons of tRNA and codons of mRNA – determines the sequence of amino acids to construct the polypeptide.
If mRNA codon is AUG, tRNA anticodon would be UAC

16. Ribosomal RNA (rRNA)
rRNA is made in the nucleolus (a cell structure found inside the nucleus)
Makes ribosomes (location in cell where proteins will be made)

17. Steps:
Ribosome attaches to start codon on mRNA

18. tRNA brings amino acids to ribosome – codon on mRNA pairs with anticodon on tRNA

19. 3. Amino acids are joined by peptide bonds

20. 4. Stop codon is reached, mRNA is released, and protein is released
Animation

23. Control of Gene Expression
Cells differ in which genes are being expressed – based on cell function – ex. nerve vs. muscle.
Levels of control in
eukaryotes include:
transcriptional
posttranscriptional
translational
Posttranslational
May rely on signals from
inside or outside of the cell

24. Transcriptional control – involves rate at which or even whether transcription occurs at all; due to:
The organization of the chromatin (form that chromosomes take in non-dividing cells)
Regulator proteins called transcription factors
Signals from inside or outside the cell

25. Posttranscriptional control – involves processing of mature mRNA
Translational control – involves life span of mRNA and ability to bind to ribosomes
Posttranslational control – involves changes needed for polypeptide to become functional

26. Gene Mutations
Definition - a change in the sequence of bases within a gene
Causes –
Mutations can be spontaneous or caused by environmental influences called mutagens.
Mutagens include radiation (X-rays, UV radiation), and organic chemicals (in cigarette smoke and pesticides).

27. Types –
Frameshift mutations –
one or more bases are inserted or deleted from a sequence of DNA
can result in nonfunctional proteins
can result in no protein at all – stop codon where there shouldn’t be one
Point mutations –
One base is substituted for another
May result in change of amino acid sequence
May not affect protein at all

28. Types of point mutations:
Silent mutation - the change in the codon results in the same amino acid
Ex: UAU  UAC both code for tyrosine
Nonsense mutation - a codon is changed to a stop codon; resulting protein may be too short to function
Ex: UAC  UAG (a stop codon)

29. Missense mutation - involves the substitution of a different amino acid, the result may be a protein that cannot reach its final shape
Ex: Hbs which causes sickle-cell disease

30. In general, mutations can have any of the following effects:
No change in proteins made or appearance
Wrong protein is made
No protein in made
New appearance may result