DNA and Protein Synthesis

- Prokaryotic DNA is located in the cytoplasm or nucleoid. They also contain small loops of DNA known as plasmids.
- The nucleus contains eukaryotic DNA. It's found in sections called chromosomes.
- Humans have 46 chromosomes in each cell—23 pairs. 23 pairs define gender: XX = girl, XY = boy.
- Chromosomes are long, coiled sections of DNA wrapped around protein.
- DNA is a large polymer consisting of numerous nucleotides (monomers) bonded together.

- There are 4 types of nucleotides. They have different bases. A+T and C+G.
- Nucleotides bind together in long chains. 

Bonds between bases—complementary base pairing—bind the two chains together.

- A-T is always bonded this way.
- C-G is always bonds this way.

DNA has a double-stranded structure. The bases in the two strands of DNA specifically bond together, as shown in the diagram.
This section is a gene.

DNA and Proteins

- DNA is a code to tell the cell what proteins to make.
- Gene: A section of DNA that codes for a protein.
- The sequence of bases in your DNA is the code.

Genes:

- Every gene holds a different code, so every gene codes for a different protein.

(each band is 1 gene)

Proteins:

- Proteins are polymers of amino acids. The sequence of amino acids is different in every protein.
- There are 20 different amino acids. You can combine them in any order to create different proteins.
- Proteins can be long/short. 
- One amino acid is coded for by three bases in DNA.
→ triplet code
- E.g., the code is 91 nucleotides long; it codes for 33 amino acids.
- E.g., if a gene is 81 nucleotides long, it codes for 27 amino acids. 

- DNA: A nucleic acid consisting of two strands coiled into a double helix.
- Genome: Entire genetic material for one organism.
- Peptide bonds: Type of bond between amino acids.
- Polypeptide: Short chain of amino acids joined together. 
- Examples of proteins: keratin and collagen.

tRNA

- t = transfer
- It's an amino acid carrier.
- Each tRNA molecule is single-stranded and forms a loop shape.
- It attaches itself to an amino acid at the top.
- It features an anticodon, which consists of three bases at the bottom.

- tRNA molecules transport amino acids to the ribosomes and guarantee their correct order of addition to the protein chain.
- The sequence on the mRNA molecule determines the order.

Protein Synthesis

Summary

- A cell must construct a protein that contains instructions for incorporating amino acids into DNA.
- The sequence of bases in DNA determines the order of amino acids in the protein. 
1) Unzip the required gene, separating two DNA strands.
→ One strand serves as a template, and free nucleotides in the nucleus use complementary base pairings to create an mRNA molecule.
2) The nucleus produces mRNA, and DNA rezips.
- The new mRNA separates from the DNA template nucleotides.
- mRNA has no nucleotides with base T; instead, it uses U.
3) mRNA moves out of the nucleus and travels to the cytoplasm.
4) mRNA attaches to a ribosome.
5) The ribosome reads the first three bases (1st codon) on the mRNA, which matches to the corresponding amino acid. This continues.
6) When the section has been completely used, the amino acids bond in the right order to form the protein.

Using the Code

- Genes are part of chromosomes and can't leave the nucleus—too big. 
- However, the cytoplasm contains ribosomes, which assemble proteins.
- ∴ A molecule called mRNA is used. 'm' = messenger.

Making mRNA:

- The gene is unzipped.
- A temporary separation occurs between two strands of DNA.

- Free nucleotides attach using the complementary base pairing rules. These float around the nucleus.
- The new strand is mRNA.
- mRNA is similar to DNA, but it is single-stranded and small enough to escape the nucleus, and it replaces base T with U.

- mRNA is a copy of the gene. It can exit the nucleus and attach to a ribosome. The ribosome reads the code on mRNA to put aa in the right order.

Summary - 2 steps in protein synthesis:

1) Making mRNA—transcription—in a nucleus.
2) Making protein—translation—in ribosome.

Discovery

- In 1953, Watson and Crick discovered the structure of DNA. They won the Nobel Prize for this.
- However, the assistance of other scientists was also necessary.
→ Rosalind Franklin and Maurice Wilkins used X-ray crystallography to take pictures of DNA.
→ Erwin Chargaff discovered there were equal numbers of A+T bases as C+G bases.