Genes and Protein Synthesis

Define gene

section of DNA which codes for one polypeptide

What is an allele?

Different forms of a gene

Where do genes sit?

On a fixed point on the chromosome (locus)

Can single bases code for amino acids?

No

Only 4 possible codes but there are 20 amino acids

4^1 = 4

Can 2 bases code for amino acids?

No

Only 16 possible codes but there are 20 amino acids

4^2 = 16

Can 3 bases code for amino acids?

Yes

64 different combinations for 20 amino acids

4^3 = 64

Name the 3 features of the genetic code

- Non - overlapping

- Degenerate

- Universal

What does non overlapping mean?

- Each base is part of one triplet only

What does degenerate mean?

Amino acids can be coded for by more than 1 triplet

64 combinations for 20 amino acids

What does universal mean?

Same triplet codes for same amino acid in all organisms

How does a gene code information?

Specific base sequence on 1 DNA strand (coding strand) controls amino acid sequence in proteins (primary structure) that are made by a cell's ribosomes so the tertiary structure so the function of the protein

Give the effects of mutations

1) Change in base sequence of DNA

2) Sequence of amino acids (primary structure) changes

3) Bonds (ionic, hydrogen, disulphide) form in different places

4) This changes the tertiary structure and shape of the protein

5) Protein may become non functional

If the protein is an enzyme the shape of the active site may change shape, substrate will no longer fit so enzyme substrate complex will no longer form

What are the types of RNA

ribosomal RNA

messenger RNA

transfer RNA

Describe rRNA

rRNA with proteins form ribosomes which are the site of mRNA translation and protein synthesis

rRNA is coded for by numerous genes in many different chromosomes

Describe mRNA

- Consist of thousands of nucleotides in a linear strand

- mRNA is formed by transcription of a gene in DNA in the nucleus

- Complementary to DNA in its base sequence

- Amino acid coded for by triplet of bases on mRNA called a codon

- mRNA has unpaired bases so is easily broken down in the cytoplasm

How many possible codons are there?

64

Describe tRNA

- Small molecule of around 80 nucleotides

- Single strand that folds back in itself

- tRNA molecules forms hydrogen bonds with complementary sections of the molecule

- Chain attaches to amino acid

- Contain anticodon

How do bases pair up in RNA

Cytosine with guanine (triple bond)

Adenine with uracil (double bond)

Describe the non coding DNA between genes

- Made of multiple repeats of base sequences called variable number tandem repeats

- Important in genetic fingerprinting

Describe the non coding DNA within genes

- Non coding sections called introns

- Prokaryotes don't contain introns in their DNA

Give differences between Prokaryotic DNA and Eukaryotic DNA

P V E

- Circular v linear

- No histone proteins associated v Histone proteins associated

- No introns v Has introns

- Shorter v Longer

Describe the DNA of chloroplasts and mitochondria

Same like prokaryotes

Define genome

the complete set of genes in a cell (including mitochondria and chloroplasts)

Define proteome

full range of proteins that a cell is able to produce

Define complete proteome

full set of proteins that can be made by an organism

Give an overview to transcription in eukaryotes

- Takes place in nucleus

- Involves the formation of pre - mRNA

- Complementary to sequence of bases on DNA

Give an overview to RNA processing in eukaryotes

- Takes place in the nucleus

- Introns are spliced from the pre - mRNA to form mRNA

- mRNA leaves the nucleus and attaches to a ribosome

Give an overview to translation in eukaryotes

- Occurs on ribosomes

- Translates to mRNA message into a specific sequence of amino acids to form a polpeptide

Explain transcription in eukaryotes

- Make pre-mRNA using DNA as a template

- The DNA molecule is separated into 2 strands by the enzyme DNA helicase so the bases are exposed

- One of the strands act as a template strand

- RNA nucleotides are found in the nucleoplasm and attach to the exposed DNA bases by complementary base pairing

- RNA polymerase forms phosphodiester bonds between adjacent nucleotides to form an RNA strand

- This is pre - mRNA and is complementary to the DNA template strand

Explain splicing of pre - mRNA

- DNA is made of coding regions called exons and non coding regions called introns

- Exons are expressed in proteins formation

- In pre - mRNA of eukaryotic cells the introns are removed by enzymes before the strand moves into the cytoplasm

- The remaining exons are joined together - splicing

- The mRNA molecule leaves the nucleus through nuclear pores

If a eukaryotic gene is longer than the mRNA strand, in terms of nucleotides, explain what causes this difference

- Introns are present in the DNA gene but are removed from the mRNA during splicing leaving only the exons

Describe proteins synthesis in prokaryotes

- Prokaryotes don't contain non coding regions so don't produce pre-mRNA that needs splicing

- Transcription takes place in the cytoplasm of the cell and involves the formation of the functional mRNA that is complementary to the DNA base sequence

- Translation occurs on ribosomes where the mRNA message is translated into the specific amino acid sequence to form a polypeptide

Give 3 features about the genetic code

- First 2 bases of the triplet usually determine the particular amino acid - any 3rd base usually has no effect. Reduces the effect of mutations in this case

- 3 stop codes that indicate the end of the mRNA section - may not code for an amino acid eg 15 nucleotides code for 4 amino acids

- AUG is the start codon which codes for Methionine and can be removed in the processing stage to make a functional protein

Explain translation

- Ribosome consists of large and small subunits

- Translation begins when they bind to the start codon, AUG

- The ribosome can fit 2 tRNA molecules at once

- Anticodon UAC and carries Met

- ATP required to attach amino acid to tRNA

- Peptide bonds form between amino acids which requires an enzyme and ATP which is hydrolysed to release energy

- Empty tRNA leaves and picks up a new amino acid

- Ribosome moves to the next codon

- The complementary base pairing of mRNA and tRNA continues until the stop codon is reached which signals completion of the chain

- Polypeptide detaches from ribosome and is free in the cytoplasm to fold into its final tertiary structure

- Many ribosomes can use the mRNA strand at the same time - polysome

What codes for the amino acid?

mRNA codon

Explain translation exam style for polypeptide formation

- mRNA associates with ribosome in cytoplasm (attaches)

- mRNA find the start codon AUG

- complementary tRNA molecule brings specific amino acid

- Anticodon on tRNA is complementary to codon on the mRNA

- Ribosome moves along to the next codon

- Amino acids are joined by peptide bonds / in condensation reactions