B7- DNA and Protein synthesis

Compare and contrast DNA in eukaryotic cells with DNA in prokaryotic cells

Similarities:

• Nucleotide structure is identical - deoxyribose attached to phosphate and a base

• Adjacent nucleotides joined by phosphodiester bonds, complementary bases joined by hydrogen bonds

• DNA in mitochondria / chloroplasts have similar structure to DNA in prokaryotes → Short, circular, not associated with proteins

Differences:

• Eukaryotic DNA is longer

• Eukaryotic DNA is linear, prokaryotic DNA is circular

• Eukaryotic DNA is associated with histone proteins, prokaryotic DNA is not

• Eukaryotic DNA contain introns, prokaryotic DNA does not

what is a homologous chromosome

• pair of chromosomes

• with the same genes

• in same loci

define the term gene

A sequence of DNA (nucleotide) bases that codes for:

• The amino acid sequence of a polypeptide

• Or a functional RNA (eg. ribosomal RNA or tRNA)

define the term allele

different versions of the same gene

define the term degenerate

more than one triplet codes for the same amino acid

what is a chromosome ?

• Long, linear DNA + its associated histone proteins

• In the nucleus of eukaryotic cells

what is a locus?

Fixed position a gene occupies on a particular DNA molecule.

what’s the difference between a:

1. triplet

2. codon

3. anticodon

1. 3 bases on strand on DNA

2. 3 bases on a strand of RNA ( complementary to DNA base sequence )

3. 3 bases on a strand of tRNA

Describe the nature of the genetic code

Triplet code→ A sequence of 3 DNA bases, called a triplet, codes for a specific amino acid

Universal → The same base triplets code for the same amino acids in all organisms

Non-overlapping→ Each base is part of only one triplet so each triplet is read as a discrete unit

Degenerate → An amino acid can be coded for by more than one base triplet

What are ‘non-coding base sequences’ and where are they found?

Non-coding base sequence - DNA that does not code for amino acid sequences / polypeptides:

1. Between genes - eg. non-coding multiple repeats

2. Within genes - introns

What are introns and exons?

• introns→ base sequence of a gene that doesn’t code for amino acids, in eukaryotic cells

• exons→ base sequence of a gene coding for amino acid sequences (in a polypeptide)

define genome

The complete set of genes in a cell (including those in mitochondria and /or chloroplasts)

define proteome

The full range of proteins that a cell can produce (coded for by the cell’s DNA / genome)

Describe the two stages of protein synthesis

1. transcription- Production of messenger RNA (mRNA) from DNA, in the nucleus

2. translation- Production of polypeptides from the sequence of codons carried by mRNA, at ribosomes

transcription process

1. DNA helicase and RNA polymerase breaks weak hydrogen bonds between DNA bases and strands separate

2. Only one DNA strand acts as a template

3. Free RNA nucleotides align next to their complementary bases on the template strand

4. In RNA, uracil is used in place of thymine (pairing with adenine in DNA)

5. RNA polymerase joins adjacent RNA nucleotides

6. This forms phosphodiester bonds via condensation reactions

7. Pre-mRNA is formed and this is spliced to remove introns, forming (mature) mRNA

how many sites are there for tRNA anticodons in a singular ribosome and why?

• 2 sites for 2 tRNA anticodons

• ensuring the 2 tRNA molecules are close enough so a peptide bond can form between 2 amino acids

translation process

1. mRNA attaches to a ribosome and the ribosome moves to a start codon

2. tRNA brings a specific amino acid

3. tRNA anticodon binds to complementary mRNA codon

4. Ribosome moves along to next codon and another tRNA binds so 2 amino acids can be joined by a condensation reaction forming a peptide bond → Using energy from hydrolysis of ATP

5. tRNA released after amino acid joined polypeptide

6. Ribosome moves along mRNA to form the polypeptide, until a stop codon is reached