Biology - Chapter 8: DNA, Genes and Protein Synthesis

Gene

A section of DNA that codes for the amino acid sequence of a polypeptide

Where are genes located

A locus on a DNA molecule

How many chromosomes do humans have?

46 (23 pairs)

The genetic code

The way in which a sequence of DNA bases stand for a sequence of amino acids. It is universal.

Why is the genetic code described as universal?

It is the same for all organisms - the same triplet codes for the same amino acid

In prokaryotic cells, what is DNA like?

Short, circular, not associated with proteins

Triplet

A sequence of three DNA bases that code for a specific amino acid

Why is the genetic code described as non-overlapping?

Each base is part of only one triplet (base sequence read sequentially)

Why is the genetic code described as degenerate?

More than one triplet codes for each amino acid

Describe the genetic code

Universal, non-overlapping and degenerate

How many naturally-occurring amino acids are there?

20

Introns

Sequences of bases that do not code for amino acids but still make up a part of the polypeptide

Exons

A sequence of triplets that codes for a sequence of amino acids

What determines the nature and development of all organisms?

Genes (genetics) and environmental factors

Stop codons

Three triplets that do not code for any amino acid and mark the end of a polypeptide chain

Histones function

Hold DNA in chromosomes together

Chromosome structure (4)

- DNA in a double helix
- This is wound around histones, forming a DNA-histone complex to fix it in position
- DNA-histone complex is coiled
- Coil is further coiled, then is packed into the chromosome

Homologous pairs

A pair of chromosomes that carry the same genes at the same loci - one is from the mother, while the other is from the father

Allele

Different versions of the same gene - different alleles have different base sequences so code for a different amino acid sequence and produce a different polypeptide and protein

Messenger RNA (mRNA)

A type of RNA that transfers the DNA code from the nucleus to the cytoplasm

Codon

The sequence of 3 bases on mRNA that code for a single amino acid - they are complementary to a triplet in DNA

Genome

All the DNA in a cell

Proteome

The full range of proteins that a cell is able to produce

Describe the structure of RNA (3)

- Polymer made up of nucleotides
- Single stranded
- Has a pentose sugar (ribose), organic base (AUCG) and a phosphate group

Two types of RNA used in protein synthesis

Messenger RNA (mRNA) and transfer RNA (tRNA)

Describe the structure of mRNA (2)

- Long strand arranged in a single helix
- Base sequence is determined by the sequence of bases on a length of DNA, made by transcription

How is the structure of mRNA suited for its function? (2)

- It is small enough to leave the nucleus through the nuclear pores and enter the cytoplasm
- It possesses information in the form of codons, which determines the amino acid sequence of a specific polypeptide that will be made

Describe the structure of tRNA (5)

- Relatively small molecule, made up of around 80 nucleotides
- Single-stranded chain folded into a cloverleaf shape
- One end of chain extends beyond the other - amino acid binding site
- Many types of tRNA, each binds to a specific amino acid
- Has anticodon: sequence of three organic bases complementary to codon on mRNA

How is the structure of tRNA suited for its function? (2)

- Specific amino-acid binding site, longer than others: part of tRNA molecule that a specific amino acid can easily attach to
- Anticodon is specific: its 3 organic bases pairs with complementary bases on codon of mRNA - lines up amino acids on mRNA template during protein synthesis

What complementary bases does RNA form?

Cytosine and guanine
Adenine and uracil (RNA) or thymine (DNA)

What can RNA join with?

DNA and other RNA by complementary base pairing

In eukaryotic cells, what is DNA like?

In nucleus: Long, linear, associated with proteins (histones) to form chromosomes
In mitochondria/chloroplasts: short, circular, not associated with proteins

What is the function of mRNA?

It transfers the genetic code from the DNA to the ribosomes (transcription) where it is used to make a protein during translation

What is the function of tRNA?

It carries the amino acids that are used to make proteins to the ribosomes

DNA vs mRNA vs tRNA (8 for each)

DNA:
1. Double polynucleotide chain
2. Largest molecule
3. Double-helix molecule
4. Deoxyribose sugar
5. Bases: ATCG
6. Found mostly in nucleus (also mitochondria and chloroplasts)
7. Quantity constant for all cells except gametes
8. Chemically very stable

mRNA:
1. Single polynucleotide chain
2. Smaller than DNA, larger than tRNA
3. Single-helix molecule in most
4. Ribose sugar
5. AUCG
6. Manufactured in nucleus, found throughout cell
7. Quantity varies from cell to cell with levels of metabolic activity
8. Less stable than DNA and tRNA

tRNA:
1. Single polynucleotide chain
2. Smallest
3. Clover-leaf shaped molecule
4. Ribose sugar
5. AUCG
6. Manufactured in nucleus, found throughout cell
7. Quantity varies from cell to cell with levels of metabolic activity
8. More stable than mRNA, less stable than DNA

Transcription

The production of mRNA from DNA

Describe the formation of mRNA by transcription (7)

1. DNA helicase is used to break the hydrogen bonds between the strands of a DNA molecule, causing the DNA strands to separate and expose the nucleotide bases
2. One strand acts as a template - template strand
3. On template strand, free RNA nucleotide bases pair with complementary nucleotides from nucleolus in nucleus and align by complementary base pairing
4. Uracil used instead of thymine
5. RNA polymerase moves along strand joining adjacent RNA nucleotides by catalysing condensation reactions between nucleotides, forming phosphodiester bonds and form a pre-mRNA molecule
6. When RNA polymerase reaches a particular sequence of bases (terminator sequence), it detaches - pre-mRNA formed
7. Pre-mRNA then spliced to remove introns, leaving only exons, which are joined together to form mRNA

tRNA vs mRNA (6)

1. tRNA of standard length, mRNA is longer
2. tRNA is cloverleaf shaped, mRNA is linear
3. tRNA has an amino acid binding site, mRNA does not
4. tRNA has anticodons, mRNA has codons
5. tRNA has base-pairing, mRNA does not
6. only 20 kinds of tRNA, different mRNA for each gene

Do eukaryotic and prokaryotic DNA need to be spliced?

Eukaryotic DNA does - it has introns, which needs to be removed from pre-mRNA to form mRNA
Prokaryotic DNA does not - it only has exons, so transcription directly produced mRNA

Translation

The production of polypeptides from the sequence of codons carried by mRNA

Describe the role of ATP is translation (2)

Releases energy through respiration to attach amino acids to tRNA and to attach amino acids together with peptide bonds

Describe the process of translation (6)

1. mRNA attaches to ribosomes/RER
2. tRNA anticodons bind to complementary mRNA codons
3. tRNA brings a specific amino acid - sequence of codons determines sequence of amino acids
4. The amino acids join by peptide bonds with the use of ATP
5. tRNA is then released
6. Ribosome moves along mRNA to form the polypeptide

Describe the role of tRNA in translation (3)

- tRNA has an anticodon complementary to codon on mRNA
- Carries specific amino acid and transfers it to ribosome
- Forms the correct sequence of amino acids along a polypeptide chain.

Explain how a change in a sequence of DNA bases could result in a change in the structure of a protein (3)

- Change in sequence of amino acids / primary structure
- Change in hydrogen/ionic/disulfide bonds leads to change in tertiary structure (and so change in active site if an enzyme)
- Substrate cannot bind as no longer complementary - E-S complexes cannot form

Give 2 advantages of showing the genetic code as base sequences on mRNA rather than triplets on DNA.

1. Ribosomes assemble polypeptides using mRNA code
2. DNA has two strands, each with a different complementary base sequence

What do stop codons do? (2)

They stop translation, resulting in the detachment of the polypeptide chain from the ribosome.

Where does transcription take place?

At the nucleus

Where does translation take place?

At the ribosomes in the cytoplasm

Describe the role of ribosomes in translation (4)

- mRNA attaches to ribosome
- mRNA codon binds to complementary anticodon on tRNA
- The ribosome moves along mRNA
- Amino acids bind to each other and form peptide bonds through condensation reactions, forming a polypeptide

Describe the role of RNA polymerase in transcription

It joins adjacent RNA nucleotides together by catalysing the condensation reaction to form phosphodiester bonds, forming a strand of mRNA

How do genes code for a protein?

- They have base sequences of DNA
- The base sequence determines the sequence of amino acids
- One triplet of bases codes for an amino acid

Where are introns found?

Between genes

What happens if there are more copies of a gene? (2)

1. There is more mRNA as more transcription.
2. This results in more translation occurring and more of the protein being produced (and a faster rate of reaction if an enzyme)

Why is transcription necessary?

DNA is too large to leave the nucleus