Topic 4: Genes and Protein Synthesis Flashcards
Describe the differences between prokaryotic and eukaryotic DNA
- Located in nucleus in eukaryotes, nucleoid in prokaryotes
- Is a chromosome molecule in eukaryotes, is a nucleoid molecule in prokaryotes
- Linear in eukaryotes, circular in prokaryotes
- Associated with histone proteins in eukaryotes, not in prokaryotes
- Longer in eukaryotes
- Eukaryotes have introns, prokaryotes do not
What is a gene?
A section of DNA at a fixed position (locus) that contains the coded information for making polypeptides and functional RNA (e.g ribosomal RNA, transfer RNA)
What is a locus?
The fixed position of a gene
What gives the phenotype of an organism?
A combination of proteins (coded for by DNA) interacting with their environment
What is an intron?
What is an exon?
Intron - non-coding sections of DNA that occur between and within genes
Exon - sections of DNA that code for polypeptides
How is each amino acid coded for?
How do we know?
1 amino acid is coded for by a DNA triplet (3 bases)
There are 20 amino acids with their own code and only 4 bases. If 1 base = 1 amino acid, there could only be 4 amino acids. If 2 bases = 1 amino acid, there could only be 6 codes.
3 bases gives 64 codes, which is enough
Explain the properties of the genetic code
- Degenerate: most amino acids are coded for by multiple triplets
- Non-overlapping: every base belongs to 1 triplet and is read only once
- Universal: with minor exceptions, each triplet codes for the same amino acid in all organisms (indirect evidence for evolution)
What is a chromatid?
How many chromatids are in a chromosome at any time?
One thread of a chromosome after DNA replication.
After DNA replication, chromosomes have 2 sister chromatids joined at the centromere, but usually it only has 1
Describe the structure of a chromosome
- DNA double helix wound around histone proteins to fix its position.
- DNA-histone complex is coiled, looped and coiled further before being packed into chromosome
- One chromosome has a single DNA molecule
What are homologous chromosomes?
A pair of chromosomes, one maternal and one paternal set. Contain the same genes at the same loci but are not genetically identical (have different alleles)
Describe the number of chromosomes in a cell
The number of chromosomes is the same for normal individuals of a species, but varies between species. Humans have 46
What is the diploid number?
The total number of homologous pairs of chromosomes in a cell
What is a haploid cell?
How are they produced?
Cells with half the chromosome number.
During meiosis, the halving of chromosome number is done so each gamete receives one chromosome from each homologous pair - each receives one gene for each characteristic
When the gametes combine, the diploid state with homologous chromosomes is restored
What is an allele?
An alternative form of a gene.
Each individual inherits one allele from each parent. When they’re different, alleles have different base sequences so produce different polypeptides.
Any mutation forms an allele
What is the genome?
The complete set of genes in a cell, including mitochondria and/or chloroplasts
What is the proteome?
The full range of proteins produced by the genome.
Sometimes called the complete proteome, where the proteome is the proteins produced by a given type of cell under a certain set of conditions
Describe the structure of messenger RNA (mRNA)
- A strand of bases in a single helix, complementary to DNA
- Small enough to leave the nucleus via nuclear pores where DNA is not
- 3 mRNA nucleotides are a codon, codes for 1 amino acid, complementary to a DNA triplet
- Easily hydrolysed so only exists while manufacture of a particular protein is needed
Describe the function of mRNA
Used to transfer information on DNA in the nucleus to the cytoplasm where, in conjunction with a ribosome, it is translated and acts as a scaffold to form a protein on
Describe the structure of transfer RNA (tRNA)
- A single-stranded chain folded into a clover-leaf shape, made from around 80 nucleotides
- One end of the chain extends beyond the other (amino acid binding site)
- At the opposite end is the anticodon: 3 bases complementary to an mRNA codon
- Binds to a specific amino acid so have many types (must be as many molecules as there are coding triplets)
Describe the function of tRNA
To bring amino acids coded for by the mRNA molecule to the ribosome in the correct order, in order to form a polypeptide
Describe the differences between DNA, mRNA and tRNA
- DNA is a double polynucleotide chain, mRNA and tRNA are single
- DNA is the largest, then mRNA, then tRNA
- DNA is a double helix, mRNA is a single helix, tRNA is clover shaped
- DNA has deoxyribose sugar, mRNA and tRNA have ribose
- DNA has A,T,G,C, mRNA and tRNA have A,U,G,C
- DNA is only found in nucleus, mRNA and tRNA are manufactured in nucleus but found throughout cell
- DNA quantity is constant in all cells but gametes, mRNA and tRNA quantities vary between levels of metabolic activity
- DNA is chemically very stable, then tRNA, then mRNA (hydrolysed within a few days)
What are the stages of protein synthesis?
- Transcription: gene is transcribed onto an mRNA molecule from DNA template
- Translation: production of polypeptides by sequence of mRNA codons in a ribosome
Describe the steps of transcription in protein synthesis
- DNA helicase breaks hydrogen bonds between DNA bases to unwind + expose required gene
- Free RNA nucleotides in nucleus pair with DNA bases
- RNA polymerase attaches to start of a polynucleotide strand + moves along, joining RNA nucleotides, forming a pre-mRNA molecule
- As RNA polymerase adds nucleotides, DNA strands rejoin behind it
- pre-mRNA is spliced to form an mRNA molecule that is small enough to leave nucleus via nuclear pore
- Outside nucleus, mRNA is attracted to ribosomes + becomes attached ready for translation
What is splicing?
Eukaryotes produce pre-mRNA, which must be spliced before translation to remove introns, as they would prevent polypeptide synthesis
Not necessary in prokaryotes as they don’t have introns
Describe the process of translation in protein sythesis
- mRNA associates with a ribosome in cytoplasm - 1st 2 codons are on ribosome + attract complementary tRNA molecules, carrying specific amino acids
- Anticodons on tRNA form hydrogen bonds with mRNA codons
- Peptide bond formed between 2 adjacent amino acids (catalysed by peptidyl transferase), also requires ATP hydrolysis
- Ribosome moves forward 1 codon, new tRNA enters + first leaves ribosome
- Another peptide bond is formed = polypeptide
- Ribosome continues along mRNA until it reaches a stop codon, then detaches due to release factor
- Polypeptide chain is released + folded into a protein
