8.DNA genes and protein Flashcards
Define a gene
A section of DNA on a chromosome coding for one or more polypeptides / functional RNA
Define a locus
The position of a gene on a chromosome
Describe the features of the genetic code
- degenerate code
- non overlapping
- universal
Why is the genetic code known as degenerate
Because most amino acids are coded for by more than one triplet
Why is the genetic code known as non-overlapping
Each base is only read once, in one triplet
Why is the genetic code known as universal
Each triplet codes for the same amino acids in all organisms
Some triplets don’t code for an amino acid, what are these known as
Stop codons, they mark the end of a polypeptide chain
Define an exon
A coding sequence within a gene
Define an intron
A non-coding sequence within a gene
What are the differences in DNA in prokaryotic cells and eukaryotic
PROKARYOTIC
-DNA molecules are shorter and circular
-they are not associated with protein molecules
EUKARYOTIC
-DNA molecules are longer and linear
-they are associated with proteins called histones
When are chromosomes visible as distinct structures
When a cell is dividing (undergoing mitosis)
What is a single thread of a chromosome called
Chromatid
What is a homologous pair
A pair of chromosomes (one maternal one paternal) that have the same gene loci and therefore determine the same features
Define diploid
A term applied to cells in which the nucleus contains two sets of chromosomes
Define haploid
A term referring to cells that contain only a single copy of each chromosome
What is the diploid number in humans
46
Define allele
One of a number of alternative forms of a gene
What is mRNA
A type of RNA that transfers the DNA code from the nucleus to the cytoplasm for protein synthesis where it acts as a template
Define codon
The sequence of three bases on mRNA that codes for a single amino acid
Define genome
The complete set of genes in a cell including those in mitochondria and/or chloroplasts
Define proteome
The full range of proteins produced by the genome
What does RNA stand for
ribonucleic acid
Describe the structure of RNA
A polymer made of repeating mononucleotide sub-units forming a single strand. Each nucleotide is made of:
- ribose
- an organic base (AGCU)
- a phosphate group
What is tRNA
- relatively small
- single stranded
- folded into clover leaf shape
- three base sequence (anticodon) on one end
- carries an amino acid
- each tRNA is specific to one amino acid and has an anticodon that is specific to that amino acid
What is different about the bases in RNA
Thymine is replaced by uracil
What is transcription
The process of making pre-mRNA using the DNA as template
Describe the process of transcription
- an enzyme acts on a specific region of the DNA causing the two strands to separate and expose the nucleotide bases in that region
- nucleotides on template strand pair with complementary nucleotides from the pool present in the nucleus
- RNA polymerase moves along the strand and joins nucleotides together (via condensation reaction making phosphodiester bonds ) to form a pre-mRNA molecule
- DNA strand rejoins behind this
- RNA polymerase reaches the stop triplet it detaches
What is the stop triplet
A particular sequence of bases on the DNA that causes RNA polymerase to stop bonding nucleotides on pre-mRNA molecules
Describe how pre-mRNA is made into RNA
Via splicing, a process where introns are removed and the functional exons are joined together
Why is splicing necessary
To get rid of introns (non coding parts of DNA) which would prevent the synthesis of a polypeptide
Why is splicing not necessary in prokaryotic cells
Most prokaryotic cells do not have introns, transcription results directly in mRNA so splicing is unnecessary
How does mRNA leave the nucleus
Via a nuclear pore as mRNA molecules are too large to diffuse out the nucleus where it attaches to a ribosome in the cytoplasm
What is translation
The synthesis of a polypeptide
Describe translation
- ribosome attaches to starting codon at one end of the mRNA
- tRNA molecule, carrying a specific amino acid, with the complementary anticodon moves to the ribosome and pairs with the codon on the mRNA
- another tRNA complementary anticodon pairs with the next codon on the mRNA, carrying a specific amino aid
- the ribosome moves along the mRNA binging the two tRNA molecules together
- two amino acids on the tRNA are joined by a peptide bond using an enzyme and ATP (which is hydrolysed to provide energy)
- as this happens the tRNA is released from its amino acid and is free to collect another amino acid from the amino acid pool in the cell
- this process continues until a ribosome reaches a stop codon causing the mRNA and the last tRNA molecule to separate
