3.4.1 DNA, Genes And Chromosomes Flashcards
Describe prokaryotic DNA
Found in the cytoplasm
They possess a single, circular chromosomal DNA molecule (that can be referred to as a nucleoid)
Can have one or more plasmids, which are small circular DNA molecules eg. For antibiotic resistance
The DNA is double stranded
It is not associated with any proteins
It is shorter than eukaryotic DNA
Describe eukaryotic DNA
Found in the nucleus
It is linear in the form of chromosomes
It is associated with histone proteins to form chromosomes
It is longer than prokaryotic DNA
It is double-stranded
What are chromosomes?
Chromosomes are a compact x-shaped form of chromatin that is formed and visible during mitosis.
They are made of one very long, condensed DNA molecule associated with proteins in eukaryotic cells.
What are histones?
Proteins associated with eukaryotic DNA that organise DNA and fix it into position so that it is condensed to fit in the nucleus.
What is chromatin?
The tightly coiled combination of DNA and histone proteins, that makes up chromosomes.
It is the condensed structure of DNA to be packaged into the nucleus.
What is the centromere?
The point that joins two chromatids to form a chromosome, that divides to separate during mitosis when the spindle attaches, to separate chromatids.
What is the gene loci/locus?
The location of a specific gene on a chromosome
What are homologous chromosomes?
The same type of chromosome, so genes are in the same place in each. One homologous chromosome of each type is inherited from each parent.
Why do sister chromatids have to be identical?
So that when separated, genetically identical cells are produced during mitosis.
Where is mitochondrial DNA found?
In the matrix of the mitochondria
Where is chloroplast DNA located?
In the stroma of chloroplasts
Describe mitochondrial and chloroplast DNA
Short
Circular
Not associated with any proteins
(Similar to prokaryotic DNA)
What is a gene?
A gene is a base sequence of DNA that codes for the amino acid sequence of a polypeptide or functional RNA molecule.
What is mRNA base sequence used for?
The base sequence on mRNA is used by the ribosomes to form a polypeptide chain.
What does tRNA do?
Carries amino acids to the ribosome based on the base sequence of mRNA.
What does rRNA do?
Forms part of the structure of ribosomes.
What do genes control?
The shape and behaviour of a protein depends on the exact sequence of amino acids in the polypeptide chain it is formed from.
Therefore the genes in DNA molecules control protein structure and function, by determining the exact sequence in which the amino acids are joined together.
How many genes code for an mRNA molecule?
One gene codes for one mRNA molecule, which codes for one polypeptide.
What is a locus?
The position of a gene on a chromosome
What are alleles?
Different forms of the same gene that have slightly different nucleotide sequences. They occupy the same position/locus on the chromosome.
Why is the DNA code called a triplet code?
The DNA nucleotide base code found within a gene is a triplet code, meaning each sequence of 3 (triplet) of bases codes for one amino acid.
How many amino acids are in proteins?
20
What do start and stop codons signal?
Where a gene begins and ends
What does the start codon do?
Codes for an amino acid (methionine) that signals the start of the gene, and the amino acid sequence. It is later removed from the protein as it is not needed for the structure.
What does the stop codon do?
Does not code for an amino acid, but marks the end of a polypeptide chain so causes ribosomes to detach and stops translation.
What are the 3 features of the genetic code?
Degenerate
Universal
Non-overlapping
Define degenerate
Some amino acids are represented by more than one codon.
Why is the genetic code degenerate?
There are 20 different amino acids, and 4 different bases.
Therefore 3 bases are needed to make enough combinations to code for 20 amino acids.
4^3= 64
Why is it advantages that the genetic code is degenerate?
If a mutation occurs, even though the triplet of bases will be different, it may still code for the same amino acids and therefore have no effect on the protein.
Define universal
The same triplet of bases codes for the same amino acid in all organisms
Why is it advantages that the genetic code is universal?
It makes genetic engineering possible
Define non-overlapping
Each base in a gene is only part of one triplet of bases that codes for one amino acid, so each codon is read as a discrete unit.
Why is it advantages that the genetic code is non-overlapping?
If a mutation occurs, it will only affect one codon and therefore one amino acid.
What is the genome?
The entire set of genetic information in an organism
