DNA, Genes & Chromosomes (AQA A Level Biology): Flashcards
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- FrontProkaryotic DNA
Define the term nucleoid
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Define the term nucleoid
The nucleoid is the region of a prokaryotic cell that contains the single, circular chromosomal DNA molecule.
Describe three features of DNA in prokaryotic cells
Short
Circular
Not associated with proteins (unlike eukaryotic DNA, which associates with histones)
Define the term plasmid
A plasmid is a small circular DNA molecule that contains a few genes, found in addition to the main chromosomal DNA of a prokaryotic cell.
True or False?
Prokaryotic DNA is associated with histone proteins
False.
The DNA within prokaryotic cells is not associated with any proteins. It is eukaryotic DNA that associates with histone proteins.
The DNA in prokaryotic cells is not associated with any .
The DNA in prokaryotic cells is not associated with any proteins.
Which type of gene is often found on plasmids?
Genes for antibiotic resistance are often found on plasmids.
True or False?
A prokaryotic cell contains a single, circular chromosomal DNA molecule
True.
Prokaryotes possess a single, circular chromosomal DNA molecule, sometimes referred to as a nucleoid.
Why are genes that are needed often, quickly or in emergencies located on plasmids?
Plasmids are more accessible to the proteins required for gene expression.
This means the genes they carry can be expressed quickly when needed.
Prokaryotes usually also contain one or more small circular DNA molecules called .
Prokaryotes usually also contain one or more small circular DNA molecules called plasmids.
How does DNA in prokaryotic cells differ from DNA in eukaryotic cells in terms of its association with proteins?
Prokaryotic DNA is not associated with proteins.
Eukaryotic DNA is associated with histone proteins.
Define the term chromosome
A chromosome is a DNA molecule together with its associated proteins (histones).
Describe three features of DNA found in the nucleus of eukaryotic cells
Very long
Linear
Associated with proteins called histones
Define the term histones
Histones are large, positively charged globular proteins that organise and condense DNA.
What is the role of histone proteins?
To organise and condense the DNA tightly so that it fits into the nucleus.
Define the term chromatin
Chromatin is the tightly coiled combination of DNA and proteins that makes up chromatids and chromosomes.
How does the DNA in mitochondria and chloroplasts differ from DNA in the nucleus?
It is shorter, circular and not associated with histones — similar to prokaryotic DNA.
True or False?
Histones are negatively charged proteins
False.
Histones are positively charged globular proteins, which allows them to associate with the negatively charged DNA.
DNA is coiled around proteins to form chromatin.
DNA is coiled around histone proteins to form chromatin.
True or False?
Eukaryotic DNA in the nucleus is linear
True.
In the nucleus of eukaryotic cells, DNA molecules are very long and linear.
Besides histones, what other proteins are associated with eukaryotic DNA?
Enzymes used in copying and repairing the DNA.
Unlike nuclear DNA, the DNA in mitochondria and chloroplasts is short and , like prokaryotic DNA.
Unlike nuclear DNA, the DNA in mitochondria and chloroplasts is short and circular, like prokaryotic DNA.
Describe three features of the DNA found in mitochondria and chloroplasts
Short
Circular
Not associated with protein
What is meant by mtDNA?
mtDNA is mitochondrial DNA, the DNA found within the mitochondria of eukaryotic cells.
What is meant by cpDNA?
cpDNA is chloroplast DNA, the DNA found within the chloroplasts of eukaryotic cells.
Where in the mitochondrion is mtDNA located?
In the matrix of the mitochondrion.
Where in the chloroplast is cpDNA located?
In the stroma of the chloroplast.
True or False?
The DNA in mitochondria and chloroplasts is similar to the DNA of prokaryotes
True.
Like prokaryotic DNA, it is short, circular and not associated with protein.
The DNA in mitochondria and chloroplasts is short, circular and not associated with .
The DNA in mitochondria and chloroplasts is short, circular and not associated with protein.
True or False?
Chloroplast DNA is linear and associated with histone proteins
False.
Chloroplast DNA is short, circular and not associated with protein, like prokaryotic DNA.
Mitochondrial DNA is located in the of the mitochondrion.
Mitochondrial DNA is located in the matrix of the mitochondrion.
Define the term gene
A gene is a base sequence of DNA that codes for the amino acid sequence of a polypeptide or a functional RNA molecule.
What two things can a gene code for?
The amino acid sequence of a polypeptide
A functional RNA molecule (e.g. rRNA and tRNA)
Name the three types of functional RNA molecule
mRNA (messenger RNA)
tRNA (transfer RNA)
rRNA (ribosomal RNA)
What is the role of rRNA?
Ribosomal RNA molecules form part of the structure of ribosomes.
What is the role of tRNA?
Transfer RNA carries amino acids to the ribosome.
What is the role of mRNA?
The base sequence on messenger RNA is used by ribosomes to form polypeptide chains.
True or False?
A gene can only code for a polypeptide
False.
A gene codes for the amino acid sequence of a polypeptide OR a functional RNA molecule.
A gene is a base sequence of that codes for a polypeptide or a functional RNA molecule.
A gene is a base sequence of DNA that codes for a polypeptide or a functional RNA molecule.
As well as coding for polypeptides, genes can code for RNA molecules such as tRNA and rRNA.
As well as coding for polypeptides, genes can code for functional RNA molecules such as tRNA and rRNA.
How do genes control protein structure and function?
Genes determine the exact sequence in which amino acids join together during protein synthesis.
This sequence determines the shape and behaviour of the protein.
Define the term locus
A locus is the fixed position of a gene on a chromosome.
Define the term allele
An allele is one of the different forms of a gene.
What is the plural of locus?
Loci
True or False?
Different alleles of a gene occupy the same locus
True.
Different alleles have slightly different nucleotide sequences but still occupy the same position (locus) on the chromosome.
How do different alleles of the same gene differ from one another?
They have slightly different nucleotide sequences.
The fixed position of a gene on a chromosome is called its .
The fixed position of a gene on a chromosome is called its locus.
Different forms of the same gene are called .
Different forms of the same gene are called alleles.
True or False?
Alleles of the same gene are found at different loci
False.
Alleles of the same gene occupy the same locus on the chromosome.
In terms of loci, define a gene
A gene is a length of DNA that codes for a single polypeptide or protein.
Define the term triplet (in the genetic code)
A triplet is a sequence of three DNA bases that codes for one amino acid.
How many DNA bases code for one amino acid?
Three (a triplet)
How many different triplets are possible, and why?
64 different triplets.
There are four bases (A, G, C, T), giving 4^3 = 64 possible combinations.
What is meant by the genetic code being degenerate?
Degenerate means that multiple triplets can code for the same amino acid.
What is the benefit of the genetic code being degenerate?
It can limit the effect of mutations, as a change in base may still code for the same amino acid.
What is meant by the genetic code being universal?
Almost every organism uses the same code, so the same triplet codes for the same amino acid in all living organisms.
State the three features of the genetic code
Universal
Non-overlapping
Degenerate
True or False?
There are 64 amino acids commonly found in proteins
False.
There are 64 triplets but only 20 amino acids that commonly occur in proteins.
Besides amino acids, what can some triplets code for?
Start and stop signals, which tell the cell where genes start and stop.
A sequence of three DNA bases that codes for one amino acid is called a .
A sequence of three DNA bases that codes for one amino acid is called a triplet.
Because multiple triplets code for the same amino acid, the genetic code is described as .
Because multiple triplets code for the same amino acid, the genetic code is described as degenerate.
Define the term intron
An intron is a non-coding sequence of DNA found within a gene.
Define the term exon
An exon is a coding sequence of DNA found within a gene.
In what form is non-coding DNA found between genes?
As non-coding multiple repeats — the same base sequences repeated multiple times.
Define the term splicing
Splicing is the process by which introns are removed and exons are joined together to form mRNA.
State the two locations of non-coding DNA in a eukaryotic genome
Between genes, as non-coding multiple repeats
Within genes, as introns
True or False?
Introns code for amino acid sequences
False.
Introns are non-coding; it is the exons that code for amino acid sequences.
Non-coding sequences found within a gene are called .
Non-coding sequences found within a gene are called introns.
The coding sequences within a gene are called .
The coding sequences within a gene are called exons.
What molecule is produced when the whole gene (introns and exons) is transcribed?
pre-mRNA
True or False?
Splicing removes exons from pre-mRNA
False.
Splicing removes the introns; the exons are joined together to form mRNA.
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