An in-depth informative booklet on Genetic Information including protein synthesis, meiosis, natural selection, species and taxonomy, investigating diversity and much more. Also includes required practicals for this topic, useful summary questions, exam style questions and the specification for thi...
Genetic Information
Variation and Relationships between Organisms
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1
, DNA, Genes and Chromosomes
A reminder of the structure of DNA
A short section of a DNA molecule
• DNA is a double stranded molecule, each strand is a polynucleotide made up of individual
nucleotides.
• Each nucleotide comprises a phosphate, deoxyribose sugar and a nitrogenous base.
• The bases from one strand pair in a complementary fashion with the bases on the other strand,
Adenine pairs with Thymine, and Guanine with Cytosine.
• DNA molecules are many millions of base pairs long.
Where is DNA found?
1. Prokaryotic (bacterial) cells have a single molecule of DNA formed into a circle. Some cells
also have smaller loops called plasmids. The DNA is naked and not associated with proteins.
2. The DNA in the chloroplasts and mitochondria of eukaryotic cells is also a short, circular
molecule not associated with proteins.
3. The DNA in the nuclei of eukaryotic cells is a very long linear molecule which is associated
with histone proteins. Together a DNA molecule and the histone proteins form a chromosome.
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,Chromosomes
Label this diagram to show how DNA is organised into
chromosomes
• The DNA molecule is wrapped around histone proteins to fix it into position, the DNA histone
complex is then coiled into solenoids and then further coiled, this means that a very long molecule
of DNA can be condensed into a single chromosome, a lot of genetic information can be stored in
the nucleus of each cell.
• When a cell is in interphase the chromosomes are dispersed throughout the nucleus and are not
visible as separate structures.
• A chromosome can only be seen as a distinct structure when the nucleus is about to divide and the
DNA molecule is already replicated by semi-conservative replication. Each sister chromatid contains
a genetically identical DNA molecule. The two sister chromatids are held together by a centromere.
• Diploid organisms inherit a full set of chromosomes from each parent, as a consequence diploid
cells have homologous pairs of chromosomes.
• Humans have a diploid number 2n = 46 however other species have different diploid numbers,
dogs have 78 chromosomes and potato plants have 48.
Label a pair of homologous chromosomes on this
diagram of human chromosomes
One member of the pair is inherited from the
mother and is called the maternal chromosome
and one member of each pair is inherited from
the father and is called the paternal
chromosome.
Homologous pairs carry the same genes at the
same locus but they do not necessarily carry the
same alleles of the gene. Homologous pairs are
not genetically identical.
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, What is a gene?
• A gene is a base sequence of DNA that codes for either:
i. The amino acid sequence of a polypeptide.
ii. A functional RNA (including rRNA and tRNA).
• A gene occupies a fixed position called a locus on a particular DNA molecule. One DNA molecule
carries many genes.
• In eukaryotes most of the DNA molecule does not code for amino acid sequences in polypeptides
(only 2% in humans). There are non-coding multiple repeat sequences between genes.
• Not all of the base sequence within genes codes for amino acid sequences. The parts of the genes
which code for amino acid sequences are called exons, they are separated by one or more non-
coding sequences call introns.
Coding sequences
of the gene
Non-coding sequences of the gene
Definitions
Genome The complete set of all of the genes in a cell, including the genes in the nucleus, mitochondria
and chloroplasts.
Proteome The full range of all the proteins that a cell is able to produce using its genome.
Allele some genes have two or more alternative forms, each allele has a different sequence of bases and
consequently codes for a different polypeptide.
Gene pool all the different alleles of all the genes found within a population.
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