Unit 4 - Genetic information, variation and relationships between organisms
Summary
Summary AQA Biology: A Level Year 1 and AS - Topic 4 - Genetic information, variation and relationships between organisms
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Unit 4 - Genetic information, variation and relationships between organisms
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AQA
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AQA Biology A Level Student Book
Summary of Topic 4: Genetic information, variation and relationships between organisms from the AQA Kerboodle textbook. No need to worry about summarising long chunky paragraphs of information or worrying about revision notes.
Unit 4 - Genetic information, variation and relationships between organisms
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Definitions- Topic 4- Genetic information, variation and Relationships between Organisms
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Flashcards61 Flashcards
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Some examples from this set of practice questions
1.
Adaption
Answer: A feature of an organism that increases its chance of survival in its environment.
An adaptation may be anatomical, physiological or behavioural.
2.
Allele
Answer: A version of a gene.
3.
Anticodon
Answer: A sequence of three nucleotide bases at one end of a tRNA molecule that is
specific to an mRNA codon.
4.
Artifical classification
Answer: A type of classification that divides organisms into groups based on
analogous characteristics such as leaf shape, number of legs and type of wing.
5.
Binomial system
Answer: A universal system of naming organisms that consists of two parts: the
generic name and the specific name, e.g. Homo sapiens.
6.
Biodiversity
Answer: The variety of genes, species and habitats within a particular area.
7.
Cellular proteome
Answer: The proteins expressed in a given type of cell.
8.
Chromatid
Answer: One strand of a replicated chromosome.
9.
Chromosome
Answer: A structure consisting of a long, coiled molecule of DNA and its associated
proteins, by which genetic information is passed from generation to generation.
10.
Chromosome mutation
Answer: A change to the number or structure of chromosomes that can
occur spontaneously.
Content preview
TOPIC 4: GENETIC INFORMATION, VARIATION AND
RELATIONSHIPS BETWEEN ORGANISMS
CHAPTER 8: DNA, genes and protein synthesis
8.1 Genes and the genetic code
What is a gene?
● A gene is a section of DNA that contains the coded information for making
polypeptides and RNA
● Coded information is in the form of specific sequences of bases along the DNA
molecules
● Enzymes are proteins. They control chemical reactions so they are responsible for an
organism's development
● Genes along with environmental factors determine the nature and development of
all organisms
● Locus: section of DNA where the gene is located
The genetic code
The minimum of three bases codes for one amino acid. Reasoning:
● Only 20 different amino acids regularly occur in proteins
● Each amino acid must have its code of base on DNA
● Only 4 different amino acids are present in DNA
● If each base coded a different amino acid only 4 different amino acids could be coded
for
● Using a pair of bases 16 (4^2) different codes are possible which is still inadequate
● Three bases produce 64(4^3) different codes, more than enough to satisfy the
requirements of 20 amino acids
As the code has three bases for each amino acid, each one is called a triplet. As there are 64
possible triplets and only 20 amino acids it follows that some amino acids are coded for by
more than one triplet
Features of the genetic code
● A few amino acids are coded for by only a single triplet
● The remaining amino acids are coded for between 2 and 6 triplets each
● The code is known as a ‘degenerate code’ because most amino acids are coded for by
more than one triplet
● A triplet is always read in one particular direction along the DNA strand
● The start of a DNA sequence that codes for a polypeptide is always the same triplet.
● Three triplets do not code for any amino acids. These are called stop codons and
mark the end of a polypeptide chain
● The code is no-overlapping. Each base in the sequence is read only once.
● The code is universal.
, Much of the DNA in eukaryotes does not code for the polypeptide. E.g. genes in the
sequence that are non-coding make multiple repeats of bases sequence for amino acids.
These are known as introns. The coding sequences are known as exons.
8.2 DNA and chromosomes
Chromosomes structures
● Chromosomes are only visible as distinct structures when a cell is dividing
● When they first become visible at the start of cell division chromosomes appear as
two strands joining at a single point
● Each three is called a chromatid because DNA has already replicated to give two
identical DNA molecule
● DNA in chromosomes are held by histones
DNA is a double helix. The helix is wound around histones to fix it in positions. The
DNA-histones complex is then coiled. The coil is looped and further coiled before being
coiled into the chromosomes.
Homologous chromosomes
● Humans are the result of the fusion of a sperm cell and egg cell, which contributes
one complete set of chromosomes to the offspring
● Each is derived from maternal chromosomes and paternal chromosomes
● The area is known as homologous pairs and the total number is referring to a diploid
● A homologous pair is always 2 chromosomes that carry the same genes but not
necessarily the same allele f the genes
What is an allele?
● An allele is an alternative version of a gene
● Each individual inherits one allele from each of their parents
● If an allele is different, each allele has a different base sequence therefore a different
amino acid sequence so produces a different polypeptide
● Any changes in the base sequence produce a new allele
8.3 Structure of ribonucleic acid
Transferring the coded information
● Synthesis of proteins takes place in the cytoplasm. However eukaryotic cells’ DNA is
confined in the nucleus
● For coded information of the DNA in the nucleus to be transferred to the cytoplasm
where it is translated into proteins sections of the DNA code are transcribed onto a
single-stranded molecule called RNA
Types of RNA
1. mRNA: also known as messenger RNA. transfer the DNA code from the nucleus to
the cytoplasm hence acting as a messenger
2. tRNA: also known as transfer RNA: Attaches to an amino acid
● Codon: sequence of 3 bases on mRNA that codes for a single amino acid
● Genome: complete set of genes in a cell
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