AQA AS Level Biology Unit 2 - DNA & Meiosis

Notes on the second topic of dna and meiosis from the aqa AS biology course

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Structure of DNA

DNA - has a double helix structure and is the chemical that determines inherited characteristics. It contains vast amounts on information in the form of the genetic code. Despite its complex structure, DNA is made up of just three basic components that combine to form a nucleotide

Nucleotide structure: Individual nucleotides of DNA are made up of three components:

  • a sugar called deoxyribose
  • a phosphate group
  • an organic base belonging to one of two different groups:               
  • (a) single-ring bases - cytosine (c) and thymine(T)                            
  • (b) double - ring bases - adenine (A) and guanine (G)

The deoxyribose sugar, phosphate group and organic base are combined as a result of condensation reactions to give a single nucleotide (mononucleotide). Two mononucleotides combine as a result of a condensation reaction between the deoxyribose sugar of one mononucleotide and the phosphate group of another. The new structure is called a dinucleotide. The continued linking in this way forms a long chain known as a polynucleotide.

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DNA structure

DNA is made up of two strands of nucleotides (polynucleotides). Each of the strands is extremely long and they are joined together by hydrogen bonds formed between certain bases.

In its simplest form DNA can be thought of as a ladder in which the phosphate and the deooxyribose molecules alternate to form the uprights and the organic bases pairs together to form the rungs

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The pairing of bases

The organic bases contain nitrogen and are of 2 types, Those with a double ring structure (adenine and guanine) have longer molecules than those with a single ring structure (cytosine and thymine). It follows that if the rungs of the DNA ladder are to be the same length the base oairs must always be made up of one of each type.

  • Adenine always pairs with thymine by the means of 2 hydrogen bonds (adenine is complementary to thymine)
  • Guanine always pairs with cytosine by the means of 3 hydrogen bonds(guanine is complementary to cytosine)
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The double helix

The ladder like arrangement of the 2 polynucleotide chains are twisted. The uprights of the phosphate and deoxyribose wind around one another to form a double helix. They form the structural backbone of the DNA molecule. For each complete turn of the helix, there are ten base pairs

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The function of DNA

DNA is the hereditary material responsible for passing genetic information from cell to cell and generation to generation. There are almost infinite varieties of sequences of bases along the length of a DNA molecule. (It is this variety that provides the immense diversity within living organisms.)

The DNA molecule is adapted to carry out its funsctions in a number of ways:

  • It is very stable and can pass from generation to generation without change
  • It's 2 separate strands are joined only with the hydrogen bonds which allow them to separate during DNA replication and protein synthesis
  • It is an extremely large molecule and therefore carries an immense amount of genetic information
  • By having the base pairs within the helical cylinder of the deoxyribose phosphate backbone the genetic information is to some extent protected from being corrupted by outside chemical and physical forces

The function of the DNA molecule depends on the sequence of base pairs that it posesses. This sequence is important to everything it does and to life itself

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DNA and chromosomes

In prokaryotic cells (bacteria) the DNA molecules are smaller, form a circle and are associated with protein molecules. Prokaryotic cells therefore dont have chromosomes,

In eukaryotic cells the DNA molecules are karger, form a lin (are linear) and occur in association with proteins to form structures called chromosomes.

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Chromosome Structure

Chromosomes are only visible in distinct structures when a cell is dividing. For the rest of the time they are widely dispersed throughout the nucleus. When visible, chromosomes appear as two threads, joined at a single point. Each thread is called a chromatid.

The DNA in chromosomes is held in position by proteins, The considerable length of DNA found in each cell is highly coiled and folded.

DNA-protein complex is then coiled. The coild is looped and further coiled before being packed into the chromosome. In this way a lot of DNA is condensed into a single chromosome. This single DNA molecule has many genes along its length. Each gene occupies a specific position along the DNA molecule.

Although the number of chromosome is always the same for normal individuals of a species, it varies from one species to another. In almost evey species, there is an even number of chromosomes in the cells of adults. This is because chromosomes occur in homologous pairs.

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Homologous chromosomes

Homologous chromosomes are when one of each of the set of chromosomes derieved from the chromosomes provided by the father (sperm) and the other is derived from the chromosomes provided by the mother (the egg) - paternal chromosomes

These are known as homologous pairs and the total number is referred to as the diploid number. (in humans its 46)

A homologous pair is always 2 chromosomes that determine the same genetic characteristics but 'determining the same genetic characteristics' is not the same as being identical'

E.g a homologous pair may each possess the information for eye colour and blood group, but one chromosome may carry the codes(alleles) for blue eyes and blood group A whereas the other carries the codes (alleles) for brown eyes and blood group B. During meiosis the halving of the number of chromosomes is done in a manner which ensures that each faufhter cell recieves one chromosome from each homologous pair. In this way each cell receives one set of information for each characteristic of the organism. When these haploid cells combine, the diplois state is restored, with paired homologous pairs, is restored

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What is an allele?

Genes are sections of DNA that contain coded information in the form of specific sequences of bases. Each gene exists in 2, or more, different forms. Each of these forms is called an allele. Each individual inherits one allele from each parent. These two allels may be the same or they may be different, each allele will code for a different polypeptide.

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Meiosis is the production of four daughter nuclei, each with half the number of chromosomes as the parent cell.

Meiosis involves two nuclear divisions tha normally occur one after teh other:The process of meiosis:

  • 1. In the first division (meiosis 1) the homologous chromosomes pair up and their chromatids wrap around each other. Equivalent portions of these chromatids may be exchange in a process called crossing over. By the end of their stage the homologous pairs have been separated, with one chromosomes from each pair going into the two daughter cells.
  • 2. In the second meiotic division (meiosis 2) the chromatids move apart. At the end of meiosis 2, four cells have been formed. In humans each of these cellls contains 23 chromatids
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Meiosis and genetic variation

In addition to halving the number of chromosomes, meiosis also produces genetic variation among the offspring, allowing an organsim to adapt and survive in a changing world. Meiosis bring about this genetic variation in the following ways:

  • independant segregation of homologous chromosomes
  • recombination of homologous chromosomes by crossing over

Key words:

  • gene - a section of DNA that codes for a polypeptide
  • locus - the position of a gene on a chromosome or DNA molecule
  • allele - one of the different forms of a particular gene

Independant segregation of homologous chromosomes:

  • During meiosis 1 each chromsome lines up alongside its homologous partner in a random order. One of each pair will pass to each daughter cell (which one of the pair goes into the daughter cell and with which one of any other pairsm depends on how the pairs are lined up in the parent cell. Because the line is random, so is the combination of chromosomes passed on to the daughter cell. - independent segregation
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Variety from new genetic combinations

Each member of a homologous pair of chromosomes has exactly the same characteristics and therefore determines the same characteristics (e.g. blood group or eye colour). However, alleles of these genes may differ(brown and blue eyes/group A or groups B) The random distribution and consequent independant assortment of these chromosomes therefore produces new genetic combinations. This happens in stages:

  • stage 1: One of the pair of chromosomes includes the gene for eye colour and carries one allele for brown eyes and one for blue eyes. The other chromosome carries the gene for blood group and the allele for blood group A and the allele for blood group B. There are two possible arrangements, P and Q of the two chromosomes at the start of meiosis.
  • Stage 2: At the end of meiosis 1, the homologous chromosomes have segregated into two separate cells.
  • Stage 3: At the end of meiosis 2, the chromosomes have segregated into chromatids producing 4 gametes for each arrangement. The actual gametes are different, depending on the original arrangement of chromosomes in stage 1. (brown and a, brown and b, blue and a, blue and b)
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Genetic recombination by crossing over

During meiosis 1 each chromosome lines up alongside a homologous parter. Then:

  • The chromatids on each pair become twisted around one another
  • During this twisting process tensions are created and portions of the chromatid break off
  • These broken portions then rejoin with chromatids of its homologous partner
  • Usually it is the equivalent portions of homologous chromosomes that are exchanged
  • In this way new genetic combination are produced

If recombinations doesnt occur only 2 types of cell ocur, whereas if recombination does occur then 4 types of cells are produces. Crossing over increases genetic variety.

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What is a gene?

  • Genes are sections of DNA that contain the coded information for making polypeptides.
  • The coded information is in the form of a specific sequence of bases along the DNA molecule
  • Polypeptides combine to make proteins and so genes determine the proteins of an organism
  • Enzymes are proteins. A enzymes control chemical reactions they are responsible for an organism's development and activities
  • Genes determine the nature and development of all organisms.
  • A gene is a sequence of DNA bases that determine a polypeptide
  • a polypeptide is a sequences of amino acids
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The triplet code

Scientists suggest that there must be a minimum of three bases that coded for each amino acid. There reasoning was as follows:

  • Only 20 amino acids regularly occur in proteins
  • Each amino acid must have its own code of bases on the DNA
  • One 4 different bases (adenine, thymine, guamine, cytosine) are present i DNA
  • If each bases coded for a different amino acid, only four different amino acids could be coded for
  • Using a pair of bases, 16 (4 squared) different codes are possible which is inadequate
  • Three bases produce 64 (4 squared) different codes morethan enough to satisfy the requirements of 20 amino acids

As the code has three bases it is called the triplet code. As there are 64 possible codes and only 20 amino acids, it follows that some amino acids have more than one code. In eukaryotes much of the nuclear DNA does not code for amino acids. These sections are called introns and can occur within genes and as multiple repeats between genes

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Features of a triplet code

Further experiments have revealed the following features of the triplet code:

  • A few amino acids have only a single triplet code
  • the remaining amino acids have between 2 and 6 triplet codes
  • The code is known as a 'degenerate code' because most amino acids have more than one triplet code
  • The triplet code is always read in one particular direction alons the DNA strand
  • The start of a sequence is always the same triplet code. The amino acidsmethionine. If the first methionine molecule does not form part of the final polypeptide, it is later removed
  • Three triplet codes do not code for any amino acids. These are called 'stop codes' and mark the end of the polypeptide chain. They act in much the same way as a full stop at the end of a sentence
  • The code is non-overlappiong i.e each base in the sequence is read only once. Thus six bases numbered 123456 are read as triplets 123, 456 rather than as triplets 123, 234, 345, 456
  • The code is universal i.e with a few minor exceptions it is the same in all organisms
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kath toman


v. helpful - but no info on topic 8.2; 'what is a gene?' and 'the triplet code'...

Bethan Harris


Hiya. Ive added the extra few sub-topics in - thanks :) they're at the end if you were to need them. :) **



good resource, cheers



Very useful. Thank you soooo much!

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