AQA Biology Unit 2: 8 DNA and Meiosis

• DNA (Deoxyribonucleic acid) is the chemical that determines inherited characteristics
• DNA is made up of 3 components that form a nucleotide
• A nucleotide is made up of deoxyribose (sugar), a phosphate group and organic base
• The two types of base are:
  • Single ring bases: Cytosine (C) and Thymine (T)
  • Double ring bases: Adenine (A) and Guanine (G)
• The deoxyribose, phosphate group and organic base are combined due to condensation reactions
• A dinucleotide is formed from two mononucleotides and if the mononucleotides continue linking they form a polynucleotide

DNA Structure

• Made up of two strands of nucleotides that are joined together by hydrogen bonds between the bases
• In a ladder shape the deoxyribose and phosphates for the poles and the bases for the steps

Pairing of Bases

• The bases contain nitrogen and are complementary of each other
• The double ring structured bases (A and G) have longer molecules than the single ring structured bases (C and T)
• A and T are paired by 2 hydrogen bonds, C and G are paired by 3 hydrogen bonds
• A and T will always have the same number as each other just like C and G to each other
• The DNA forms a double helix in which every turn has 10 bases

Function of DNA

• It passes genetic information from cell to cell and generation to generation
• There is almost an infinite amount of sequences for bases along the DNA
• DNA is stable and difficult to be changed as well as being large carrying much genetic information
• The separate strands are able to split apart during protein synthesis and DNA replication due to being held by hydrogen bonds
• Bases protected by the <span>deoxyribose-phosphate backbone from chemical/physical forces</span>

What is a Gene?

• Genes are sections of DNA that contains information for polypeptide production
• The information has a specific sequence of bases along the DNA molecule
• Polypeptides form proteins such as enzymes
• Enzymes control chemical reactions so are responsible for organism development and activities
• A Polypeptide is basically a sequence of amino acids

The Triplet Code

• 20 amino acids occur regularly in proteins
• Each amino acid has its own code of bases, some have more than one code
• The code is known as the degenerate code due to some amino acids having more than one code
• The code is also non overlapping

• In eukaryotic cells the DNA molecules is large, linear \and occurs in association with proteins to from chromosomes
• In prokaryotic cells the DNA molecules are smaller, form a circle and are not associated with protein molecules so they have no chromosomes

Chromosome Structure

• Only visible when a cell is dividing and appear as two threads (each known as a chromatid) joined at a single point (centromere)
• DNA in chromosomes is held in place by proteins
• In DNA the helix is wound around proteins, this DNA-protein complex is the coiled
• The coil is looped and further coiled to pack into the chromosome
• The single molecule of DNA in a chromosome contains many genes that occupy specific positions on the DNA molecule
• The number of chromosomes in species varies however they are always in pairs called homologous pairs

Homologous Chromosomes

• In sexually produced organisms half the DNA comes from each parent
• One of each pair of chromosomes comes from each parent, they have the same gene loci and are known as homologous pairs
• Cells that contain two sets of chromosomes in the nucleus are known as diploids
• A homologous pair possess information for the same thing e.g. eye colour but the chromosomes may carry different alleles e.g. brown colour or blue colour
• During meiosis the halving of chromosomes ensures that each daughter cell receives one chromosome from each homologous pair
• When the haploid cells combine they form a diploid

What is an Allele?

• Allele: One of the forms of a gene
• You receive one allele from each parent, different alleles code for different polypeptides
• Any differences in base sequence of an allele can result in different sequence of amino acids being coded causing different polypeptide production

Why is Meiosis Necessary?

• Meiosis produces four daughter nuclei each with half the number of chromosomes as the parent cell
• In sexual reproduction two gametes fuse to create the offspring with the full amount of chromosomes needed
• Meiosis is needed as by halving the number of chromosomes it makes sure the offspring with have the right amount of chromosome and it won't have doubled
• During meiosis the chromosome pairs separate so only once chromosome enters the gamete (haploid)
• When two haploids gametes fuse the diploid number is restored

The Process of Meiosis

• It involves two nuclear divisions that occur straight after each other:
  • Meiosis 1: Homologous chromosomes pair up and their chromatids wrap around each other. Equivalent portions of the chromatids can be exchanged in crossing over. By the end the homologous pairs have separated with one chromosome from each pair going into one of two daughter cells
  • Meiosis 2: Chromatids move apart and four cells are formed each with a single chromatid
• Meiosis produces genetic variation allowing offspring to adapt and survive to the environment
• Meiosis brings Genetic Variation by:
  • Independent segregation of homologous pairs
  • Recombination of homologous pairs by crossing over

• Gene: section of DNA that codes for a polypeptide
• Locus: the position of a gene on a chromosome/ DNA molecule
• Allele: one of the different forms of a particular gene

Independent Segregation

• When homologous pairs arrange themselves it is done randomly
• The combination of chromosomes into the daughter cell is completely random

Variety from New Genetic Combinations

• The independent segregation of the chromosomes produces new genetic combinations
• Gametes produced from meiosis will be genetically different due to the different combination of maternal and paternal chromosomes
• Each gamete had a different make up and due to random fusion variety is produced

Genetic Recombination by Crossing Over

• The chromatids of each pair become twisted around each other
• During this tensions are created and portions of chromatids break off
• These broken portions then rejoin with the chromatid of its homolgous pair
• New genetic combinations are produced
• Recombination: when broken off portions of chromatid recombine with another chromatid
• Crossing over increases genetic variety as it can produce four cells with different genetic composition