Biology Unit 2

BIOLOGY UNIT 2

7. Variation

7.1_Investigating Variation

·       When one species differs from another, this is called interspecific variation.

·       However, members of the same species also differ from each other, this is intraspecific variation.

SAMPLING

• Involves taking measurements of individuals selected from the population of organisms being investigated.
• Can we be sure the sample is representative?
• Sampling bias – Investigators may be making unrepresentative choices when sampling. E.g. may avoid areas in cow dung or stinging nettles.
• Chance – Individuals may, by pure chance, not be representative.

-Best way to eliminate sampling bias is to eliminate as much human involvement as possible.

-Random sampling – Divide area into grid with numbers > Use random number table to obtain coordinates. > Take samples from these coordinates.

-Cannot completely remove chance but can minimise its effect:

-Use a large sample size – more reliable data.

-Analysis of data collected – Statistical tests can be used to determine to what extent chance may have influenced results – allow us to determine whether variation is due to chance or another cause.

CAUSES OF VARIATION

·        Variation is the result of two main factors:

-         Genetic differences

-         Environmental influences

Genetic differences:

·        Mutations: Sudden changes to genes and chromosomes may, or may not, be passed to next generation.

·        Meiosis: Special form of nuclear division formes gametes. This mixes up genetic material before it’s passed to gametes, all of which are therefore different.

·        Fusion of gametes: In sexual reproduction, offspring inherit some chacteristics of each parent so are different from both. Which gametes fuse is random, adding to variety.

·        Variety in asexually reproducing organisms can only be increased through mutations.

Environmental influences:

·        Genes set limits, but it’s largely the environment that determines where, within the limits, an organism lies.

·        E.g. a buttercup may have genes to allow it to grow tall, but if there is low light in an area, it will not grow properly.

·        Variation is usually due to a mixture of genetic differences and environmental influences.

·        Difficult to distinguish between effects of many genetic and environmental influences so it is very difficult to draw conclusions about what causes variation in any particular case.

8. DNA and meiosis

8.1_Structure of DNA

NUCLEOTIDE STRUCTURE

Individual nucleotides have 3 components:

-Deoxyribose (sugar)

-Phosphate group

-An organic bas belonging to one of 2 groups

  *Single-ring bases – cytosine (C) and Thymine (T)

  *Double- ring bases – Guanine (G) and Adenine (A)

·        The deoxyribose sugar, phosphate group and organic base are combined (through a condensation reaction), to give a mononucleotide.

·        Two mononucleotides may cobine in a condensation reaction between the Deoxyribose sugar of one nucleotide and the phosphate group of another, to form a dinucleotide.

·        A long chain of these is a polynucleotide.

DNA structure

• In 1953, James Watson and Francis Crick worked out the structure of DNA, following on from the work of Rosalind Franklin on the x-ray diffraction patterns of DNA.
• DNA is made up of two strands of nucleotides (polynucleotides), each of which are very long and joined by hydrogen bonds between certain bases.
• Simplified, DNA is a ladder where phosphate and Deoxyribose form the uprights, while the organic base pairs make the rungs.

Pairing of bases

• Organic bases contain nitrogen.
• Double-ring structures (A & G) have longer molecules than Single-ring structures (C & T).
• Therefore, for the rungs of ladder to be same length, base pairs must always form from one double-ring and one single-ring structure.

-Adenine always pairs with Thymine by two hydrogen bonds.

-Guanine always pairs with Cytosine by two hydrogen bonds.

• Adenine is complementary to Thymine and Guanine is complementary to Cytosine.
• Quantities of Adenine and Thymine are always the same, as are the quantities of Guanine and Thymine, but the ratio of A & T and G & C varies between species.

The double helix

• The ladder arrangement of two polynucleotide chains is twisted to form a double helix.
• The phosphate and Deoxyribose form the backbone of the DNA molecule.
• For each complete turn of the helix, there are 10 base pairs.

The function of DNA

• DNA is the hereditary material responsible for passing genetic information between cells and generations.
• There are around 3.2 billion base pairs in the DNA of a typical mammalian cell.
• This means there is an almost infinite variety of sequences of bases along the length of a DNA molecule, which provides immense genetic diversity.

DNA molecule adapted to carry out functions in number of ways:

-Very stable and can pass between generations without change.

-Two separate strands are joined only with hydrogen bonds, allowing them to separate during DNA replication and protein synthesis.

-Extremely large molecule so carries immense amount of genetic information.

-Having the base pairs within the helix means that the genemtic information is somewhat protected from being corrupted by outside chemical and physical forces by the deoxribose-phosphate backbone.

• The function of DNA depends on the sequence of base pairs it possesses.

8.2_The triplet code

WHAT IS A GENE?

• Section of DNA that contains coded information for making polypeptides.
• Coded information in form of specific sequence of bases along DNA molecule that determines a polypeptide which is a sequence of amino acids.
• Polypeptides combine to make proteins so genes determine proteins of an organism.
• Enzymes control chemical reactions so are responsible for an organisms development and activities.
• Genes determine nature and development of all organisms.

THE TRIPLET CODE

·        Must be a minimum of 3 bases coding for each amino acid:

-Only 20 amino acids regularly occur in proteins

-Each amino acid must have own code of bases on DNA.

-Only 4 different bases are present in DNA.

-If each base coded for different A.acid, only 4 could be coded for.

-Using a pair of bases – 16 (4²) codes possible – still inadequate.

-3 bases produce 64 (4³)codes – more than enough for 2 A.Acids.

• Three bases = triplet code.
• 64 codes and only 2 amino acids = some amino acids have more than one code.
• In eukaryotes much of nuclear DNA doesn’t code for Amino acids,
• These sections are called introns and can occur within genes and as multiple repeats between genes.

INTRONS

8.3_DNA and chromosomes

Prokaryotic cells – e.g. bacteria – DNA molecules are smaller, form a circle and not associated with protein molecules. Do not have chromosomes.

Eukaryotic cells – DNA molecules are larger, form a line and occur in association with proteins to form chromosomes.

CHROMOSOME STRUCTURE

• Chromosomes only visible as distinct structures during cell division.
• Rest of the time – widely dispersed throughout nucleus.
• When visible – two threads joined at a single point. Each thread is a chromatid.
• DNA in chromosomes held in positions by proteins.
• Around 2m of DNA in every human cell is highly coiled and folded.

-Helix wound around proteins to fix in position.

-DNA- protein complex then coiled.

-The coil is looped and further coiled.

Coil packed into chromosome.

• Single DNA molecule has many genes along length.
• Each gene in a specific position.
• Same number of chromosomes for normal individuals of a species e.g. humans have 46, potato plant have 48 an dogs have 78.
• Almost all species have an even number of chromosomes in cells of adults because of homologous pairs.

HOMOLOGOUS CHROMOSOMES

• Reason for pairs – fusion of sperm & egg in sexual reproduction.
• One of each pair from mother(maternal), one from father(paternal).
• Total number of homologous pairs is the diploid number. (46 in humans).
• A homologous pair is always 2 chromosomes that determine same genetic characteristics – ‘determining the same characteristics’ is not being identical – Homologous pair may posses info on eye colour and blood group, one chromosome blue and A, one green and B (alleles).
• During meiosis, the halving of the number of chromosomes is done in such a way that ensures each daughter cell gets a chromosome from each homologous pair – each cell receives one set of information for each characteristic.
• When these haploid cells combine, diploid state with homologous chromosomes is restored.