Biology Key points Topic 3

• Zygotes are formed when two gametes fuse during fertilisation
• The DNA of an organism is its genome
• Genes contain the code to make specific proteins
• Proteins are polymers made up of a chain of amino acids
• Gametes only have one set of chromosomes so are haploid
• When two haploid gametes fuse (n+n) we get a diploid zygote
• Gametes are produced by meiosis
• Meiosis results in the formation of four haploid gametes which are all genetically different from each other.

1. Chromosome in the diploid cell make copies of themselves. DNA replication occurs (X-shape)

2. Two sets of chromosomes pair up

3. Each pair of chromosomes separate so there are two cells containing half of the chromosomes

4. The two chromosomes (X-shapes) spli so there are four haploid cells

Mitosis and Meiosis

Function:  Growth, repair/replace and asexual reproduction

                 Produce gametes for sexual reproduction

No. of cells prosuced: 2

                                     4

Types of cells produced: Body cells (diploid) - 2n

                                        Gametes (haploid) - n

Indentical or different: Identical

                                    Different

No. of divisions: 1

                           2

• A DNA molecule consists of two strands which are twisted into a helix, called a double helix
• Thw two strands join together by pairs of bases
• There are four types of bases A, T, C and G
• A always pairs with T
• C always pairs with G
• These matching base pairs are called complimentary base pairs
• The bases are joined together by weak hydrogwn bonds
• Hydrogen bond = weak force of attraction between a slightly negative part of a base and a slightly negative part of a base.
• Two hyrdrogen bonds form between A and T; three hydrogen bonds form between C and G

• DNA is a polymer consisting of monomers called nucleotides
• A nucleotide consists of three parts: Base, sugar and phosphate group.
• The sugar and phosphate form the backbone of the DNA strands
• The order of bases in a gene determines which protein is produced.

• Adding detergent - breakes down cell surface and nuclei membranes (releases DNA from the cells)
• Adding salt - makes DNA molecules clum together
• Filtering the mixture - separates cell debris from filtrate (e.g. cell walls)
• Adding the protease enzyme - to break down proteins (found in chromosomes)
• Adding cold ethanol - DNA is insoluble in ethanol so it forms a precipitate

• RNA consists of nucleotides, these consist of; a different sugar, bases C, G, A and U and a phosphate group
• RNA consists of a single strand of nucleotides
• There are two types of RNA - mRNA (messenger RNA) and tRNA (transfer RNA)

• DNA - specific base sequence provides the code for the sequence of amino acids
• mRNA - takes the code from the DNA in the nucleus to the ribosomes
• tRNA - brings specific amino acids to the ribosomes so that they can be placed in the correct sequence according to the base sequence on the mRNA 

• Transcription - the process by which the genetci code in one starnd of DNA molecules is used to make mRNA
• Translation - the process by which the genetic code in a molecule of mRNA is used to make a polypeptide
• Template strand - the strand of DNA molecule that RNA polymerase uses to make mRNA
• RNA polymerase - an enzyme that creates mRNA from DNA
• Codon - a set of three bases found in DNA and RNA. The genetic code is formed from patterns of codons
• Polypeptide - a chain of amino acids

1. RNA polymerase unwindes the DNA molecule

2. mRNA synthesis: nucleotides added to growing strand of messenger RNA molecule

3. DNA rewinds into double helix structure

4. mRNA moves through nuclear pore in nuclear membrane to the cytoplasm

5. tRNA molecule brings in the correct amino acid to the ribosome

6. Anti-codon on the tRNA matches with the correct codon on the mRNA and drops off the amino acid

7. tRNA leaves the ribosome

8. tRNA molecule is recharged with another amino acid of the same type, ready to take part in protein synthesis

• A change in the bases of a gene creates a genetic variation or mutation

Causes of Mutations:

• DNA is not copied correctly
• The environment e.g. radiation and certain chemicals

Mutations that Change the Function of Proteins:

Haemoglobin

• Consists of 4 polypeptides of two kinds (a and ß). The polypeptides join and fold in a specific way to form a globular protein which carries oxygen.
• When a single base is changed in the DNA for the ß polypeptide an incorrect amino acod is placed in the sequence that results in incorrect folding of the polypeptide
• This leads to a genetic disorder, called sickle cell disease. Red blood cells change shape. They stick together and can block blood vessels causing extreme pain.
• A second type of mutatiom in haemoglobin can cause red blood cells to break apart and results in shortness of breath.

Mutations that Alter the Amount of Protein that is Produced

• These mutations are due to changes in the non-coding (does not code for a polypeptide) regions of DNA.

1. This may result in RNA polymerase not binding well, reducing transcription. Less protein is produced. e.g. Beta athalassaemia

2. If a mutation causes RNA polymerase to bind better, less mRNA and therefore more protein is produced. e.g. excess of protein PSA produced in men with prostate cancer.

• Each chromosome in a pair may carry alternative forms of the same gene. These alternative forms are called alleles.
• For example, the gene for eye colour has alleles that code for blue and brown. For each gene an individual might have two matching alleles and is said to be homozygous for that characterstic.
• If both alternative alles are present the individual is said to be heterozygous for the characteristic
• A dominant allele is an allele that controls the development of a characteristic even when it is only present on one of the chromosomes in a pair
• A recessive allele only controls the development of a characteristic if it is present on both of the chromosomes in a pair.

• The alleles of an organism are its genotype e.g. RR, Rr or rr
• What an organism looks like is its phenotype e.g. red or white
• Genetic diagrams and Punnett squares can be used ot explain the inheritance of one gene (monohybrid inheritence)
• A family pedigree chart shows how genotypes and their resulting phenotypes are inherited in families e.g. inheritence of genetic disorders.

Mendel's Laws of Inheritence:

• Each gamete receives only 1 factor for characteristic
• The version of a factor received is random
• Some versions of a factor are more powerful than others and always affect the offspring

In modern terms:

• Each gamete receives one allele for a characteristic
• The allele is received randomly
• Some alleles are dominant and always affect the offspring

ABO Blood groups:

• Red blood cells have 3 types of markers i.e. A,B and O
• 3 allles are responsible 1ᴬ, 1ᴮ and 1ᴼ
• 1ᴼ is recessive to both 1ᴬ and 1ᴮ
• 1ᴬ and 1ᴮ are codominant
• Co-dominant - when both alleles of a gene affect the phenotype

• These are caused by genes inherited differently in males and females e.g. red-green colour blindness and haemophilia which are common in men.
• These genes occur on the sex chromosomes, usually only on the X chromosome. Men only have one allele for these genes.

• Some cone cells in the retina do not work properly. Females can be carriers and not affected.

• In 2003 the first complete human genome was decoded i.e. the 3.3 billion complementary base pairs in one set of 46 human chromosomes was listed.

Further work has revealed:

• Individual genes have been decoded
• DNA from a wider range of people as been mapped
• Over 90% of the sequence of DNA bases in humans is the same

Uses of the Human Genome:

• Indicate the risk of developing certain diseases
• Help identify which medicines might work best in individual patients

• Gene mutation = change in a gene that creates a new allele
• This involves a change in the base sequence of DNA
• This is more likely to happen when DNA is damaged e.g. by radiation and certain chemicals. Blue eyes are a mutation

• Most human characteristics are controlled by more than one gene
• Eye colour is affected by several genes. The most important gene is the OCA2 gene.
• It controls how much melanin is prodiced i.e. a little melanin = blue eyes; lots of melanin = brown eyes

1. Harmful e.g. when an essential protein cannot be produced

2. Small effect on the phenotype

3. No effect

• Variation = differences in the characteristics of organisms

There are two causes of variation:

• Genetic variation is caused by the inheritence of different alleles during sexual reproduction e.g. eye colour, cystic fibrosis
• Environmental variation occurs when charcateristics are affected by their surroundings e.g. plant growth is affected by how much light and minerals are present.
• Acquired characteristics are characteristics changed by the environment during the life of an individual. These characteristics will not be passed on to the next generation. e.g. length of hair, mass, etc

• Continuous variation - data can be any value within a certain range e.g. length of a pea pod, height of a person

- no distinct catagories

- tends to be a number (quantitative)

• Discontinuous variation - data can only take a limited set of values e.g. number of peas in a pod, blood group

- distinct categories

- tends to be descriptive (qualitative)

• Continuous data for variation often forms a bell-shaped curve, known as a normal distribution curve.
• This occurs where:

- The most common value is the median value of the whole range

- The further a value is from the median, the fewer individuals have that value.