Biology IGCSE: Inheritance

Genome definition

The entirety of an organisms DNA

Gene Definition

Section of DNA

Where are chromosomes?

In the nucleus

Chromosome Definition

Long strand of DNA

Describe the structure of DNA

Double helix with base pairs A-T and G-C connected to sugar phosphate backbone (Adenine, Thymine, Guanine and Cytosine)

Describe the structure of RNA

Single stranded with base pairs A-U and G-C connected to singular sugar phosphate backbone (Adenine, Uracil, Guanine and Cytosine)

Describe the process of transcription (protein synthesis)

• Occurs in the nucleus and aims to produce a copy of a section of DNA in the form of mRNA

1. Part of DNA molecule unwinds when hydrogen bonds between base pairs break

2. Exposes template strand of the gene that codes for wanted protein

3. mRNA nucleotides in nucleus bind to complementary nucleotides on template strand

4. mRNA nucleotides join to other nucleotides to form single strand of mRNA

5. mRNA molecule leaves nucleus through nucleus pore

• New mRNA strand is complementary code of DNA code from original gene

Describe the process of translation (protein synthesis)

• Occurs in ribosomes in cytoplasm and aims to produce chain of amino acids which will form a protein

1. mRNA molecule attaches to a ribosome

2. In cytoplasm there are tRNA molecules which have triplet of unpaired bases called anticodons and amino acids at the other end (each anticodon corresponds to specific amino acid)

3. Anticodon on each tRNA molecule pairs with complementary codon on mRNA, bringing specific amino acid

4. Another tRNA molecule attaches to its complementary codon on mRNA , peptide bond is formed between neighbouring amino acids

5. Process continues until STOP codon on mRNA molecule is reached and translation stops, amino acid chain coded for by mRNA is complete

6. Amino acid chain is folded to form final protein molecule eg. enzyme

Dominant allele definition

An allele that will always show up in a organism's phenotype if present

Recessive allele definition

An allele that will only show up in an organism's phenotype if dominant allele is not present

Homozygous definition

having 2 identical alleles of a particular gene

Heterozygous definition

having 2 different alleles of a particular gene

Phenotype definition and example

physical characteristic (hair colour)

Genotype definition and example

genetic makeup (AA, Aa, or aa)

Codominance definition

neither allele in a gene pair are dominant, equally dominant

Sex alleles of male cell

XY

Sex alleles of female cell

XX

What increases chances of genetic mutation?

Exposure to ionising radiation (eg. gamma rays, x-rays and ultraviolet rays) and chemical mutagens (eg. chemicals in tobacco)

Diploid number of cells in humans

46

Haploid number of cells in humans

23

Mutation definition

change in DNA sequence

What are phenotypic features the result of?

polygenic inheritance (more than one gene)

Explain Darwin's Theory of Evolution by natural selection

Variation - there is variation in a population due to random mutations

Survival - Those with mutation survive as they have an advantage

Reproduce - Individuals with advantageous allele survive and reproduce, passing on mutation

Frequency - frequency of advantageous allele increases in next generation

Explain the process of mitosis

1. Each chromosome in nucleus copies itself exactly

2. Chromosomes line up along centre of cell where cell fibres pull them apart

3. Cell divides into two (cytokinesis) and each cell has copy of each chromosome

• Two genetically identical daughter cells are produced (diploid)

What is mitosis used for?

growth, repair, asexual reproduction

Explain the process of meiosis

1. Chromosomes duplicate and line up in centre of cell

2. Cells divide twice so only one copy of each chromosome passes to gamete

• Four haploid daughter cells are produced and are genetically different

What is meiosis used for?

production of gametes (sperm and egg cells)

How does change in DNA affect phenotype?

Alters sequence of amino acids in a protein

Name three ways sequence of DNA can be changed and describe them.

Insertion - new base inserted into DNA sequence

Deletion - base is deleted in DNA sequence

Substitution - base is substituted in place of another

Describe how antibiotic resistance occurs in large populations of bacteria.

Variation - random mutation gives a bacteria allele that codes for antibiotic resistance

Survival - All bacteria dies from antibiotics except those with allele

Reproduce - Surviving bacteria are more likely to reproduce and pass on resistance alleles to offspring

Frequency - frequency of bacteria with allele increases in next generation leading to strain of resistant bacteria