Edexcel IGCSE Biology - Inheritance

what is a genome?

the entire DNA of an organism

what is a gene?

a small section of DNA that codes for a specific protein

what is a chromosome?

a long length of DNA coiled up - consists of numerous genes

where are genes located in eukaryotic cells?

nucleus

how many chromosomes are there in a human cell?

46 → 23 pairs → one in each pair from mother/father

what does DNA stand for?

deoxyribonucleic acid

what is the structure of DNA?

two strands coiled to form a double helix

what are the building blocks of DNA?

nucleotides

what is a DNA nucleotide composed of?

• a nitrogen containing base

• a sugar (deoxyribose)

• a phosphate group

what four bases are there in DNA?

• adenine

• thymine

• cytosine

• guanine

what is complementary base pairing?

• DNA bases are always paired, and they only have one base which they pair to

• Adenine + Thymine

• Cytosine + Guanine

which bonds join DNA bases together?

hydrogen

what does RNA stand for?

Ribonucleic acid

what is the structure of RNA?

single helix

what are the components of an RNA nucleotide?

• a phosphate group

• a sugar - ribose

• a base

which bases make up RNA?

• Adenine

• Uracil

• Cytosine

• Guanine

which bases are complementary in RNA?

• Adenine + Uracil

• Guanine + Cytosine

what are the differences between RNA and DNA?

• DNA - double helix; RNA - single helix

• RNA has Uracil instead of Thymine

• DNA - deoxyribose sugar; RNA - ribose sugar

• RNA is shorter than DNA

what are the 2 stages of protein synthesis?

• transcription

• translation

what is a codon?

a group of 3 bases

what is an anticodon?

a group of 3 unpaired bases on a tRNA that is complementary to a codon on mRNA

why does transcription have to take place?

DNA is found in the nucleus and can’t be taken out due to its size, but protein synthesis takes place in the cytoplasm with ribosomes

describe the process of transcription

• DNA helicase unwinds and separates the DNA strands in a gene

• RNA polymerase binds to one of the strands on a non-coding region before the gene

• free-floating mRNA nucleotides line up with their complementary base pairs and make an RNA copy of the gene

• the mRNA molecule moves out of the nucleus and binds with a ribosome in the cytoplasm

what does the RNA polymerase do?

produces the complementary mRNA strand by attaching the nucleotides together

describe translation

• a tRNA with the complementary anticodon to the first codon on the mRNA attaches to it, bringing the corresponding amino acid

• this process continues until the mRNA strand ends

• the ribosome attaches the amino acids together with peptide bonds, forming a polypeptide

• the amino acid chain separates from the tRNAs and mRNA and coils up to form a protein

What is tRNA?

tRNA is an RNA molecule that contains an anti codon and a corresponding amino acid

what is an allele?

a different form of the same gene

what is a consequence of alleles?

people inherit different characteristics

how many alleles does a person have for each characteristic?

2 - one maternal, one paternal

what is polygenic inheritance?

a characteristic that is controlled by two or more genes working together - vast majority

what is monohybrid inheritance?

the inheritance of single gene that controls a single characteristic - rare

what is a dominant allele?

an allele that is always expressed in the phenotype (capital letter)

what is a recessive allele?

an allele that is only expressed in the phenotype if it is in the presence of another (same) recessive allele

what does homozygous mean?

2 identical copies of the same allele for a gene

what does heterozygous mean?

two different alleles for a gene

what does phenotype mean?

the expressed characteristics of an organism

what does genotype mean?

all the alleles that an organism has for a particular characteristic

what is codominance?

when you have two different dominant alleles that are both expressed

what genetic diagrams can you use to demonstrate/work out monohybrid inheritance? (mainly)

Punnett squares - also include alleles, gametes, genotypes, phenotypes

what is a family pedigree?

a genetic diagram that shows the history of a trait in a family

how can you determine if a trait is recessive from looking at the phenotypes present in a family pedigree?

if the trait skips generations - is present in children but not parents - it suggests that parents must be carriers and so the gene is recessive

how can you determine if a trait is dominant from looking at the phenotypes present in a family pedigree?

if the characteristic does not skip generations

which pair of chromosomes controls the sex of a person?

23rd pair - XY

which pair of chromosomes makes the sex of a person female?

XX

which pair of chromosomes makes the sex of a person male?

XY

what is a diploid cell?

a cell with two copies of each chromosome - in humans, 46 chromosomes/23 pairs

what is a haploid cell?

a cell with one copy of each chromosome - in humans, 23 chromosomes

what are the two process of cell division?

• mitosis

• meiosis

when does mitosis occur?

• growth

• repair

• cloning

• asexual reproduction

where does mitosis occur?

all body cells

what does mitosis produce?

2 genetically identical diploid cells

how many divisions are there in mitosis?

1

when does meiosis take place?

production of gametes

where does meiosis take place?

ovaries/testes/anther

what does meiosis produce?

4 genetically different haploid cells

how many divisions occur in meiosis?

2

describe the process of mitosis

• the DNA in a cell duplicates and then forms x-shaped chromosomes - each arm is a duplicate of the other (chromatid) - there are 46 sets of duplicates

• the chromosomes line up at the centre of the cell and cell fibres pull them apart

• the two arms of each chromosome go to opposite sides of the cell and membranes form around each set

• the cytoplasm divides

describe the process of meiosis

• the DNA in a cell duplicates and then forms x-shaped chromosomes - each arm is a duplicate of the other (chromatid) - there are 46 sets of duplicates

• these chromosomes each line up in the middle of the cell - the mother’s and father’s are next to each other

• crossing over occurs - some bits of mothers and fathers swap over

• these pairs are split in half, producing two genetically different diploid cells

• each of the 23 chromosomes now line up in the middle of the new cell

• the arms are pulled apart

• you end up with 4 genetically different haploid cells

what does random fertilisation produce?

genetic variation of offspring

what are the two causes of variation within a species?

• genetic - e.g. eye colour, blood group

• environmental - e.g. health, intelligence

• usually a mixture of both

what is a mutation?

a random, rare change in the genetic material - DNA base sequence of a gene - that can be inherited

what do mutations do?

• change the DNA base sequence of an organism

• produces a genetic variant (altered version of gene)

• can lead to a change in the protein that the gene codes for - this could stop the production of an enzyme or change the shape of it’s active site

why might a mutation have no effect?

• most common

• occurs in an unimportant region of DNA

• a mutated codon may code for the same amino acid - not alter shape or function of protein

• may occur in a recessive allele

why might a mutation have a slight effect?

• may cause a change of amino acid

• however this may only have a slight effect on the protein’s structure and function

why might a mutation have a significant effect?

• may cause an entirely different protein to be formed which can no longer carry out its function

• can be harmful or beneficial

what can the incidence of mutations be increased by?

• exposure to ionising radiation - gamma and x-rays

• exposure to chemical mutagens - like tobacco

what is continuous variation?

variation in a population that is influenced by genetics and the environment

what is discontinuous variation?

variation in a population that is controlled by a single gene and has no environmental influence

describe Darwin’s theory of evolution by natural selection

• variation arises in a population due to mutation, random inheritance of alleles and environmental factors

• some organisms may possess characteristics which make it easier for them to survive/survive for longer

• these organisms are more likely to reproduce and pass on these advantageous alleles to their offspring

• this process repeats over many generations and the prevalence of the allele increases

give an example of natural selection and briefly summarise it

antibiotic resistance - some bacteria may have mutated to be resistant to antibiotics; they are more likely to survive and reproduce; over time the prevalence of the allele increases in the population

what is a negative effect of antibiotic resistance?

make infections difficult to control