Edexcel IGCSE Biology; Inheritance

Genome definition

The entire genetic material of an organism

Gene definition

Section of a molecule of DNA that codes for a specific protein

Allele definition

Different versions of the same gene

Where are genes located within a nucleus

Genes arrange along the lengths of a chromosome, found in nucleus

What is a DNA molecule

Two strands coiled to form a double helix, with the strands being linked by a series of paired bases

What is a double helix

Two polynucleotide strands wrapped around each other in a spiral shape

What are the possible DNA bases

Adenine (A)

Thymine (T)

Cytosine (C)

Guanine (G)

What is an RNA molecule and how is it different to DNA

Singled Stranded, Contains Uracil (U) instead of Thymine (T)

What does DNA stand for?

deoxyribonucleic acid

Diploid vs Haploid cells

Diploid cells contain two sets of chromosomes

Haploid cells contain only one set of chromosomes

How many chromosomes do humans have

23 pairs - 46

How many chromosomes do gametes have

23 individual chromosomes

How are nucleotides joined?

Sugar phosphate backbone - phosphate group bonds to sugar in nucleotide above/below

Structure of a nucleotide

phosphate group, sugar, nitrogenous base

What are the complementary base pairs

Adenine and Thymine

Guanine and Cytosine

What is polynucleotide

Contains many nucleotides linked together in a long chain

What is a nucleotide

Monomer - basic repeating unit of DNA

Name for a group of 3 bases for DNA, mRNA, tRNA

Triplet, Codon, Anticodon

Where are proteins synthesised?

Proteins are synthesised in the ribosomes in the cytoplasm

What are the stages of protein synthesis?

1. Transcription

2. Translation

What is the process of transcription and why is it needed?

Once a gene is activated, it must be copied into a form that can leave the nucleus, using mRNA, as DNA is too big to move out.

mRNA is also a sequence of bases, but it is shorter and single-stranded. It uses U instead of T as well.

RNA polymerase is the enzyme that joins together the base sequence to make mRNA.

1. It binds to a region of non-coding DNA

2. It then unzips the two DNA strands, and RNA polymerase moves along one of the strands of DNA

3. It uses the coding DNA in the gene as a template to make the mRNA. Base pairing ensures the mRNA is complementary to the gene

4. It then moves out of the nucleus and joins with a ribosome in the cytoplasm

What is mRNA and how does it differ from DNA

Messenger RNA:

single stranded nucleic acid

made from ribonucleic acid not deoxyribose

cannot form a double strand

Deoxyribose vs ribose

Sugar in DNA nucleotides

Sugar in RNA nucleotides

What is the problem with amino acids?

There are lots of different combinations for the same amino acids - only the first two bases of the codon affect the amino acid created

How many possible combinations are there of the 3 bases?

64 (4^3)

What does a ribosome look like

Large subunit, small subunit make up te ribosome, with the tRNA going into the large and the mRNA going into the small

What is tRNA?

tRNA is transfer RNA, found as free molecules in the cytoplasm.

It can carry specific amino acids found also in the cytoplasm, with an anticodon that corresponds to a specific codon on the mRNA.

What is the process of transcription?

Once the mRNA is bound to a ribosome, the protein can be assembled.

1. The order in which amino acids are brought to the ribosome matches the order of the codons in the mRNA

2. tRNA has a structure called an anticodon, which is complementary to the codon for the amino acid. This ensures amino acids are brought to the ribosome in the correct order.

3. The amino acids are joined together by the ribosome, which makes a protein.

What joins each amino acid in a protein

Peptide bond

When does mitosis occur?

Growth, repair, cloning and asexual reproduction

What is the diploid and haploid number of chromosomes in human cells?

Diploid - 46; Haploid - 23

What is the definition of mitosis

Nuclear division giving rise to genetically identical cells

What happens before mitosis?

Before mitosis, the cell is in interphase. It gets bigger and makes more organelles and replicates the DNA.

These copies are joined together and known as sister chromatids.

How does fertilisation work

Sperm cell fuses with egg cell in fallopian tube

Sperm forms zygote by passing genetic information to egg

Both gametes must be haploid

Process of meiosis

1. The cell duplicates its DNA so there’s enough for a new cell. One arm of the X chromosome is an exact copy of the other

2. In the first division, chromosomes line up in pairs in the centre. One in each pair comes from the mother/father.

3. The pairs are pulled apart, so each cell only has one copy of each chromosome.

4. Each new cell will have a mixture of both parents, which is important as it creates genetic variation.

5. In the Second division, they line up again in the centre. The arms are pulled apart

6. Four haploid gametes are made. They will each have a single set of chromosomes. They are all genetically different.

What is recombination of the chromosomes in meiosis

also called genetic recombination, is the process that introduces genetic diversity into the gametes during meiosis.

Why is their halving involved in meiosis

Must be halved when gametes are formed

Otherwise there would be double number of chromosomes after they join at fertilisation in zygote

What are homologous chromosomes?

Two genetically similar chromosomes, one from each parent - same genes but could be different versions

Mitosis (Num. daughter cells, Genetic difference, Diploid/Haploid daughter cells, Stages of cell division, Movement at equator)

2, Genetically Identical, Diploid, One stage, Line up along equator

Meiosis (Num. daughter cells, Genetic difference, Diploid/Haploid daughter cells, Stages of cell division, Movement at equator)

4, Genetically different, Haploid, 2 stages, Pair up at equator

What is meiosis used for?

Production of gametes, Increase genetic variation

How does meiosis introduce variation

Forming new combinations of maternal and paternal chromosomes every time gamete is made, so when they fuse randomly at fertilisation offspring will be different

Where does genetic variation come from

Random fusion of gametes at fertilisation creates genetic variation between zygotes

Ways of causing phenotypic variation

Genetic:

Random fusion of gametes at fertilisation creates genetic variation between zygotes

Environmental:

scarring, weight, height (e.g. plant growing to reach more light)

Discontinuous vs continuous variation

discontinuous = characteristics fall into certain groups

with no overlap (e.g. blood group) - determined by genetics only ; will give step-like graph if plotted

continuous = characteristics show a range (e.g. height) - determined by genetic and environmental factors ; will give smooth bell curve if plotted

How is height affected by both genetic and environment

Continuous variation:

Tall parents pass genes for child to be tall

Have genetic potential to be tall

However if diet is poor then they will not grow well

So environment has impact on height

Why is the tRNA anticodon important for its function?

Anticodon is complementary to mRNA codon for an amino acid, pairing ensures amino acids are brought in correct order

Why are nuclear pores necessary for RNA polymerase to carry out its function

RNA polymerase is protein - produced by ribosomes in cytoplasm

Nuclear pores allow it to move from cytoplasm to nucleus to carry out its function

What is non overlapping DNA

• each triplet / codon codes for one amino acid / eq (1)

• codons are discrete / independent of each other / nucleotides /

bases are not shared between codons / eq (1)

• example of triplet reading frames, e.g. CAT TCA / eq

Translation stage

mRNA arrives at) ribosome (1)

• mRNA has codons / is a template (1)

• ribosome moves along mRNA strand (1)

• tRNA brings amino acids (to ribosome) (1)

• anticodon (on tRNA) binds with codon (on mRNA) / tRNA binds

with mRNA (1)

• amino acid chain produced / amino acid joined / polypeptide (1)

Why might mutation not affect phenotype

Protein can be coded with multiple codons

Mutations will not change encoded aminos acid

Means protein won't change

Chromosome definition

Thread-like structures of DNA that carry genetic information in the form of genes, located in the nucleus of cells

Dominant vs recessive

Dominant - always expressed, even if only one is present (CAPITAL)

Recessive - only expressed if dominant allele is absent - so needs two to be expressed (lower case)

Homozygous vs heterozygous

Homozygous - two alleles of gene are the same

Heterozygous - individual has two different alleles - both dominant and recessive

Genotype vs phenotype

Genotype - Combination of alleles that control a characteristic

Phenotype - Observable, physical characteristics of an organisms

Codominant definition

Both of the alleles of a characteristic are expressed in the phenotype - neither are recessive; only n heterzygous

Polygenic inheritance vs monohybrid inheritance

Polygenic - Characteristics are controlled by more than one gene - interaction of multiple

Monohybrid - controlled by single gene

How to draw a Punnett Square

For monohybrid inheritance

Show genotype on top and left for each individual

Go inwards one, separate into separate alleles

Complete 4 squares, combining each alleles with one from the other

Write the genotypes created, the associated phenotype, the ratio, and therefore the probability

What does pure breeding mean

An individual has homozygous pair of alleles for that characteristic

What is a family pedigree

Trace patterns of inheritance of specific characteristic through generations;

Square is male, circle is female

Horizontal lines show male and female have produced children

What are the sex chromosomes

Sex is determined by entire chromosome pair

XX- female

XY- male - only father can pass on Y gene

Mutation definition

Random changes in the sequence of DNA bases in a gene or chromosome, leading to genetic variation

How to increase rate that mutations occur

Factors known as mutagens - environmental factors

Ionising radiation like X-Rays

Chemical Mutations, like in cigarette smoke

Increase rate of mutations increases likelihood of cells becoming cancerous

What is a silent mutation

alters a base but does not change the amino acid

Frame shift

genetic mutation that shifts the "reading" frame of the genetic message by inserting or deleting a nucleotide;

Types of genetic mutation

Substitution - base in DNA sequence randomly swapped for a different base, only changes amino acid for group of thee baes where mutation occurs - no knock on

Frame shift mutations:

Insertion - New base randomly inserted into DNA sequences

Deletion - base is randomly deleted from sequence

Consequences of a mutation

Most mutations have no effect on phenotype

Some significantly alter protein coded, affecting ability to perform function - e.g. if shape of active site changes, no longer form enzyme-substrate complex

Some cause development of alleles, new phenotypes

Some cause survival advantage

Sickle cell anemia

Gene mutation changes haemoglobin , so RBCs become stiff and sickle shaped when they release O2

Get stuck in narrow blood vessels, blocking flow

May cause stroke, blindness, damage to lungs kidney or heart

Darwin's theory of evolution

Change in inherited characteristics and frequency of phenotype of population over time through process of natural selection

Ones most suited to environment have higher chance of survival and reproduction

So characteristics passed to offspring at higher rate than those less suited

Beneficial characteristics become more common in population, species changes

This is 'survival of the fittest'

Evolution definition

the change in the frequency of a phenotype in a population over many generations

Example of Darwins Theory of Evolution

Black peppered moths were originally lighter coloured

Pollution means bark of trees is darker, dark moths can camouflage

So get darker over time

Stages of selective breeding in animals

Humans select individuals with desired characteristics and breed together

From offspring select those with best characteristics, breed together

Repeat generation after generation, selecting only individuals with desired characteristics for breeding

Eventually so little variation all offspring will have desired characteristics

Selective breeding definition

Selecting and breeding parent organisms to pass on specific traits to offspring

Ethical considerations of selective breeding

Young without desired may be put down

Breeding can be encouraged naturally by adding hormones

Or artificially - artificial insemination

Could cause black market

Selective breeding in plants

Select individuals with desired characteristics, remove stamen and stigma of rest

Using brush, transfer pollen from stamen of one desired to stigma of other

Grow seeds, select individuals with best

Repeat for many generations

Eventually little variation

What are antibiotics

Chemical substances made by certain fungi or bacteria that affect the working of bacterial cells, either by disrupting their structure or function or by preventing them from reproducing

How is antibiotic resistance caused?

Bacteria have random mutations in DNA

May give resistance to antibiotic

Therefore more adapted to environment, all else with be killed when in presence of antibiotic

Survive more, reproduce more

Pass on allele more

Repeated every generation

Proportion of bacteria with resistant allele increases until most have it

Preventing antibiotic resistance

Avoid overuse - not in non-serious infection

Not for viral infections

Finish whole course so all are killed, none are left to mutate

Reduced in industries like agriculture

Reducing spread of resistant strains

Good hygiene - handwashing, hand sanitisers

Isolation of infected patients to prevent spread of resistant strains

Why do antibiotics not work on viruses?

Antibiotics work by disrupting cell functions like respiration

However viruses do not carry out cell function or have cell walls, cell membranes or cell organelles

Why does deletion change protein produced but substitution does not

Substitution may just change final base - three bases all code for same amino acid - degeneracy/redundancy of DNA

However deletion causes frame shift mutation - knock on effect

Writing frame for natural selection

VMASRA

Variation in a species due to random genetic mutation

A - gives advantage to species

S - Survive more

R - So reproduce more

A - Advantageous Alleles are passed down to the next generation, repeated over many generations

Then mention the increase in number and increase in frequency/proportion with allele

Why would the predicted ratio of offspring not be obtained

Same size too small (not enough crosses)

Random fusion of gametes at fertilisation

Some may be more likely to die

Mutations may have occured

How to ensure flowers only self pollinate

Enclose stamen and stigma in a bag/isolate plants, pollinate by hand with a brush; prevent cross pollination

Differences between sexual and asexual reproduction

Sexual requires two gametes

Sexual has genetic variation

Sexual involves fertilisation

Sexual is slower

Difference between dominant allele and recessive allele (3)

Dominant is always expressed, expressed in heterozygous but recessive is not, recessive is only expressed if homozygous recessive - in absence of dominant

How to tell if an organism have a characteristics that isn't immediately obvious

Look at family history

Genetic Test

Look for combination of symptoms

How can family pedigree show recessive allele is present

Condition present in offspring but not parents; skips generations; carriers present

Differences between selective beeding and natural selection

Selective breeding is:

Faster

Done by humans

Characteristic may not even be advatangeous or affect survival

Why might mutation have little effect on phenotype

• change in base may code for same amino acid (1)

• amino acid may not be involved in active site (1)

• enzyme still made/still functions/equivalent (1)

• could be recessive allele (1)

• so not expressed in phenotype (1)

How does mutation affect functioning of enzyme

• change in the order of bases/equivalent (1)

• leads to different codon (1)

• different amino acid in protein (1)

• different-shaped enzyme/change to active site/enzyme not made/equivalent (1)

Natural selection definition

the process whereby organisms better adapted to their environment tend to survive and produce more offspring

Why might offspring not have desired characteristics

Recessive alleles also contribute to genotype and therefore phenotype

Random fusion of gametes at fertilisation

Mutations

Genes are randomly selected

How to selectively breed fish for fast growth

Select fish that you can see are growing quickly - have desired characteristic

Breed them together, repeat over several generations

Alleles for fast growth are passed on