topic 6-inheritance

difference between meiosis and mitosis

meiosis leads to non-identical cells being formed while mitosis leads to identical cells being formed.

sexual reproduction example

Sexual reproduction involves the joining (fusion) of male and female gametes:

• sperm and egg cells in animals

• pollen and egg cells in flowering plants.

what happens in sexual reproduction

In sexual reproduction there is mixing of genetic information which leads to variety in the offspring

what does the formation of gametes involve

The formation of gametes involves meiosis.

what is asexual reproduction

Asexual reproduction involves only one parent and no fusion of gametes. There is no mixing of genetic information. This leads to genetically identical offspring (clones). Only mitosis is involved.

meiosis and fertilisation

meiosis halves the number of chromosomes in gametes and fertilisation restores the full number of chromosomes.

how do cells in reproductive organ divide

Cells in reproductive organs divide by meiosis to form gametes.

what happens when a cell divides to form gametes

• copies of the genetic information are made

• the cell divides twice to form four gametes, each with a single set of chromosomes

• all gametes are genetically different from each other.

fertilisation

Gametes join at fertilisation to restore the normal number of chromosomes. The new cell divides by mitosis. The number of cells increases. As the embryo develops cells differentiate.

Advantages of sexual reproduction:

• produces variation in the offspring

• if the environment changes variation gives a survival advantage by natural selection

• natural selection can be speeded up by humans in selective breeding to increase food production.

Advantages of asexual reproduction:

• only one parent needed

• more time and energy efficient as do not need to find a mate

• faster than sexual reproduction

• many identical offspring can be produced when conditions are favourable

• .

how can some organisms reproduce

Some organisms reproduce by both methods depending on the circumstances.

examples of organisms that can reproduce by both methods

• Malarial parasites reproduce asexually in the human host, but sexually in the mosquito.

• Many fungi reproduce asexually by spores but also reproduce sexually to give variation.

• Many plants produce seeds sexually, but also reproduce asexually by runners such as strawberry plants, or bulb division such as daffodils.

dna

The genetic material in the nucleus of a cell is composed of a chemical called DNA. DNA is a polymer made up of two strands forming a double helix.

where can the DNA be found

The DNA is contained in structures called chromosomes.

what is a gene

A gene is a small section of DNA on a chromosome. Each gene codes for a particular sequence of amino acids, to make a specific protein.

genome of an organism

The genome of an organism is the entire genetic material of that organism.

importance of studying human genome

The whole human genome has now been studied and this will have great importance for medicine in the future.

benefits to studying human genome

• search for genes linked to different types of disease

• understanding and treatment of inherited disorders

• use in tracing human migration patterns from the past.

what is DNA

polymer of molecules called nucleotides -DNA as a polymer made from four different nucleotides.

what does each nucleotide consist of

Each nucleotide consists of a common sugar and phosphate group with one of four different bases attached to the sugar.

the four different bases

A,C, G, T

whats a key fact of DNA strands

they are complementary which means that the same bases always pair on the opposite strands A-T C-G

how is a particular amino acid made

A sequence of three bases is the code for a particular amino acid

how is a particular protein assembled

The order of bases controls the order in which amino acids are assembled to produce a particular protein.

what do the long strands of DNA consist of

The long strands of DNA consist of alternating sugar and phosphate sections. Attached to each sugar is one of the four bases.

what is the DNA polymer made of

The DNA polymer is made up of repeating nucleotide units.

how many different types of mino acids

20

how is the shape of the protein determined

the specific order of the amino acids determne the shape of protein

protein synthesis

1. the first stage takes place in the nucleus and the second stage takes place in the cytoplasm

2. the first stage is called transcription -the base sequence of the gene is copied into a complementary template molecule (messenger RNA or mRNA which is a single stranded molecule which is a single stranded molecule)

3. the mRNA will pass out of the nucleus and into the cytoplasm

4. the second stage of protein synthesis is called translation In this stage, the mRNA molecule attaches to a ribosome and amino acids are brought to the ribosome on carrier molecules(they are called trasnfer RNA or tRNA)

5. the ribosomes now reads the triplet of bases on the mRNA and uses this to join together the correct amino acids in the correct order

6. When the protein chain is complete it folds up to form a unique shape. This unique shape enables the proteins to do their job as enzymes, hormones or forming structures in the body such as collagen.

how is the sequence of the protein determined

the base sequence of a gene determines the amino acid of a protein.When the protein chain is complete it folds up to form a unique shape. This unique shape enables the proteins to do their job as enzymes, hormones or forming structures in the body such as collagen.

mutation

a random change to a base -Mutations occur continuously. Most do not alter the protein, or only alter it slightly so that its appearance or function is not changed.

why does sometimes mutations have no effect

this is because different base triplets can sometimes encode for the same amino acid -the mutation has had no effect on the protein’s shape or function

what happens when a mutation has an effect

A few mutations code for an altered protein with a different shape. An enzyme may no longer fit the substrate binding site or a structural protein may lose its strength.

non-coding parts of DNA

Not all parts of DNA code for proteins. Non-coding parts of DNA can switch genes on and off, so variations in these areas of DNA may affect how genes are expressed.

alleles

different forms of genes

what are the characteristics controlled by a single gene

what do the alleles do

The alleles present, or genotype, operate at a molecular level to develop characteristics that can be expressed as a phenotype.

difference between dominant and recessive allele

A dominant allele is always expressed, even if only one copy is present. A recessive allele is only expressed if two copies are present (therefore no dominant allele present).

difference between homozygous and heterezygous

If the two alleles present are the same the organism is homozygous for that trait, but if the alleles are different they are heterozygous.

what are most characteristics due to

Most characteristics are a result of multiple genes interacting, rather than a single gene.

how are inherited disorders passed on

Some disorders are inherited. These disorders are caused by the inheritance of certain alleles.

cystic fibrosis

(a disorder of cell membranes) is caused by a recessive allele.

polydactyly

(having extra fingers or toes) is caused by a dominant allele.

what happens in embryo screening

embryos are tested to see if they have the alleles for inherited disorders. embryos which do not have the defective alleles are implanted into the woman. these can develop into healthy offsprings

issues with embryo screening

-expensive and that money can be used elsewhere in the health service

-often a large number of embryos are created but only small number are implanted. that means that some healthy embryos are destroyed and people think that is unethical

-in the future we may be able to screen embryos to produce offsprings with desirable features. many people think this is unethical

gene therapy

correcting faulty alleles and use this to treat inherited disorders and at the moment this is experimental

make sure to learn family trees

what sex chromosomes do males have

XY

what sex chromosomes do females have

XX

how many pairs of chromosome determine sex

22 pairs control characteristics only, but one of the pairs carries the genes that determine sex.

variation

Differences in the characteristics of individuals in a population is called variation

what may the differences be due to

:

• the genes they have inherited (genetic causes)

• the conditions in which they have developed (environmental causes)

• a combination of genes and the environment.

variation through phenotype

• all variants arise from mutations and that: most have no effect on the phenotype; some influence phenotype; very few determine phenotype.

Mutations occur continuously. Very rarely a mutation will lead to a new phenotype. If the new phenotype is suited to an environmental change it can lead to a relatively rapid change in the species.

evolution

s a change in the inherited characteristics of a population over time through a process of natural selection which may result in the formation of a new species.

what does the theory of evolution by natural selection state

The theory of evolution by natural selection states that all species of living things have evolved from simple life forms that first developed more than three billion years ago.

what happens if two populations of one species become soo different

If two populations of one species become so different in phenotype that they can no longer interbreed to produce fertile offspring they have formed two new species.

what is selective breeding

Selective breeding (artificial selection) is the process by which humans breed plants and animals for particular genetic characteristics.

how long have humans been doing this s

. Humans have been doing this for thousands of years since they first bred food crops from wild plants and domesticated animals.

what does selective breeding involve

Selective breeding involves choosing parents with the desired characteristic from a mixed population. They are bred together. From the offspring those with the desired characteristic are bred together. This continues over many generations until all the offspring show the desired characteristic.

The characteristic can be chosen for usefulness or appearance:

• Disease resistance in food crops.

• Animals which produce more meat or milk.

• Domestic dogs with a gentle nature.

• Large or unusual flowers.

what can selective breeding lead to

Selective breeding can lead to ‘inbreeding’ where some breeds are particularly prone to disease or inherited defects.

genetic engineering