bio6

What does DNA stand for?

deoxyribonucleic acid

what does DNA determine?

inherited characteristics and what proteins the cell produces (which in turn determines the type of cell it is)

what is DNA contained in?

chromosomes

What is a gene?

A section of DNA that codes for a particular sequence of amino acids which are put together to make a specific protein

How many amino acids are there?

20

What is the genome of an organism?

entire genetic material of that organism

Why is understanding the human genome important?

- allows scientists to identify genes linked to different types of diseases

- knowing genes linked to inherited diseases could help develop effective treatments

- genomes can trace the migration of certain populations

describe the structure of DNA?

polymer made from four different nucleotides in two strands forming a double helix shape

what is a nucleotide?

repeating units consisting of a common sugar and phosphate group with 1 of 4 bases

what are the 4 bases?

Adenine, Thymine, Guanine, Cytosine

How are the strands of DNA held together?

hydrogen bonds

describe the make up of strands of DNA?

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

What is each amino acid coded for by?

A sequence of three bases in the gene

What is complementary base pairing?

When base A always pairs up with base T, and base C always pairs up with base G

what does the order in which amino acids are assembled depend on?

the order of bases in the gene

what is the role of the non-coding parts of DNA?

switching genes on and off, so they control whether or not a gene is expressed (used to make a protein)

what is the role of mRNA?

A molecule of mRNA carries the genetic message from a gene (DNA) to ribosomes that translate it into protein. The correct amino acids are brought to the ribosomes in the correct order by carrier molecules

what happens once a chain of amino acids has been assembled?

it folds into a unique shape, enabling the protein to perform its task

how does the structure of DNA affect the protein made?

-Different bases

-Different amino acid sequence

-Different protein

-Different function

Describe protein synthesis

mRNA copies the code for the gene from the DNA in the nucleus

mRNA then carries this code to the ribosomes in the cytoplasm

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

Carrier molecules (tRNA) bring the bases to the ribosomes in the correct order

When the protein chain is complete, it folds up to form a unique shape which helps the protein to carry out its function

what are examples of proteins?

Enzymes, hormones and structural proteins

describe how DNA code is carried to the ribosomes

the order of bases in the nucleus is copied into the mRNA which moves to the ribosomes carrying the code. Here, the chain of amino acids is assembled according to the order of the bases

mutations occur...

continuously

what is a mutation?

change to the sequence of bases in DNA

What do mutations do?

They change the sequence of the DNA bases in a gene which produces a genetic variant (different form of the gene)

They sometimes lead to changes in the protein that a gene codes for

what are 2 examples of how a mutation can affect a protein's ability to perform its function?

1) shape of an enzyme's active site may change so substrate can no longer bind to it

2) structural proteins could lose their strength if their shape is changed, making them useless at providing structure and support

what happens if there is a mutation in the non-coding DNA?

it can alter how genes are expressed

what are the 3 types of mutation?

substitution, deletion, insertion

What is an insertion mutation?

Where a new base is inserted into the DNA base sequence where it shouldn't be, changing the way a group of 3 bases are read, which can change the amino acids they code for.

They have knock-on effect on bases further on in the sequence so can change more than one amino acid

What is a deletion mutation?

where a random base is deleted from the DNA base sequence

They change the way the base sequence is read and have knock-on effects further down the sequence

What is a substitution mutation?

when a random base is changed to a different base

has no knock-on effect

Compare sexual and asexual reproduction

sexual reproduction involves the fusion of male and female gametes. Because there are two parents, the offspring contain a mixture of their parents genes (genetically different)

in asexual reproduction there is only one parents. There is no fusion of gametes, no mixing of chromosomes and no genetic variation between parents and offspring. The offspring are genetically identical to the parent (clones)

what type of cell division is involved in asexual reproduction?

mitosis

how are gametes produced?

meiosis

describe meiosis

before division, the cell duplicates its genetic information and the chromosomes arrange into pairs

the pairs line up in the centre of the cell

they are pulled apart so each new cell only has one copy of each chromosome (some of fathers and mothers chromosomes go into each new cell)

the chromosomes line up again

the cells divide again so four gametes are produced, each with half the amount of random chromosomes

what happens after meiosis in the formation of gametes?

the gametes fuse during fertilisation and the resulting new cell (with normal number of chromosomes) divides by mitosis many times to produce lots of new cells in an embryo

what are the advantages of sexual reproduction?

- Produces variation in the offspring

- if the environment changes, variation increases the survival chance of a species

- because individuals with better adapted characteristics have a higher chance of survival, they are more likely to breed successfully and pass on traits to offspring (natural selection)

- natural selection can be sped up by humans in selective breeding (allows us to produce animals with desirable characteristics which can be passed on to offspring) to increase food production

What are the advantages of asexual reproduction?

- only one parent needed

- time and energy efficient as no need to find a mate

- faster than sexual reproduction

- many identical offspring can be produced when conditions are favourable

name 3 examples of organisms which reproduce by sexual and asexual reproduction

- malaria parasites reproduce asexually in the human host, but sexually in the mosquito

- fungi reproduce asexually by spores but also sexually to give variation

- many plants produce seeds sexually but also reproduce asexually by runners (stems that grow horizontally on the surface of the soil away from a plant forming identical plants along the runner) such as strawberry plants if bulb division (new bulbs form from the main bulb and divide off, each growing into a new identical plant) such as daffodils

what is a gamete?

reproductive cell

How many pairs of chromosomes do humans have?

23

What chromosomes do males have?

XY

what chromosomes do females have?

XX

what is an allele?

Different form of a gene (capital and lower case letter)

What is a dominant allele?

A form of a gene that is fully expressed, even when two different alleles are present (capital letter)

What is a recessive allele?

An allele that is masked when a dominant allele is present (lower case letter)

What does homozygous mean?

same alleles

What does heterozygous mean?

two different alleles

what is a genotype?

The combination of alleles you have e.g- BB Bb etc

what is a phenotype?

physical characteristics

What allele is polydactyly caused by?

dominant

What is polydactyly?

extra fingers or toes

What is cystic fibrosis?

It is a genetic disorder of the cell membranes. Causes thick, sticky mucus to build up in air passages and pancreas.

What allele is cystic fibrosis caused by?

recessive

what is a person called when they have one copy of the allele but not the disorder?

carrier

What is embryo screening?

When a cell is removed from an embryo and its genes are analysed for genetic disorders

what are the arguments against embryo screening?

- it implies that people with genetic disorders are undesirable so increases prejudice

- there may come a point where everyone wants to screen their embryos so they can pick the most desirable one

- it is expensive

what are the arguments for embryo screening?

- it helps to stop people suffering

- treating disorders costs the government and taxpayers a lot of money

- there are laws to stop it going too far (eg parents can't select sex)

what are the two types of variation?

genetic and environmental

what is genetic variation caused by?

the combining of genes from the two parents

what is environmental variation caused by?

the environment, including the conditions that organisms live and grow in, cause differences between members of the same species

what is variation?

Any difference between individuals of the same species.

what are examples of characteristics determined only by genes?

eye colour, blood group, inherited disorders

what are examples of characteristics determined by the environment?

size and colour of leaves, skin tone

what is evolution?

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 state?

All species of living things have evolved from simple life forms that first developed more than 3 billion years ago

Who came up with the theory of evolution? how?

Charles Darwin by using the observations he made on a round-the-world trip, along with experiments, discussions and new knowledge of fossils and geology

what did Darwin observe which led to his theory of evolution?

he knew that organisms in a species show wide variation in their characteristics and that organisms have to compete for limited resources in an ecosystem

explain how evolution through natural selection of variants that give rise to phenotypes best suited to their environment?

some mutations cause organisms to have different phenotypes. the organisms with the most suitable characteristics for the environment would be more successful competitors and would be more likely to survive and produce offspring which would inherit the desirable characteristics. Over time, beneficial characteristics become more common and the species changes - evolution