biology inheritance, variation and evolution

what is DNA

the chemical all genetic material in a cell is made up of

what does DNA stand for

Deoxyribonucleic acid

what does DNA contain

coded information which instructs the cell how to put an organism together and make it work

as a result what does dna determine

what characteristics you inherit

where is dna found

in chromosomes, which are found in the nucleus of animal and plant cells

how do chromosomes usually come

in pairs

what is dna’s structure

two strands coiled up together in a double helix

what is dna

a polymer

what do chromosomes contain

small sections of dna called genes

what does each gene code for

a particular sequence of amino acids which are put together to make a specific protein

how many total amino acids are used

20

what do they make up

thousands of different proteins

what is genes role (more simply)

to tell cells what order the amino acids should be put in

what does dna also determine

what proteins the cell produces → will decide what type of cell it is

what is a genome

the entire set of genetic material in an organism

what have scientists worked out

the complete human genome

why is understanding the human genome important

its an important tool for science and medicine for many reasons

what are 3 reasons of their importance

1. scientists can identify genes in genomes which are linked to different types of disease

2. knowing links between genes and inherited diseases can help us understand and develop more effective treatments

3. scientists can use genomes to trace migration of certain populations globally

what is dna made up of

dna strands called nucleotides

what is a nucleotide

polymers made up of lots of repeating units

what does each nucleotide consist of

• a sugar

• a phosphate group

• one ‘base’

what do the sugar and phosphate form

the ‘backbone’ to the dna strands

what will the sugar and phosphate groups do

alternate

what will join to each sugar

one of 4 bases - A, C, G, T

next, what will each base link to

a base on the opposite strand in the helix

what is complementary base pairing

A always pairs with T ; C always pairs with G

what decides the order of amino acids in a protein

the order of bases in a gene

what is each amino acid coded for by

a sequence of 3 bases in the gene

then, what do the amino acids do

join together to make various proteins depending on the order of the gene’s bases

what do some parts of dna do

dont code for proteins → they switch genes on and off, controlling whether certain genes are expressed (used to make a protein)

what is a ribosome

tiny structures in the cytoplasm where proteins are made

how do ribosomes make proteins

they use the code in the dna

where is this dna found

in the nucleus → they can’t move out of it due to it being really big

how is this issue of getting dna to the ribosome resolved

a molecule called mRNA is used as a messenger between the dna and the ribosome, carrying the code between the two

how is mRNA made

copying the code from dna

what do mRNA molecules result in

the correct amino acids are brought to the ribosomes in the correct order by the carrier molecules

what forms when a chain of amino acids has been assembled

it folds into a unique shape which allows the protein to form its task

what are 3 examples of types of proteins

1. enzymes → act as biological catalysts, speeding up reactions

2. hormones → used to carry messages around the body (e.g. insulin released by pancreas into blood to control blood glucose levels)

3. structural proteins → physically strong (e.g. collagen is a structural protein that strengthens connective tissues)

what may occasionally happen to a gene

it may mutate

what is a mutation

a random change in an organisms dna (it can sometimes be inherited)

how do mutations occur

continuously. they can also occur spontaneously e.g. when a chromosome isn’t replicated properly

what is chance of mutation increased by

exposure to certain substances or types of radiation

what do mutations change

the sequence of dna bases in a gene → this produces a genetic variant → as a result, this can lead to changes in the protein it codes for

what is a genetic variant

a different form of the gene

what is the effect of most mutations

very little or no effect on the protein; some will change it to such a small extent its function or appearance will remain unchanged

however, what can some mutations do

seriously affect a protein. it may code for an altered protein with a changed shape which could affect its ability for perform its function

what is an example of this

if the shape of an enzyme’s active site is changed, the substrate may no longer be able to bind to it

what will happen if theres a mutation in non coding dna

it can alter how genes are expressed

what are the 3 possible types of mutation

• insertions

• deletions

• substitutions

what is an insertion

when a new base is inserted into the base sequence where it shouldnt be

what does this result in

this changes the way groups of three bases are ‘read’, changing the amino acids they code for

how many amino acids do insertions affect

can affect more then one as they have a knock on effect on bases further on in the sequence

what is a deletion

when a random base is deleted from the dna base sequence (they act similarly to insertions)

what is a substitution

when a random base in the dna sequence is changed to a different base

what is sexual reproduction

genetic material from 2 different organisms (father and mother) is combined to produce offspring which is genetically different to either parent

what do the mother and father produce in sexual reproduction

gametes as a result of meiosis (e.g. egg and sperm cells in animals)

how many chromosomes does a gamete contain in humans

23 chromosomes

how many chromosomes are in a normal cell

46 (or 23 PAIRS) → a gamete has just one of each chromosome rather then two

what happens when the egg (female) and sperm (male) cells fuse together (or fertilise)

they form a cell with the full number of chromosomes (half from father, half from mother)

what does sexual reproduction involve

the fusion of male and female gamete, producing a mixture of their parent’s genes

what is the result of receiving a mixture of genetic information

produces genetic variation in offspring

what else can reproduce in this way

flowering plants - they also have egg cells, but their version of sperm is pollen

how many parents are there in asexual reproduction

1 → so offspring are genetically identical to the parent

how does asexual reproduction happen

mitosis → an ordinary cell makes a new cell by dividing in half

what does mitosis produce

a new cell which has exactly the same genetic information as the parent cell → a clone

what produces asexually (3)

• bacteria

• some plants

• some animals

meiosis only occurs in humans in…

the reproductive organs (ovaries in females, testes in males)

steps of meiosis (5)

1. before the cell starts dividing, it duplicates its genetic material forming 2 armed chromosomes (one arm of each chromosome is an exact copy of the other arm)

2. after replication, the chromosomes arrange themselves into pairs

3. in the first division of meiosis the chromosome pairs line up in the centre of the cell

4. the pairs are then pulled apart so each new cell has only one copy of each chromosome (some of the father’s and mother’s chromosomes go into each new cell)

5. in the second division, the chromosomes line up again in the centre of the cell. the arms of the chromosomes are pulled apart.

what is produced as a result

4 gametes, each containing only a single set of chromosomes

are the gametes genetically identical or different

the chromosomes all get shuffled up during meiosis and each gamete only gets half of them randomly → as a result, they are each genetically different to the others

after fertilisation, when two gametes fuse, what happens to the remaining new cell

it divides by mitosis to create a copy of itself → this repeats many times to produce lots of new cells in an embryo

what will happen to these new cells

they start to differentiate into different types of specialised cells which make up a whole organism

what does variation increase the chance of

a species surviving a change in environment - they have a survival advantage

as a result, what is more likely of species which are genetically varied

better chance of survival → more likely to successfully breed → more likely to pass their genes on

what is this process called

natural selection

what process can be used to speed up natural selection

selective breeding

what does selective breeding allow

to produce animals with desired characteristics

what is selective breeding

individuals with desired characteristics are bred to produce offspring with the desired characteristics

what will selective breeding increase

food production e.g. meat, milk

what are advantages of asexual reproduction (compared to sexual) (4)

1. only needs one parent

2. uses less energy because organisms dont have to find a mate

3. faster

4. many identical offspring can be produced in favourable conditions

what are 3 examples of organisms which can reproduce sexually and asexually

1. malarial parasites

2. plants

3. fungi

how do mosquitos transfer parasites

when a mosquito carrying a parasite bites a human

how do parasites reproduce sexually

when its in the mosquito

how do parasites reproduce asexually

when its in the human host

how do fungi species reproduce

they release spores, which can become new fungi when they land in a suitable place

how can spores be created

sexually or asexually

how do fungi reproduce asexually

these spores form fungi which are genetically identical to the parent

how/why do fungi reproduce sexually

these spores are often formed in response to an unfavourable change in the environment → by introducing variation, it increases the chance that the population will survive the change

how do plants reproduce asexually

in many different ways

• e.g. strawberry plants grow ‘runners’ (stems that grow horizontally on soil surface away from a plant). at various points, a new identical strawberry plant will form

• bulb plants (e.g. daffodils) produce new bulbs off the main bulb and divide off - each new bulb will be identical to the parent

how do plants reproduce sexually

with seeds

how many pairs of chromosomes are in each human body cell

23

out of these, how many are matched pairs of chromosomes that control characteristics

22

what is the 23rd pair

either XX or XY

what do these 2 chromosomes decide

your sex (female or male)

what do males have

XY - the Y causes male characteristics

what do females have

XX - the XX combination causes female characteristics

what happens when making sperm

the X and Y chromosomes are drawn apart in the first division of meiosis

what are the chances of getting a X or Y chromosome

50% chance each sperm cell gets an X chromosome, 50% chance each sperm cell gets a Y chromosome

how is this similar to making eggs

similar process, but the original cell has two X chromosomes, so all eggs have 1 X chromosome

what can be used to find the probability of getting a boy or girl

a genetic diagram e.g. punnett square