4.6 Inheritance, variation and evolution

Where does reproduction occur other than within large organisms?

Reproduction occurs in cells (eg. cell division by mitosis)

Define sexual reproduction

The process of combining the genetic material of two parents to produce offspring that are genetically different from the parents.

Because there are two parents, the offspring contain a mixture of their parents’ genes, which is why the offspring inherits genes from both parents.

In sexual reproduction, how do the mother and father produce gametes?

Cells in the parents’ reproductive organs divide by meiosis to form genetically different gametes

Give examples of gametes

Female: egg cells in animals and plants

Male: sperm cells in animals and pollen in plants

Describe what mitosis is, the three stages of the cell cycle and its purpose

Mitosis is a type of cell division that produces two genetically identical daughter cells from a single parent cell. Each daughter cells has the same number of chromosomes as the parent cell

Stage 1 (interphase): the cell grows and increases its number of sub cellular structures. The DNA replicates to form two copies of each chromosome

Stage 2 (mitosis): the chromosomes line up in the middle of the cell and the cell fibres pull one chromatid from each replicated chromosome to either side of the cell

Stage 3 (cytokinesis): the cytoplasm and cell membrane divide to form two identical daughter cells

Purpose: growth and development in multicellular organisms, repair and replacement of cells, asexual reproduction

What does sexual reproduction require

The fusing of male and female gametes. In sexual reproduction there is mixing of genetic information, leading to variety in the offspring

What does the formation of gametes involve?

Meiosis

Why does asexual reproduction only result in genetically identical offspring?

Because it involves only one parent and no fusing of gametes.

There is no mixing of genetic information so all of the offspring share the same genetic information as the parent. Only mitosis is involved

What type of cell division is involved in asexual reproduction?

Mitosis

What is the difference between the cells formed from meiosis and mitosis?

Meiosis: non identical cells formed

Mitosis: identical cells forme

What is meiosis and its purpose?

Meiosis is a type of cell division that produces gametes with half the number of chromosomes of the original cell

Purpose: to produce gametes for sexual reproduction to create genetic variation in offspring

Describe the process of meiosis

Meiosis involves two divisions forming four non-identical daughter cells

1. The DNA is replicated (by replicating all of the chromosomes in the cell). This means that each chromosome now has two chromatids.

2. The replicated chromosomes (with two chromatids) line up in the centre of the cell; which chromosome is on the left or right of the cell is completely random

3. The first division occurs: one replicated chromosome from each homologous pair is pulled to each side of the cell. The cell membrane and cytoplasm divides forming two non-identical daughter cells

4. The chromosomes of each new daughter cell line up at the centre of the cell again. One chromatid from each chromosome is pulled to each side of the cells and the cytoplasm and cell membrane divide again.

5. This leaves four genetically different haploid (only 23 chromosomes, not in pairs) daughter cells: each gamete is genetically different as they all have different chromosomes due to the

(Do not need to know this in this much detail)

Define diploid cell

Has two copies of chromosomes - one from each parent

(Each cell has 43 chromosomes, 23 from each parent)

Define halploid cell

Has only one copy of each chromosome

eg. gametes

What is the function of a chromosome

They are located in pairs in the nucleus of animal cells

A chromosome is a long molecule of DNA that carries genes

Genes are instructions that determine characteristics and control how the cell functions

What’s a chromatid?

A chromatid is one half (one arm) of a replicated chromosome

What is the technical term for pairs of chromosomes?

Homologous chromosomes

Describe fertilisation briefly and what happens after

Gametes join at fertilisation to restore the normal number of chromosomes (forming a zygote which has 46 chromosomes).

The new cell divides by mitosis, the number of cells increases forming an embryo. As the embryo develops cells differentiate and continues to divide, forming a fetus.

Why do gametes join at fertilisation?

To restore the normal number of chromosomes

Where and by which process are gametes formed?

Cells in reproductive organs divide by meiosis to form gametes

Describe simply meiosis (3 points)

• copies of the genetic material are made

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

• all gametes are genetically different from eachother

Explain the advantages of sexual reproduction

The greatest advantage of sexual reproduction is variation in offspring.

• produces variation in offspring

• if the environment changes, variation gives a survival advantage by natural selection; if some organisms are killed by the environment, there is a high chance that at least some will survive due their variation.

• natural selection can be sped up by humans in selective breeding to increase food production (eg. breeding animals that produce a lot of meat)

Describe the disadvantages of sexual reproduction

• As meiosis and a mate is required, it requires a lot of time and energy

• Cannot be done alone

Describe selective breeding and why this is done

Selective breeding is the process by which humans choose which organisms to breed in order to produce offspring with desirable traits.

Humans do this to speed up natural selection to increase food production

Define asexual reproduction

A method of reproduction that involves a single parent, so offspring produced are genetically identical to the parent

What is the process by which asexual reproduction occurs?

Cells divide by mitosis; the new cell has the exact same genetic information as the parent cell meaning that it is a clone

Why are the offspring of asexual reproduction genetically identical?

There is no fusion of gametes and no mixing of chromosomes, so therefore there is no genetic variation between parent and offspring

Describe advantages of asexual reproduction

• Only one parent is needed

• More energy and time efficient as the parent does not need to find a mate

• Much faster than sexual reproduction

• Many identical offspring can be produced when conditions are favourable

Describe the disadvantages of asexual reproduction

• Cannot lead to genetic variation as there is no mixing of genetic material

This means that if one of the offspring is not suited to the environment, they all won’t be so they will be easily wiped out by environmental change or disease

Describe three organisms that can reproduce both asexually and sexually and what makes these organisms produce in either way

These organisms can reproduce asexually or sexually depending on the conditions

• Malarial parasites 

Reproduce asexually in the human host but sexually in the mosquito

• Fungi

Produce asexually by spores but also reproduce sexually to create variation

• Some plants

Many plants produce seeds sexually

Strawberry plants produce seeds sexually but also produce runners for asexual reproduction (buds on the runners grow into shoots and roots which are genetically identical)

Daffodils reproduce asexually by bulb division (the parent bulb grows underground and small bulbs form around the bulb and can develop into roots and shoots becoming a new independent plant)

Describe sexual reproduction in strawberry plants

• Pollen is produced from the male sex organ and transferred to the female sex organ by insects

• The pollen fuses with the female egg to form a zygote

• The zygote develops into a seed

What is DNA?

A chemical that makes up the genetic material of a cell

It determines how an organism develops, functions and reproduces

Describe the structure of DNA

The genetic material of a nucleus is composed of a chemical called DNA.

DNA is a polymer made up of two strands forming a double helix; it is contained in structures called chromosomes

Describe the structure of DNA in terms of the nucleotide

DNA is a polymer made up of two strands; the backbone of these two long strands of DNA consist of alternating common sugars and phosphate groups, linked together.

The two strands of DNA attract eachother (due to chemical bonding) forming a double helix.

The rungs of this double helix are made up pairing nitrogenous bases; one base from each strand pairs with the base from the other strand.

Define genome

The complete set of genetic material in an organism

Why is it important to understand the human genome?

• allows scientists to identify genes in the genome that are linked to different types of disease, which can allow them to inform patients whether they may be predisposed to certain types of disease (eg. a patient may have gene that makes them more likely to develop breast cancer. If they know this, they will be informed to get regular checkups to potentially detect the disease at an early stage)

• knowing which genes are linked to inherited diseases can help scientists understand them better, which can help them to understand how to develop effective treatments for them

• can be used to trace human migration patterns in the past

Describe the order of organisation from cell to genes

Cell: basic unit of all living tissue

Within the cell, there is the nucleus

The nucleus contains the genome which is split between 23 pairs of chromosomes.

Each chromosome contains a long strand of DNA, tightly packaged.

Within the DNA there are sections called genes (these genes contain the instructions for making proteins)

What is the DNA polymer made up of?

Repeating nucleotide units (monomers)

Describe what DNA is made up of

DNA is a polymer made up of four different nucleotide

What does each nucleotide consist of?

A phosphate group, a common sugar and a base (one of four different bases)

Describe and draw the structure of a nucleotide

The nucleotides are the monomers that make up the structure of DNA.

They consist of phosphate groups and common sugars that make up the backbone of DNA strands and alternate, and a nitrogenous base (one of the four types) connected to the common sugar

(To form DNA, the base on one strand is connected to the base on the other strands. The two strands form a double helix structure).

Phosphate group: circle

Common sugar: pentagon

Nitrogenous base: rectangle

Draw a diagram of DNA (and draw the parts that it is made up of)

What are the four nitrogenous bases of DNA?

A, T, C, G

What is the complementary base pairing?

The specific pairing of nitrogenous bases in DNA

A always pairs with T

C always pairs with G

What does a sequence of three bases code for?

A particular amino acid

How is a particular amino acid coded for?

By a sequence of three bases (codon)

What does the sequence of bases control?

The sequence of bases in DNA controls the sequence of amino acids in a protein. The sequence of amino acids controls the structure and (therefore) function of a protein.

(As proteins control cell structure and function, codons (DNA) ultimately control an organism’s characteristics)

What is a sequence of three bases called and what does it do?

A codon codes for one particular amino acid

What are proteins?

Polymers made up of sequences of amino acids

There are 20 different amino acids

Define protein synthesis

the process of creating proteins from genetic information (codons made up of nitrogenous bases) in DNA

What is DNA’s biggest process and why?

Protein synthesis as it provides the template upon which proteins are coded for

What controls the function of a protein?

The protein’s amino acid chain folds up to form a unique 3D shaped protein.

The unique shape of each protein determines its function

Give examples of functions of proteins

• enzymes

• hormones

• forming structures in the body such as collagen

Explain simply how the structure of DNA affects the protein made

• DNA is made up of nucleotides

• Each nucleotide has a nitrogenous base

• A sequence of three nitrogenous bases (codon) codes for one amino acid

• The sequence of codon determines the order of the sequence of amino acids in the protein

• The protein’s amino acids chain folds up to form a unique 3D shaped protein

• The unique shape of the protein controls its function

What are the two major phases of protein synthesis?

• Transcription

• Translation

Describe protein synthesis

Transcription:

• DNA does not leave the nucleus (due to its size)

• The cell copies a gene into mRNA (messenger RNA) using DNA as a template

• The mRNA leaves the nucleus and carries the genetic code to the ribosome in the cytoplasm

• It attaches itself to the ribosome

Translation:

• The mRNA recruits tRNA (transfer RNA) that are complementary to the base sequence of the mRNA

As the tRNA are complementary to the base sequence of the mRNA, they bind to the amino acids that are needed (have been transcripted for by the mRNA) that are floating in the cytoplasm, having been absorbed from food

• The tRNA brings the correct amino acids to the ribosome

• The ribosome links the amino acids together in the correct sequence to form the protein

What is transcription?

The copying of DNA into mRNA 

What is meant by a genetic variant?

A permanent change in the DNA sequence that makes up a gene

These can be inherited, caused by environmental factors and also through problems with cell division.

They may have no affects, be beneficial, cause diseases etc.

What is coding DNA and what is non-coding DNA?

Coding DNA is split into genes which contain the instructions for making proteins so that the genes can be transcribed into mRNA which is then translated to form proteins. Therefore, coding DNA acts as a template, determining what proteins are formed

Non coding DNA act as regulators of gene expression and have structural and protective roles

Roughly how much total DNA is made up of coding DNA compared to non coding DNA?1

1%

Define phenotype

An organism’s observable physical characteristics

as a result of the interaction of its genotype with the environment

Describe how genetic variants in coding DNA influence phenotype

Coding DNA is the part of DNA that contains the instructions for protein formation; they determine what proteins are formed.

A genetic variant can change a codon, which can change the specific amino acids that the codon codes for.

If the amino acid changes, this changes the amino acid sequence of a protein, therefore altering the protein’s structure. If the protein’s structure changes, it also changes its function.

Change in the function of protein can impact phenotype. For example:

• if the function of structural proteins are changed (it may lose strength) it can impact the strength and shape of an organism’s tissues

• enzymes may no longer fit the substrate binding site

• can cause non functional proteins (as above)

What does it mean if a gene is expressed or not expressed (turned on or off)?

Expressed: It is used to code for an amino acid sequence (through the process of transcription and translation (the sequence is copied into mRNA which is translated and tRNA brings the corresponding amino acids) and therefore a protein.

Not expressed: It is not being used to code for/ make a protein

Describe how genetic variants in non coding DNA influence phenotype

Non coding DNA can control gene expression by controlling which genes are switched on and off and how strongly

Therefore a genetic variant in non coding DNA can impact how strongly or if at all a gene is expressed (how genes are expressed).

Therefore, it controls what types of proteins are made and in what amounts.

Changing the type or amounts of proteins impact phenotype. For example:

• if the function of structural proteins are changed (it may lose its strength) it can impact the strength and shape of an organism’s tissues

• enzymes may no longer fit the substrate binding site

• can cause non functional proteins (as above)

Describe the effect of a change in DNA structure

A change in DNA structure can affect the protein synthesised by a gene.

A change in the DNA sequence can change the bases/ the codons of DNA.

A change in the codons of DNA changes the particular amino acid that the codon codes for.

If the amino acid changes, this impacts the sequence of amino acids of the protein synthesised.

This impacts the shape and therefore function of the protein synthesised by a gene.

What is the carrier molecule?

tRNA (transport RNA)

What is mRNA and tRNA and what do they do?

mRNA: messenger RNA (transcription)

During transcription, a section of DNA is copied into mRNA using the sequence of codons and bases as a template. mRNA leaves the nucleus and carries the copy of the genetic information to the ribosomes where it is translated

tRNA: transport RNA (translation)

is the carrier molecules that are complimentary to the bases of the mRNA. It brings the correct amino acids to the ribosome so that the amino acids can be linked together and the protein can be synthesised

What are mutations of DNA?

Mutations are random changes in the DNA sequence

Define non functional protein

A non functional protein is a protein that cannot perform its normal job within the cell

True or false: mutations in DNA happen continuously (all the time)

True

What are the effects of mutations on protein synthesis? (long answer)

Most mutations do not alter the protein formed, or only alter it slightly so that its appearance of function is not changed.

This is because minor changes do not impact the shape of the protein that much, some codons code for the same amino acid and most mutations occur in non-coding DNA so don’t directly impact a protein’s structure, but instead whether it’s produced or not.

A few mutations code for an altered protein with a different shape. The effects of this may be that an enzyme may no longer fit the substrate binding site or a structural protein may lose its strength. Mutations can also be beneficial (eg. adaptations of camouflage, antiobiotic resistance).

True or false: genetic variants and mutations have the same impacts on proteins

True

Explain how a change in DNA structure may result in a change in the protein synthesised by a gene.

• A change in the DNA structure is a mutation

• If a mutation occurs, it changes the sequence of bases of DNA

• This may change one or more codons

• As each codon codes for one amino acid, a change of codon can cause the codon to code for a different amino acid, or even add a new amino acid to the chain, changing the sequence of amino acids

• Therefore during transcription, the copy of the gene (mRNA) will be different meaning that during translation the tRNA will bring the different amino acids

• Therefore the sequence of amino acids brought to the ribosomes will be different, meaning that the shape of the protein will be different and therefore the function of the protein synthesised will be different

When do mutations occur?

Mutations occur continuously (all the time) as DNA is constantly being copied during cell division, and it is likely that errors will occur.

What does non coding DNA do and what do variations in this cause?

Non coding DNA controls gene expression by turning genes on and off

Variations in this affect how genes are expressed

Explain what a gamete is

A gamete is a sex cell.

It is a haploid cell, meaning that it contains half the number of chromosomes as a normal body cell.

Gametes fuse together during fertilisation to form a zygote with a complete set of chromosomes.

Eg. sperm cell, egg cell, pollen (male), egg cell (female)

Explain what a chromosome is

Chromosomes are made up of tightly coiled, long strands of DNA; chromosomes carry many genes.

Explain what a gene is

A gene is a section of DNA that codes for (determines the amino acid sequence for) a specific protein, which determines the characteristic of an organisms.

Therefore, genes control our genetic characteristics.

Humans have thousands of genes, each controlling different traits.

Explain what an allele is

An allele is a version of a gene.

Most genes exist in more than one form; alleles arise by mutation and are found on the same place on a chromosome.

Explain dominant and recessive alleles

Dominant alleles are always expressed, even if only one copy is present

Recessive alleles are only expressive if two copies are present (there are no dominant alleles present).

How many alleles does each gene have in an organism?

A gene has two alleles (one from mother, one from father)

Explain what it means if an organism is homozygous or heterozygous for a gene

An organism is homozygous for a gene if it contains it has two identical alleles for that gene

An organism is heterozygous for a gene if it contains two different alleles for that gene (the dominant allele determines the characteristic in this case)

Define genotype

An organism’s complete genetic makeup: the specific combination of alleles an individual has for a particular gene

Define phenotype

An organism’s complete set of observable traits (physical characteristics) expressed by an organism due to the interaction between its genetic makeup and its environment.

True or false: a characteristic is controlled by one gene

False!

Some characteristics are controlled by a single gene (eg. red-green colourblindness in humans, fur colour in mice), however most characteristics are a result of multiple genes interacting.

What disorders are caused by the inheritance of certain alleles, what are they and are they caused by dominant or recessive alleles?

• Cystic fibrosis (a disorder of cell membranes, caused by recessive alleles)

• Polydactyly (having extra fingers or toes, caused by dominant alleles)

How many pairs of chromosomes do normal human body cells contain, and how many pairs carry the genes that determine sex?

23 pairs

1 pair of chromosomes carry the genes that determine sex, 22 pairs control characteristics only.

What are the female and male sex chromosomes?

Female: XX

Male: XY

Define genome

The genome is the complete set of genes in an organism

What influences the development of an organism’s phenotype?

the genome and its interaction with the environment

What is the differences in the characteristics in a population called?

Variation

Define variation

Differences in the characteristics between individuals in a population

What is variation in a population caused by?

Differences in

• the genes they have inherited (genetic causes)

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

• a combination of genes and the environment

True or false: there is usually minimal genetic variation within a population of a species

False.

There is usually extensive genetic variation within a population of a species

How does genetic variation come about?

Genetic variation is caused by a different alleles caused by mutations.

Most mutations have no effect on the phenotype, some influence phenotype, very few determine phenotype.

Describe the effect of mutations on phenotype.

• Most have no effect on phenotype

• Some influence phenotype

• Very few determine phenotype

Describe what can lead to a relatively rapid change in a species

• Mutations in DNA very rarely lead to a new phenotype

• However if they do, and this phenotype is suited to the environment, the organism with this phenotype will be more likely to survive and reproduce to pass on their genes

Describe evolution

A change in the inherited characteristics of a population over time through a process of natural selection.

This can lead to the formation of new phenotypes of a species in order to make them more suited to their environment; this may result in the formation of a new species

What does the theory of evolution by natural selection state about where all species evolved from?

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

What are genetic variants?

Genetic variants are different forms of the same gene (different alleles), caused by permanent changes in DNA (mutations)