Topic 4 - Genetics

What is meant by a gene?

A base sequence of DNA that codes for the production of one or more polypeptide chains and functional RNA.

What do genes occupy?

A fixed position on a DNA molecule, called its locus

What is meant by a ‘locus’ of DNA?

The fixed position on a DNA molecule which a gene occupies

What does the base sequence of each gene carry?

The coded genetic information that determines the sequence of amino acids during protein synthesis

What does the sequence of amino acids in a molecule determine?

The proteins of an organism

Define ‘allele’.

One of a number of alternative forms of a gene

What are Chromosomes?

Structures consisting of a long, coiled molecule of DNA and its associated proteins, by which genetic information is passed from generation to generation.

What does a chromosome consist of?

A long, coiled molecule of DNA and its associated proteins

How many pairs of chromosomes do humans have?

23

How many total chromosomes do humans have?

46

What are homologous pairs?

Pairs of matching chromosomes

How are homologous pairs of chromosomes similar and different to each other?

They are exactly the same size, have exactly the same genes but may have different alleles.

Describe how DNA is stored in eukaryotic cells.

Long, linear DNA that exists as chromosomes

Located in the nucleus of the cell

Double helix tightly wound around proteins called histones

Where is the DNA of a eukaryotic organism stored?

In the nucleus

What are histones?

The protein that DNA is tightly wound around in a eukaryotic cell

What is the purpose of the DNA being tightly wound around histones?

So that the molecule can fit inside the nucleus.

Describe how DNA is stored in prokaryotic cells.

Short, circular, not associated with proteins

DNA supercoils to fit inside the cell

Describe how DNA is stored in chloroplasts and mitochondria.

Short, circular, not associated with proteins

DNA supercoils to fit inside the cell

In what two organelles is DNA also found? (Excluding nucleus)

Chloroplasts and mitochondria

What is the genetic code?

The order of bases in DNA

What does the genetic code consist of?

Triplets of bases

What does each triplet of bases in the genetic code code for?

An amino acid

What is a codon?

A sequence of three bases on mRNA that codes for a specific amino acid

What does a codon code for?

an amino acid

What is at the start of every gene?

A start codon

What does a start codon code for?

The amino acid methionine.

What happens to methionine if it is not needed for the structure of a protein?

It is removed

What is at the end of every gene?

A stop codon

What is a stop codon?

3 bases that do not code for an amino acid that marks the end of the polypeptide chain

What does the stop codon do?

Cause ribosomes to detach and stop translation

What are the three key features of the genetic code?

Degenerate

Universal

Non-Overlapping

What does ‘degenerate’ mean in terms of the genetic code?

There are more possible combinations of triplets than there are amino acids. This means that many amino acids are coded for by more than one triplet.

What does ‘universal’ mean in terms of the genetic code?

The same codons code for the same amino acids in all living organisms.

What does ‘non-overlapping’ mean in terms of the genetic code?

Each base in a gene is only part of one triplet of bases. Therefore each triplet of bases is read as a discrete unit.

Why is the genetic code being degenerate an advantage?

If a point mutation occurs, the same amino acid may be coded therefore the mutation has no effect.

Why is the genetic code being universal an advantage?

Genetic engineering is possible.

Why is the genetic code being non-overlapping an advantage?

If a point mutation occurs, it will only affect one codon and therefore one amino acid.

What are introns?

Sections of DNA that do not code for amino acids

In what type of DNA are introns found?

eukaryotic DNA

What happens to introns during protein synthesis?

They are removed and spliced out of mRNA

What are exons?

Sections of DNA that do code for amino acids

What is a genome?

An organisms complete set of DNA in one cell

What is a proteome?

The full range of proteins in one cell

Describe how the genome and proteome of a cell may change.

The genome never changes, but the proteome is constantly changing depending on which proteins are needed.

What is RNA?

A polymer of a nucleotide

What is RNA formed of?

A ribose, a nitrogenous base and a phosphate group

What are the nitrogenous bases in RNA?

Adenine, Uracil, Guanine, Cytosine

What is the function of RNA?

To copy and transfer the genetic code from DNA in the nucleus to the ribosomes

What is created when RNA is combined with proteins?

Ribosomes

What is mRNA?

A copy of a gene from DNA

Where is mRNA created?

The nucleus

Why does mRNA leave the nucleus?

To carry the copy of the genetic code of one gene to a ribosome in the cytoplasm

What are the three key properties of mRNA? (In short)

Shorter than DNA

Short lived

Single stranded and linear

How long is mRNA?

The length of one gene

What is the purpose of mRNA being shorter than DNA?

It can leave the nucleus

Why is mRNA short lived?

It is only needed temporarily to help create a protein.

What occurs every 3 bases of mRNA?

a codon

Where is tRNA located?

the cytoplasm

What is tRNA?

A form of RNA that carries specific amino acids to the ribosomes

Describe the structure of tRNA.

Single stranded but fold to form a clover leaf shape held together by hydrogen bonds

What is the function of tRNA?

To attach to an amino acid and transfer it to ribosomes to create a polypeptide chain.

What is the anticodon on tRNA complementary to?

the codon on the mRNA molecule.

Compare the structure of mRNA and tRNA.

tRNA is a clover leaf shape, and mRNA is linear

tRNA has hydrogen bonds, mRNA does not

tRNA has an anticodon, mRNA has a codon

Both are single stranded

Where are proteins created?

On ribosomes

What are the two stages of protein synthesis?

Transcription and translation

What is transcription?

The first stage of protein synthesis.

What occurs during transcription? (Overall)

The formation of pre-mRNA in eukaryotes and mRNA in prokaryotes from a section of the template strand of DNA.

Describe the process of transcription.

The DNA helix unwinds to expose the bases to act as a template.

One chain of the DNA acts as a template.

DNA helicase breaks the hydrogen bonds between bases.

Free mRNA nucleotides in the nucleus align opposite exposed complementary DNA bases.

DNA Polymerase bonds together the nucleotides to create a new RNA polymer chain. One entire gene is copied.

What has to happen to pre-mRNA following transcription?

It has to be modified to become mRNA.

Describe how pre-mRNA is modified to become mature mRNA.

The introns are spliced out by a protein called splicesome, leaving behind the exons.

Describe the process of Transcription during protein synthesis.

The modified mRNA attaches to a ribosome at the start codon.

The tRNA molecule aligns opposite to mRNA, held in place by the ribosome.

The ribosome moves along the mRNA molecule, enabling the complementary tRNA to attach to the codons on mRNA.

The two amino acids that have been delivered by the tRNA molecule are joined by a peptide bond. This is catalysed by an enzyme and requires ATP.

This continues until the ribosome reaches the stop codon. This causes the ribosome to detach and translation to end.

Define what is meant by ‘meiosis’.

A type of cell division in which a parent cell divides to form four haploid cells, each genetically distinct from one another.

What does meiosis produce?

Four haploid cells that are genetically different from one another and the parent cells.

What cell(s) does meiosis start out with?

A single diploid cell

Describe how ‘independent segregation’ can lead to increased genetic diversity.

During meiosis 1, homologous pairs of chromosomes line up opposite each other at the equator of the cell. The arrangement of the pairs are random, meaning that the division into daughter cells is also random.

Describe how ‘crossing over’ can lead to increased genetic diversity.

When pairs of chromosomes line up they can exchange some of their genetic material. Crossing over occurs when one chromosome may swap places with the same part of its homologous pair leading to a different combination of alleles on the gene.

Define what a mutation is.

The change in the base sequence or quantity of DNA which occurs spontaneously during DNA replication.

List the two key types of DNA mutation that can occur.

Substitution

Deletion

Describe how a substitution mutation can lead to the change of a DNA sequence.

One nucleotide in the DNA sequence is replaced by another.

Describe how a deletion mutation can lead to the change of a DNA sequence.

A nucleotide in the DNA sequence is lost. This can cause

frame shift, were the reading frame will shift to the left.

Name two forms of chromosome mutation that may occur.

Polyploidy

Non-Disjunction

Describe what the polyploidy mutation is.

Changes can occur in the whole set of chromosomes so that an individual has three of more sets of chromosomes instead of two.

Where is polyploidy common?

In plants

Describe how a non-disjunction mutation can occur and what this leads to.

Occurs when chromosomes fail to separate correctly in meiosis. As a result the gametes and any zygotes formed will have one more or one less chromosome than they should.

Describe one example of a non-disjunction mutation.

Down’s syndrome is the result of non-disjunction where individuals have an extra chromosome 21.

Define ‘genetic diversity’.

The number of different alleles in the genes of a population.

What does natural selection require to occur?

Genetic diversity.

Define ‘natural selection’.

The process that leads to evolution in populations.

What does natural selection result in?

A species becoming better adapted to their environment.

Define ‘evolution’.

The change in allele frequency over many generations in a population.

What are the three types of adaptations?

Anatomical

Physiological

Behavioural

What is a physiological adaptation?

An adaptation that influences the processes inside the body of an organism.

Describe the process of natural selection.

New alleles of a gene are created by random mutations.

If the new allele increases the chances of the individual to survive in the environment, then they are more likely to survive and reproduce.

This reproduction passes on the advantageous allele to the next generation.

Therefore over many generations, the new allele increases in frequency within a population.

What are the two types of natural selection?

Directional and stabilising

Describe what is meant by ‘directional selection’.

One of the extremes has the selective advantage. It occurs when there is a change in the environment. The modal trait changes.

Describe what is meant by ‘stabilising selection’.

The modal trait has the selective advantage. Occurs when there is no change in the environment. The modal trait stays the same.