yr 10 bio- Genetics and evolution

Where is genetic information stored?

• Genetic information is stored inside the nucleus of an organism’s cells

• Genetic information is found in a molecule - DNA

• DNA = Deoxyribonucleic acid

• DNA contains the instructions to build proteins.

The structure of DNA

Structure is known as a double helix.

A DNA molecule is like a ladder. Each "rung" of the ladder is made up of two smaller molecules, known as bases.

DNA bases

adenine (A), thymine (T), guanine (G) and cytosine (C).

DNA base pairing rule

Each base can only join with one other type of base to form a rung of the ladder:

• adenine (A) and thymine (T) join together – A–T

• guanine (G) and cytosine (C) join together – G–C

• Each A-T or G-C combination is known as a base pair.

• The two bases in a base pair are complementary.

What is a gene

• A gene is a section of a DNA molecule 

• It provides instructions for building a specific protein. 

• Humans have about 20,000 genes.

Genes: how information is passed down from parent to offspring

• Genetic information is passed down from one generation to the next in packets or units. These are known as genes. 

• The passing down of traits via genes from one generation to the next is called inheritance.

Proteins

• Proteins are molecules

• The basic unit of a protein is an amino acid

• Amino acids bond together → proteins

• Proteins make up most of the cells & tissues of an organism’s body 

How does DNA build proteins

• DNA is stored in a cell's nucleus. 

• Each gene in a strand of DNA is a recipe for making a specific protein. 

• Proteins are built out of amino acids outside the nucleus, in the cell's cytoplasm. 

What’s the role of RNA

• Transcription is the process where a cell creates an RNA copy of a DNA sequence.

• This RNA molecule is single stranded.

• The RNA carries the genetic information from the DNA in the nucleus to the cytoplasm, where proteins are made. 

RNA bases

• RNA contains a base that's not found in DNA. 

• In place of thymine (T), RNA uses the base uracil (U).

Codons

• The bases in the RNA strand provide instructions for building a specific protein

• The cell "reads" the bases of the RNA in groups of three. 

• Each group of three bases is called a codon. 

• Codons "code" for amino acids.

How codons determine amino acids

• The set of bases in a gene make up the code for that gene. 

• These are used to create an RNA molecule, made up of a sequence of codons. 

• The codons that make up the RNA molecule determine the sequence of amino acids needed to build a protein. 

The codon wheel

There are 20 amino acids found in human proteins

Which codons code for which amino acids can be shown in the codon wheel. 

Work from the inner circle outwards to determine the 3 letter sequence of the amino acid

AUG = Met

Building a protein

• A sequence of codons determines the order in which amino acids are strung together to build a protein.

What is a mutation?

• A change in the DNA sequence of an organism

• Changes can then be passed down to new generations

• There are many types of mutations 

• Small changes in the DNA sequence can have big consequences

Types of mutations

Point mutation- a single base is changed

The three point mutations are:

• Insertion

• Deletion

• Substitution

Insertion

The addition of one or more bases, this causes a frameshift to occur - a change in the amino acid sequence.

Deletion

When one or more bases are removed from the sequence, this causes a frameshift to occur.

Substitution

When one or more bases in a sequence is replaced by the same number of bases. No frameshift occurs.

Genetic traits

• Genes play a big role in your traits. E.g. eye colour, skin colour, hair colour

• Genes are inherited from your parents

• Traits are features of an organism.

Simple inheritance patterns diagram

Dominant and recessive traits- pure breeding red and white pea plants

• In pure-breeding lines, only the characteristics of the parents will be shown in the offspring.

  • Red-flowered will produce red-flowered

  • White-flowered will produce white-flowered

• However, when a red-flowered pea plant and a white-flowered pea plant were crossed, all the offspring had red flowers.

• This means that the red characteristic is dominant and the white characteristic is recessive.

Genes and alleles

• When there is more than one possible characteristic for a particular gene, the different possibilities are called alleles.

• Alleles are variations of genes.

• In Mendel’s work it showed that pea plants have two alleles for flower colour, the red dominant allele and the white recessive allele.

Using alleles

Using our knowledge of alleles and which is dominant and which is recessive, we are able to predict the characteristics of offspring for particular genes.

For example: 

• A red pure-bred flower would be written as: RR

• A white pure-bred flower would be written as: rr

• A red flower produced by crossing a white and red would be written as: Rr

How alleles represent dominat and recessive genes

• The dominant allele is always written with a capital letter. Eg R

• The recessive allele is always written with a lower case letter. Eg r

• RR or Rr = red flowers because the red is dominant

• rr = white flowers because the white is recessive

Different types of alleles

• If the code has two of the same case letters such as RR or rr = homozygous

• If the code has two different case letters such as Rr = heterozygous

Phenotype vs genotype

A genotype is a genetic code for your characteristics 

• (invisible code in your DNA)

• eg. RR, Rr or rr

A phenotype is the visual appearance or traits produced by the genetic code 

• (what we can see looking at organisms)

• eg red flowers or white flowers.

Phenotype and genotype in pea plants (image)

What is used to predict the inheritance of traits

Punnett squares are used to determine the possible types of offspring that could result from a cross.

Punnett squares

• The top row = possible gametes produced by one of the parents

• The side column = possible gametes produced by the other parent

• In each square is a possible outcome of fertilisation.

Variation

• Differences in traits between individuals of the same species are known as variations.

• Variation is produced due to evolution.

Genetic trait

A feature that is passed down by genes from one generation to the next.

• E.g skin colour

Acquired trait

• Gained during an individual's lifetime.

• They are not determined by genes in an individual’s DNA.

Random Mutations

• Variations in traits can arise from mutations (small random changes) in DNA. 

• The changes can produce new variations in traits.

• They may be harmful, beneficial or have no impact on the organisms survival. 

• New mutations arise in each generation through reproduction

• The evolution of new species needs random mutations to produce genetic variations. For example, variations in three tree frog species.

Evolution

any change in the genetic traits in a population over many generations

Population

a group of organisms of the same species living in the same place.

Variation in evolution example

A population of beatles might begin with very little variation in colour.

In each generation, a random mutation can occur to produce a new colour. 

Why is genetic variation important

Because it is needed for a population to evolve.

• The amount of variation is genetic diversity

Why is genetic diversity important

Because it helps a species survive when its environment changes.