3.4.3 Genetic Diversity can arise as a result of mutation or during mitosis

What is genetic diversty?

a measure of the number of different alleles in genes in a population

What causes genetic diversity (5)?

1. gene mutations

2. chromosome mutations

3. crssoing over

4. random assortment

5. random fusion of gametes

Which of the causes of genetic diversity occur in meiosis

crossing over, chromosome mutations and random assortment

What is a gene mutation?

a change in the base sequence of a chromosome

When do gene mutations usually occur?

DNA replication

What happens if a gene mutation occurs in an exon?

the polypeptide coded for may be different and as the tertiary structure is different, may be non-functional

What are neutral mutations?

When the change in amino acid sequence does not affect the function of the protein

What is an allele?

a new base sequence of a gene

Give 3 types of gene mutations?

• substitution

• deletion

• addition

What is a substitution gene mutation?

When one or more bases in the gene are exchanged / subsititued for others. This only affects one triplet and the rest stay the same, changing only one amino acid

Why might some substitution gene mutations not result in a change to the polypeptide chain?

The DNA code is degenerate, so the substituted triplet may code for the same amino acid

What are mutations that don’t result in change called?

Silent mutations

If a triplet is substituted with a stop triplet, what happens to the amino acid?

The chain will be shorter

What is an addition gene mutation?

When one or more bases are added to the base sequence of a gene

What is a deletion gene mutation?

When one or more bases are deleted from the base sequence of a gene

What does an addition or deletion gene mutation result in?

a frame shift. Each triplet following the frame shift is altered with the consequence of an antirely new amino acid code

What effect does a deletion or addition gene mutation usually have?

the protein coded for will likely be non functional

What are mutagenic agents?

Factors that increase the rate of mutation

Give 4 examples of mutagenic agents

• High energy raduation

• Ionising radiation

• Tar in cigarettes

• Virusses (HPV)

What ae carcinogens?

Substances which cause mutations in oncogens that causes a cell to become cancerous

What are oncogenes?

genes asssociated with regulation of cell division

How frequently to gene or chromosome mutations take place?

randomly

When do chromosome mutations occur?

meiosis

What happens when 2 gametes fuse?

The diploid number is restored

What is the n number of a zygote?

2n

What is non-disjuntion??

When the homologous chromosomes do not separate in meiosis meaning that a gamete is diploid for one chromosome

Describe a non-disjunct chromosome

Two chromosomes of the same type

If someone had a non-disjunct chromosome and had a child, describe the chromosomes of the child

3 homologous chromosomes of that specific chromosome

What causes down syndrome?

non-disjunction of chromsome 21 (3n)

What is meiosis?

the division of a diploid cell to four haploid daughter cells (gametes)

Where does meiosis occur in humans?

gonads

Describe the meiosis daughter cells

• genetically different

• haploid

Why are meiosis daughter cells not identical?

• crossing over

• independent segregation of chromosomes

What happens when two haploid gametes fuse?

the diploid chromosome number is restored

Why is meiosis important?

chromosome numbers are kept constant through the generations

How many divisions are there in meiosis?

2

What occurs in meiosis I?

The homologous chromosomes are separated

What happens in meiosis II?

The sister chromatids are separated

When does crossing over take place?

meiosis I

When does independent segregation of homologous chromosomes occur?

meiosis I

Draw a diagram of meiosis

What type of division is meiosis I?

reducing division

How does the n number change during meiosis I?

changes from 2n to n

Describe the chromosomes before meiosis I?

each chromsome consists of 2 sister chromatids

Describe the chromosomes after meiosis I?

each chromosome consists of 2 sister chromatids

Describe the chromosomes after meiosis II

each chromosome consists of one sister chromatid

What is crossing over?

When homologous chromosomes pair and come together, wrapping round each other and essentially swapping sections

What do the chromosomes form in crossing over?

bivalent

Describe the process of crossing over?

• non sister chromatids wrap around each other

• They attatch at points called chiasmata

• They break and rejoin, swapping sections of non-sister chromatids

• New combinations of alleles

What are the points where the chromosomes rejoin called in crossing over?

chiasmata

What happens as a result of crossing over?

new combinations of alleles in the gametes

Draw a diagram of crossing over?

Describe the process of independent segregation of chromosomes

• In the first meiotic division the homologous pairs of chromosomes randomly align at the equator, the pair is the separated to opposite poles of the cell

• The paternal and maternal homologous chromsomes are separated from each other

• They are segrated independently

Draw a diagram of independent segregation

How can you calculate the number of combinations of maternal and paternal chromosomes?

2^n where n is the number of pairs of chromosomes

Complete the meiosis vs mitosis table:

Mitosis	Meiosis
Genetically identical daughter cells?
How many daughter cells?
How many divisions?
n number before and after

Mitosis	Meiosis
Yes	No
2	4
1	2
2n → 2n	2n → n

Draw a human lifecycle with n number

What does 2n represent?

diploid cells

What does n represent?

haploid cells

Draw the moss life cycle

How do you know meiosis has occured in a life cycle?

the n number changes from 2n to n