Genetic diversity can arise as a result of mutation or during meiosis

Define mutation.

A change in the DNA base sequence.

What is a substitution mutation?

One nucleotide is replaced by another with a different base

What are the three possible outcomes of a substitution mutation?

• Silent mutation

• Different amino acid produced

• Stop codon produced

What is a deletion mutation?

Loss of a nucleotide from the DNA sequence.

Why are deletion mutations often more serious?

They cause a frameshift, changing every codon after the mutation.

Alters the reading frame

Define chromosome mutation.

A change in chromosome structure or chromosome number.

What is polyploidy?

Possessing three or more complete sets of chromosomes.

What is non-disjunction?

Failure of homologous chromosomes or chromatids to separate during meiosis.

What is a mutagenic agent?

An environmental factor that increases mutation rate.

Give three examples of mutagenic agents.

• UV radiation

• Ionising radiation

• Toxic chemicals

What are the natural mechanisms to ensure the accuracy of DNA replication?

Proof reading and repairing damaged DNA

If mutation rate rises above normal rate these mechanisms are ineffective

Define meiosis.

A type of cell division producing four genetically different haploid cells.

What happens in meiosis I?

• Prophase I: chromosomes condense, homologous chromosomes pair up, nucleolus disappears, nuclear envelope disintegrates, centrioles move to opp ends of cell to form spindle fibres

• Metaphase I: chromosomes line up along equator of cell, each chromosome attaches to spindle by their centromere

• Anaphase I: spindle fibres contract + shorten pulling homologous chromosome pairs, separating them and pulling to opposite poles of cell (chromatids stay together)

• Telophase I: chromasomes uncoil nuclear envelope forms around each set nucleolus starts to reform cytoplasm divides in 2

what happens in meoisis II?

• Prophase II: Chromosomes condense, nuclear envelope breaks down, and spindle apparatus forms.

• Metaphase II: Individual chromosomes align in single file along equator; spindle fibres attach to centromeres.

• Anaphase II: Centromeres divide; spindle fibres contract, pulling sister chromatids to opposite poles.

• Telophase II & Cytokinesis: Nuclear envelopes reform, chromosomes uncoil, and cytoplasm divides into four genetically different, haploid cells.

State four differences between mitosis and meiosis.

Mitosis	Meiosis
1 division	2 divisions
2 cells produced	4 cells produced
Diploid cells	Haploid cells
No variation	Genetic variation produced

What is crossing over?

Exchange of DNA between homologous chromosomes during Prophase I. leads to recombination- new allele combo

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How does crossing over increase variation?

Produces new combinations of alleles.

What is independent segregation?

Random orientation of homologous chromosome pairs during Metaphase I.

how does indepepndent segregation increase variation?

produces different combos of maternal and paternal chromosomes in gametes

What is introducing genetic variation essential for?

Natural selection and allows evolution of species

Random fertilisation

Each parent has different alleles of a gene which get randomly assorted into gametes

Random fertilisation means all offspring have unique combinations of alleles (except MZ twins)

Leads to genetic diversity

Why is genetic diversity advantageous?

Means populations are able to adapt to changing environments

How does crossing over (recombination) happen in prophase 1?

• Homologous chromosomes pair up in cell

• Forms bivalent

• Chromatids of 2 homologous chromosomes twist around each other to form chiasmata

• At chiasmata sections of DNA are exchanged between homologous chromosomes

• During anaphase 1 homologous chromosomes pairs in bivalents separate and carry different combinations of alleles into new cells

  
  

What is a bivalent?

A pair of joined homologous chromosomes

What is a chiasmata?

The points where chromatids break and swap pieces during crossing over

How does independent segregation (random assortment) happen in metaphase 1?

• Pairs of homologous chromosomes in bivalents line up along equator of cell

• Orientation of each pair is random (maternal + paternal chromosomes can be either side of equator)

• Different combinations of maternal and paternal chromosomes end up in each gamete