GENETIC DIVERSITY: MUTATIONS AND MEIOSIS

A gene mutation is a change in the sequence of base pairs in a DNA molecule that may result in an altered polypeptide

continuously and at random

• The DNA base sequence determines the sequence of amino acids that make up a protein; • therefore, mutations in a gene can sometimes lead to a change in the polypeptide that the gene codes for

A type of mutation when a nucleotide (and therefore its base) is randomly deleted from the DNA sequence. A deletion mutation changes the amino acid that would have been coded for Responsible for changing the triplets of three bases further on in the DNA sequence

A mutation that occurs when a base in the DNA sequence is randomly swapped for a different base Unlike a deletion mutation, a substitution mutation will only change the amino acid for the triplet in which the mutation occurs

• silent mutation –> if it does not alter the amino acid sequence of the polypeptide because many codons code for the same amino acid as the genetic code is degenerate • The mutation may alter a single amino acid in the polypeptide chain The mutation creates a premature STOP codon, causing the polypeptide chain produced to be incomplete and therefore affecting the final protein structure and function

environmental factors that increase the mutation rate of cells

-high-energy radiation such as UV light -ionising radiation such as X-rays -toxic chemicals such as peroxides

Chromosome mutations involve a change in the number of chromosomes and involve non-disjunction

Non-disjunction occurs when chromosomes fail to separate during meiosis This occurs spontaneously

Meiosis produces daughter cells that are genetically different from each other and to the parent cell

This is due to the processes of independent segregation and crossing over

During meiosis I, homologous chromosomes line up in the centre of the cell The chromosomes are then separated and pulled into different cells — this is called segregation of homologous chromosomes

That means the way one pair segregates does not affect how another pair segregates This is what makes it independent — each chromosome pair has its different path by chance

the process where homologous chromosomes exchange genetic material during meiosis I

-Homologous chromosomes pair up and form bivalents -The chromatid then breaks and rejoins to the chromatid of its homologous chromosome, so that its alleles are exchanged -Crossing over leads to new combinations of alleles on each chromatid; this is called recombination

-Meiosis is a form of nuclear division that results in the production of haploid cells from diploid cells -It produces gametes in plants and animals that are used in sexual reproduction

-Meiosis I is the first division in meiosis -It separates homologous chromosomes and reduces the chromosome number by half, producing two haploid cells

-DNA replication has already occurred -DNA condenses and becomes visible as chromosomes -The chromosomes are arranged side by side in homologous pairs -Crossing over of non-sister chromatids may occur -The spindle is formed by centrioles -The nuclear envelope breaks down, and the nucleolus disintegrates

Homologous pairs of chromosomes line up randomly along the equator of the spindle Independent segregation occurs in metaphase I

The homologous pairs of chromosomes are separated as microtubules pull whole chromosomes to opposite ends of the spindle The centromeres do not divide

-The chromosomes arrive at opposite poles -Spindle fibres start to break down -Nuclear envelopes form around the two groups of chromosomes, and nucleoli reform -The cell then divides by cytokinesis, forming two haploid daughter cells — each with half the original chromosome number (but still with duplicated chromosomes) -These cells are haploid as they contain half the number of centromeres

-The nuclear envelope breaks down and chromosomes condense -A spindle forms at a right angle to the old one

Chromosomes (still made of two sister chromatids) line up in a single file along the equator of the spindle