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T4 Chromosomes, mutation

and Meiosis

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Chromosomes, Genes, Alleles and Mutations

Chromosome: In eukaryotes a sequence of DNA associatedwith protein.

In prokaryotes the protein is absent, so no chromosomesform.

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Chromosomes, Genes, Alleles and Mutations

Gene: A sequence of DNA on a chromosome that codes for aparticular characteristic and can be inherited.

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Chromosomes, Genes, Alleles and Mutations

Allele: Different forms of agene controlling the samecharacteristic.

Alleles are found at thesame location on a

chromosome and havealmost the same basesequence aseachother. E.g. thealleles for blue eyesand green eyepigment.

The position of a geneon a homologouschromosome is its locus

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Chromosomes, Genes, Alleles and Mutations

Genome: All the DNA in an organism or cell

Karyotyping: A way of organising the genome so that pairs ofchromosomes are lined up in order of size.

22 of the 23 pairs are homologous = the chromosomes within a

pair are the same length. 1 pair is non-homologous = the chromosomes within the pair

are not the same length. This pair carries gender specificinformation: sex chromosomes.

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Chromosomes, Genes, Alleles and Mutations

Chromosome mutation: a change in the number of genes,e.g. Downs syndrome results from an extra no. 21chromosome. This gives body cells with a total number of 47chromosomes.

Occurs due to non-disjunction and is also known as trisomy 21

Diagnosis: amniocentesis or chorionic villus sampling (placentatissue)

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Chromosomes, Genes, Alleles and

Mutations

Gene Mutation: A change in the base sequence of a gene.

There are many types of mutation. One involves substitutingone base for another.

Base substitution in sickle Cell Anemia: Red blood cells need the amino acid glutamic acid. This has the

triplet base code GAG.

In sufferers of sickle cell anemia, the base sequence of the geneinstead codes as GTG, which is the triplet base code for valine.

The base substitution of T instead of A causes an incorrect amino

acid to be translated into the protein. This leads to malformation of red blood cells and as a result

sufferers develop the symptoms of sickle cell anemia.

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Sickle Cell: Base Substitution can be anadvantage.

Sickle cell and Malaria:

The mutation of red blood cells in sickle cell has a protective effectagainst malaria.

This has helped the mutation survive in the population because insome areas it becomes an advantage to have sickle cell.

This explains why the mutation is prevalent in people originally fromthose countries where malaria is widespread.

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Meiosis

Meiosis starts with 46 chromosomes in one parentcell and ends with 4 daughter cells eachcontainong 23 chromosomes.

This is called a reduction division.

The number of chromosomes in the parent cell isthe diploid number (2n).

The number of chromosomes in the daughtercells is the haploid number (n).

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Meiosis: 2 divisions

In all cells both mitosis and meiosis start at the end ofinterphase.

In both processes during interphase chromosomes

replicate.

Replicated chromosomes associate with each-otherand join at the centromere, condensing to becomesister chromatids.

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Lining up - a major difference between mitosis and

meiosis

IMPORTANT:

In mitosis: homologous parental chromosomes stay

apart in metaphase.

In the first part of meiosis: homologous parentalchromosomes associate together in metaphase.

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Meiosis I: The first division

Prophase I: sister chromatids form and the spindle starts to extend

across the cell.

Chromatids of homologous chromosomes associate together

and form a bivalent.

Metaphase I: Bivalents line up on the equator of the cell - the

metaphase plate.

Anaphase I: Bivalents separate and are pulled to opposite endsof the cell by spindle fibres.

Remember: sister chromatids are still joined at the centromere

Telophase I and cytokinesis: Nuclear envelope reforms, cytoplasm

divides and 2 cells are formed.

The cell enters a very short interphase before the second division.

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Meiosis II: the second division

Prophase II: Nuclear membrane breaks down and spindle

forms.

Metaphase II: Sister chromatids line up on the metaphaseplate (equator).

Anaphase II: Sister chromatids separate at the centromereand move to opposite poles of the cell and are calledchromosomes again.

Telophase II and cytokinesis: nuclear membrane reforms andcytoplasm divides to form in total 4 daughter cells. Eachdaughter cell has an original or copy of one of a homologouspair of chromosomes.

Therefore in each cell there will be 23 single chromosomes =haploid.