Mitosis and Meiosis

Chromosomes One linear strand of DNA is coiled tightly around proteins. This is how 3m of DNA material can fit into a nucleus

In body cells (somatic cells, not gametes) each cell has 2 copies of every chromosome=diploid number Humans have 46 in total -22 pairs are the same in males and females and the 23rd pair determines the sex (XX for females and XY for males)

The cell cycle Series of events in which cells grow and divide to produce two identical daughter cells -Interphase, Mitosis, Cytokinesis Includes growth and division

First up there is interphase!!!

Sister chromatids Before S phase chromosomes are composed of one DNA molecule During S phase chromosomes replicate – two DNA molecules These molecules are called chromatids Sister chromatids are held together by a centromere Sister chromatids end up in two genetically identical daughter cells

Mitosis-division of the nucleus 4 phases Produces 2 identical daughter cells from the parent cell Each daughter cell has the exact same genetic information as parent Same number and type of chromosomes as parent

Prophase Chromatin coils tightly in chromosomes- becomes visible Nuclear membrane disintegrates Centrosomes (organelle) move to opposite poles of the cell-produce microtubules that link poles of the cell

Metaphase Microtubules attach to sister chromatids at centromere Sister chromatids lie in the equator of the cell pulled in line by the microtubules

Anaphase Two sister chromatids split at the centromere Chromosomes move to opposite poles (ends) of the cell pulled by microtubules that contract

Telophase Chromosomes elongate to form chromatin and re not visible Chromosomes are at each pole Nuclear membrane begins to reform around each set of chromosomes

Cytokinesis-division of cytoplasm and cell Animal Cells Inward pinching of cell membrane to form cleavage furrows Plant Cells Plant cells have a firm cell wall and form a cell plate

Meiosis Sexual reproduction is the fusion of male and female gametes (sex cells) to form a zygote (fertilised egg) A fertilised egg contains a combination of genetic material from both parents-50% of inherited characteristics from the maternal (mother’s side) and 50% from the paternal (father’s side) Our cells contain genetic information from both parents through a process called meiosis

For humans to pass on genes there needs to be a method to divide a parents 46 chromosomes (diploid) to 23 (haploid)=meiosis Meiosis-produces gametes with half the number of chromosomes-they only have one copy of each chromosome in each cell (haploid) Gamete- sex cells (sperm and ovum-egg) This is so when gametes (sperm and egg cells) come together at fertilisation the total number of chromosomes, 46, will be restored

Meiosis occurs in reproductive organs called gonads (ovaries and testes) Meiosis=2 stages -Meiosis 1 is one cell divides into 2 – Crossing over occurs which swaps regions of DNA between matching chromosomes (homologous chromosomes) to make 4 unique chromatids- produces variation in offspring – Homologous chromosomes (matching) are separated -Meiosis 2 is 2 cells into 4 (haploid) - sister chromatids are separated

Results in a change of the DNA code (base sequence) or chromosome number Can cause a change in hair colour, skin colour = harmless Others can impede or stop cell function, others can affect cell division (cause tumours and cancer) = harmful If a change during meiosis, can be passed on to offspring- variation in species Mutagens (environmental factors) can cause mutation during DNA replication-UV light, cigarette smoke, X rays

Inherited characteristics Gregor Mendel in the 19 th Century paved the way for the understanding of how certain conditions and genes are passed on to the next generation We have homologous pairs of chromosomes (one from our mother and one from our father) For every gene on one chromosome there is a corresponding gene on the other.ie. For hair colour or eye colour These different forms of the same gene are called alleles

X and Y chromosomes carry different genes Genes carried on either the X or Y chromosome are ‘ sex-linked’ If a recessive gene is carried on either the X or Y chromosome of males it will be expressed in the phenotype (the appearance of the organism) Why?

Because the gene is the only allele present Females have two X’s (XX) so they have two alleles of the same gene so the gene can be recessive or the female can be a ‘ carrier’ of the X-linked recessive disease

These alleles can be dominant or recessive Dominant -only needs to occur on one allele for the trait to be expressed eg. Brown hair (B) Recessive -needs to occur on both alleles to be expressed. eg. Blonde hair (b)

Homozygous-an individual has two identical alleles. Eg. BB (B for brown hair) or bb (for blonde hair) Heterozygous-an individual has one of each type of allele. Eg. Bb Will the above person have brown or blonde hair?