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Transcript
Genetics: What is inherited? Chromosomes and how they are passed down Heredity Heredity is the action of inheritance: gaining material or characteristics from our parents We gain material from our parents’ DNA The DNA we gain is packaged into cells in the gonads, called sex cells Half of your DNA is from your mother, and the other half from your father Chromosomes “gene carriers” Structures where DNA is organised into easily accessibly sections, called GENES Organising DNA The DNA in just ONE of your cells would be many kilometres long if you were to stretch it out. It must be organised to resist tangling and breakage. Representing chromosomes Stylised, showing gene banding Stylised, showing shape and overall structure Highly stylised representation within cells Photograph taken from microscope, showing gene banding (from stain) How many chromosomes? Normal human cell = 23 pairs (46 in total). 22 AUTOSOME pairs, and 1 sex chromosome pair When chromosomes are paired, the cell is DIPLOID (also written as 2n) In sex cells (eg. Sperm, egg), there is only one copy of each chromosome, so there are 23 in total (in humans). This is a HAPLOID cell. http://www.personal.psu.edu/staff/d/r/drs18/bisciImages/haploidDiploid.png Karyotypes A picture made, which puts chromosomes present in a cell in order, from largest pair (1) to smallest pair (22 in humans), and finally the sex chromosomes. Used to quickly diagnose chromosomal disorders Usually include gene banding Each matching pair is called a HOMOLOGOUS pair http://sciencegirlsrock.files.wordpress.com/2011/05/karyotype.jpg Errors in the karyotype If there are any chromosomes missing or extra, the problem probably occurred during the creation of the sex cells, in a process called MEIOSIS Meiosis The formation of new sex cells Only have ONE of each chromosome (haploid) Two sex cells meet in fertilisation and give rise to a GAMETE (a new diploid cell) Can include a phenomenon called CROSSING OVER, where the arms of two chromosome swap information Who remembers MITOSIS? The reproduction of somatic cells (ie. Not sex cells) Occurs in growth regions Requires duplication of chromosomes Gives rise to TWO IDENTICAL diploid cells The phases of Mitosis can be remembered using the following: Interphase Prophase Metaphase Anaphase Telophase IPMAT MEIOSIS Gives rise to FOUR daughter cells, all haploid Daughter cells are NOT genetically identical, as there has been random assortment of chromosomes that have been placed in each Sometimes tangles occur, and genetic information CROSSES OVER from one chromosome to another, producing yet another possible point of difference MEIOSIS The great gene shuffling machine © 2007 Paul Billiet ODWS Copyright Dr. Michael Knee Source: http://hcs.osu.edu/hcs300/ Meiosis performs two functions It halves the number of chromosomes to make haploid sets It shuffles the genes to produce new combinations (recombinations) © 2007 Paul Billiet ODWS Meiosis and sexual reproduction Meiosis is needed to produce sex cells (gametes) with unpaired sets of chromosomes (haploid) Sex cells are used in fertilisation At fertilisation two sets of genes come together to form a hybrid with a set of paired chromosomes (diploid) The hybrid, whilst similar to the parents, is unique © 2007 Paul Billiet ODWS Haploid and Diploid Karyotypes of somatic cells show paired sets of chromosomes The origin of the pairs are the maternal and paternal chromosomes of the egg and the sperm The number of types of chromosomes of a species is constant = n So the diploid (paired set) = 2n © 2007 Paul Billiet ODWS The sexual reproduction life cycle Diploid (2n) Fertilisation Meiosis Haploid (n) © 2007 Paul Billiet ODWS Meiosis a two step process Meiosis 1 is the reduction division Meiosis 2 resembles mitosis © 2007 Paul Billiet ODWS Meiosis 1: Early Prophase 1 Chromosomes condense Homologous pairs linked by chiasmata (chiasma sing.) © 2007 Paul Billiet ODWS Meiosis 1: Late Prophase 1 Spindle fibres form and spread out between the centrioles © 2007 Paul Billiet ODWS Meiosis 1: Metaphase 1 The pairs of chromosomes line up on the equator The orientation of the maternal and the paternal chromosomes is random © 2007 Paul Billiet ODWS Meiosis 1: Anaphase 1 Maternal and paternal chromosomes segregate (pulled separate on the spindle) They move to opposite poles © 2007 Paul Billiet ODWS Meiosis 1: Metaphase 1 revisited The pairs of chromosomes could orientate in different ways © 2007 Paul Billiet ODWS Meiosis 1: Anaphase 1 revisited Resulting in different combinations of chromosomes This means there are 2n combinations In humans this means 223 or over 8 million combinations © 2007 Paul Billiet ODWS Meiosis 2: Prophase 2 Now the cells are haploid The chromosomes do not decondense at the end of meiosis 1 Each chromosome has still two chromatids © 2007 Paul Billiet ODWS Meiosis 2: Prophase 2 Spindles form again © 2007 Paul Billiet ODWS Meiosis 2: Metaphase 2 The chromosomes line up on the spindle equator independently © 2007 Paul Billiet ODWS Meiosis 2: Anaphase 2 The sister chromatids separate on the spindle Each cell will receive a copy of each chromosome type (i.e. it will receive n chromatids all different) The genes on the different chromosomes are recombined © 2007 Paul Billiet ODWS (shuffled) Meiosis 2: Telophase 2 Four haploid sex cells are produced © 2007 Paul Billiet ODWS Crossing over During prophase 1 not only do the homologous pairs link They exchange genetic material The genes on each chromosome are not identical they may be alleles Alleles are different versions of a gene e.g. Ear shape gene has two alleles the ear lobe allele and the no ear lobe allele © 2007 Paul Billiet ODWS Meiosis: Anaphase 1 So when the pairs are separated, the alleles of the genes on the same chromosome are recombined (reshuffled) Genes on the same chromosome are called linked genes © 2007 Paul Billiet ODWS Meiosis: Prophase 2 Each cell is haploid (n) The sister chromatids are no longer identical copies © 2007 Paul Billiet ODWS Meiosis: Anaphase 2 At anaphase 2 the chromatids segregate (separate) randomly Even greater variation is achieved in the sex cells © 2007 Paul Billiet ODWS Meiosis: Telophase 2 Thus an infinite variety of sex cells is possible Combined with random mating between males and females an infinite variety of individuals is conceived at fertilisation © 2007 Paul Billiet ODWS