DNA, GENES & CHROMOSOMES UNIT 4 BIOLOGY DNA, GENES & CHROMOSOMES

D.N.A (deoxyribose nucleic acid) Watson and Crick model  2 long strands of sub-units called NUCLEOTIDES wound around each other to form a DOUBLE HELIX Each nucleotide is made up of SUGAR – PHOSPHATE – BASE subunits with the sugar being deoxyribose The spine of the DNA is made up of 4 nitrogenous bases: Adenine (A) Thymine (T) Guanine (G) Cytosine (C) The Base-Pairing Rule states: A bonds with T C bonds with G The code is organised in to triplets called CODONS Each codon codes for an amino acid which will in turn form a protein Over a metre of DNA in each cell, twisted and coiled into 46 CHROMOSOMES Not all the length of DNA codes for proteins Parts of DNA that do code for proteins are called GENES and the set of total genes for an organism is called its GENOME

CHROMOSOMES In eukaryotic organisms the DNA molecules and associated molecules are contained in the nucleus as tangled fibres of CHROMATIN They are coiled around proteins closely associated with DNA called HISTONES When cells are about to divide the chromosomes thicken and double in genetic material becoming joined at the CENTROMERE (see right) They form two sister CHROMATIDS Usually only visible at this stage Eukaryotic chromosomes exist in pairs known as HOMOLOGOUS PAIRS Along the length of each DNA molecule, particular regions (GENES) code for different proteins that determine particular characteristics or TRAITS The location of a particular gene on a chromosome is referred to as its LOCUS and on homologous chromosomes the corresponding gene is found at the same locus These alternative forms of the gene are called ALLELES

Codons – Is this DNA? Packaging of DNA: 2 meters of DNA is packaged into a cell no bigger than a full stop DNA is wrapped around protein units, like string of beads This structure is called CHROMATIN During cell replication, the chromatin folds many times so that is is very condensed Codons – Is this DNA?

CHROMOSOMES cont...... KARYOTYPES Karyotypes are photographic images of chromosomes arranged and ordered into matched pairs from largest to smallest. Each organism is characterised by the number of chromosomes in each cell (eg. Humans= 46) Genetic disorders such as Trisomy 21 and whole chromosome disorders can be determined via a karyotype In the nucleus of a somatic cell there will be 23 homologous pairs. 22 AUTOSOMES and 1 pair of SEX CHROMOSOMES (XX=female/XY=male) The DIPLOID number of chromosomes are all chromosomes with both pair members present (Human diploid number = 46) The HAPLOID number of chromosomes are chromosomes with only one pair member present (Human haploid number = 23) The only haploid cells in humans is the gametes (sperm & ova)

MITOSIS, MEIOSIS & THE CELL CYCLE

MITOSIS STAGES OF MITOSIS P.M.A.T.I OR I.P.M.A.T 4 Phases of Mitosis 1. PROPHASE 2. METAPHASE 3. ANAPHASE 4. TELOPHASE Mitosis is the process used by cells to make new, genetically identical, cells. This is predominantly used in growth and repair INTERPHASE is a phase in a non-dividing cell where all the chromosomes exist in a single chromatid form. Cells spend the majority of their lives in this form P.M.A.T.I OR I.P.M.A.T

UNDERSTAND THE PROCESS DON’T MEMORISE THE STEPS MITOSIS STAGES OF MITOSIS PROPHASE  chromosomes become shorter and fatter by coiling. Repeated coiling is called supercoiling  microtubules grow from the poles of the cell and the centrioles  these form a spindle called the MITOTIC SPINDLE METAPHASE  spindle microtubules attach to the chromosome centromeres  chromosomes are moved to the equator. Each chromatid moves to the opposite pole ANAPHASE  the pairs of sister chromatids separate, centromeres split and the microtubules pull them to opposite poles TELOPHASE  chromatids at each pole gather together and become enclosed in a nuclear membrane  the whole cell splits at the equator and forms 2 new cells  each cell has the SAME number of chromosomes as the parent cell and contains the same genetic information  cell enters INTERPHASE again UNDERSTAND THE PROCESS DON’T MEMORISE THE STEPS

MITOSIS VIDEO: CRASHCOURSE MITOSIS VIDEO: THE EUKARYOTIC CELL CYCLE

MEIOSIS Meiosis is a type of cell division that produces gametes (sex cells) Cells produced are haploid (contain HALF the original chromosome number) Process accounts for great genetic variation due to  RANDOM ASSORTMENT  CROSSING OVER 2 cell divisions: 1st: PROPHASE I METAPHASE I ANAPHASE I TELOPHASE I 2nd: PROPHASE II METAPHASE II ANAPHASE II TELOPHASE II

MEIOSIS

MEIOSIS STAGES OF MEIOSIS PROPHASE I METAPHASE I ANAPHASE I  cell has 2n (diploid) number of chromosomes  chromosomes become visible  homologous pairs line-up  crossing over occurs and exchange of genetic material (see right) METAPHASE I  spindle microtubules move homologous pairs to the equator of the cell  orientation of paternal and maternal chromosomes at either side of the equator is random (RANDOM ASSORTMENT) ANAPHASE I  Homologous pairs are separated. One chromosome of each pair moves to each pole TELOPHASE I  Nuclear membranes form around chromosomes  cell membranes form around each nucleus  reduction of chromosome number from diploid to haploid is completed (2 haploid cells now formed)  cytokinesis occurs

MEIOSIS STAGES OF MEIOSIS part 2 PROPHASE II  nuclear envelope begins to disintegrate  spindle microtubules form  chromosomes (with 2 chromatids) condense and become visible METAPHASE II  chromosomes line up along the equator (in a straight line) ANAPHASE II  centromeres separate and chromatids are moved to opposite poles TELOPHASE II  chromatids reach opposite poles  nuclear envelope forms  cell membrane re-forms  cytokinesis occurs At this stage there are 4 GENETICALLY DIFFERENT daughter cells They contain HALF the original chromosome number (haploid) Phase II is very similar to mitosis