headstart biology AS

• diff protein responsible for diff characteristics 

how does a cell know which type of protein to produce?

• genes on chromosomes responsible for types of protein produced 
• genes= sets of coded instructions for building protein (1 gene codes 1 protein)

how does the code work?

• unravel chromo. ---> very long molecule called DNA (deoxyribonucleic acid) ---> made from 2 chemicals (bases ---> A, T, C, G 
• gene= short section of DNA 
• sequence of bases determines order in which amino acids put in protein 

mutations are changes in the code 

• base order changed --->sequence of amino acids change ---> diff. shape + possibly function
• e.g. cystic fibrosis: gene with 3 bases missing= missed amino acid in protein= bad function
• natural + most harmful but some are not = beneficial + inc. chances of survival 

inc. rate of mutation 

• mutagens = ionising radiation Xrays+ UV, chemicals can damage/destroy DNA molecules 

cancer is caused by mutations 

• celll division is controleed by proteins produced from genes 
• mutations occur in genes ---> proteins produced wont work properly---> cells divide uncontrollably ---> mass of cells (tumour) ---> invade other areas (cancer)

DNA found on chromosomes 

• DNA found in nucleus of eukaryotic cells + cytoplasm of prokaryotic cells 
• wound up in chromo. to fit (containing 100's-100s of genes)

homologous pairs 

• humans- 23 homologous pairs of chromo. 2 no. 1s, 2 no. 2s, 2 no. 3s...
• one from each pair from father + mother - same size + carry same genes (homologous) 
• usually have diff. alleles (diff. versions of same gene)

chromo. shown as X shape 

• one chromo. attached to identical copy of itself 
• only X shape after DNA replication (cell division)  
• chromatid = each side of chromosome + identical copies 
• middle bit where attached= centromere 

mitosis needed for growth + repair 

• damaged tissue- cells around it divide by mitosis to replace 
• also produces new tissue for growth 

asexual reproduction invoves mitosis 

• singles organism provides offspring by splitting in two organsims 
• offspring + parent genetically identical 
• most cells divide by mitosis 

DNA copies itself then cell divides once 

• every DNA molecule (chromo.) replicates so new cell has full copy of DNA ---> new molecule remains attached to original at centomere ---> each DNA molecule supercoiled + compact --> chromo. appears as 2 chromatids lying side by side joined at centomere (X) ---> nuclear membrane breaks down ---> chromosomes line along equator ---> centromere splits + chromatids separate ---> dragged to opposite cell ends ---> nucleus membrane forms around each set of chromatids ---> cytoplasms divide ---> 2 identical cells 

sexual reproduction involves meiosis

• offspring genetically diff. form parents + eachother = variation in pop. 
• parents produce gametes with 1 set of genetic material 
• gametes are haploid in meiosis 
• fertilisation gametes join to form zygote with 2 sets of genetic material= diploid 
• zygote grows by mitosis + embryo formed 

DNA copies itself then cells divide twice 

• only cells are testes + ovaries with homologous pair of chromo (same)
• cells divide to produce gametes (sperm + egg) ---> DNA replicates itself so 46 chromo. become 2 chromatids joined by centromere ---> sort into 23 pairs of homo. pairs ---> pairs separate ---> one of each pair goes to diff. sides of cell ---> in both new cells chromatids separate ---> cytoplasm divides to form 2 cells ---> 4 haploid cells produced from every orginal diploid