headstart biology AS

HideShow resource information
  • Created by: charlie
  • Created on: 01-09-13 17:34

the genetic code

  • 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 
1 of 5

the genetic code part 2

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)
2 of 5

DNA and chromosomes

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 
3 of 5

cell division- mitosis

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 
4 of 5

cell division- meiosis

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  
5 of 5


No comments have yet been made

Similar Biology resources:

See all Biology resources »See all cell structure resources »