Biology- Unit 2- DNA and meiosis

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  • Created by: FireDwarf
  • Created on: 11-01-14 15:49
What is DNA?
Chemical that determines inherited characteristics and contains vast amounts of infomation in the form of genetic code.
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Nucleotide structure- what are the 3 componants of a mononucleotide?
deoxy-ribose, organic base, phosphate.
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What are the 4 organic bases?
C & T (single ring base) and A & G (double-ring base)
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How are the d-oxyribose, phosphate & organic base joined?
By condensation reactions.
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How do two mononucleotide's join?
They join together via condensation, between the phosphate group of one of the nucleotides and the de-oxyribose of the other.
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What do they join together to form?
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What is DNA made up from? How are they joined?
Two strands of polynucleotide which are joined together by hydrogen bonding between complementory organic bases.
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What is complementory organic bases?
bases which shapes are complementory to each other, allowing them to bond, and join the two strands together.
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what are they?
A- T (2 hydrogen bonds) G-C (3 H bonds).
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What is the double helix?
the two strands are twisted.
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How many bases per turn of this helix?
10 bases
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What is the function of DNA?
Material responsible for passing on genetic infomation from cell to cell.
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How is DNA adapted to its job?
Very stable and very protected & long.
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How does it become stable & safe?
The strands are joined by H bonds, numerous and allow to seperate (in DNA replication or protien synesis), bases are inside the helix which offers protection.
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Why isit important DNA is protected?
Bases form the genetic code- important- if genetic code changes then primary structure of protiens change- this changes protiens tertiary shape- cant do function.
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What is a gene?
Sections of DNA which contain coded infomation for making polypeptides.In the form of specific sequence of bases.
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So how does the triplet code fit into this?
each triplet of bases code for a specific amino acid.
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How did they work out it was a triplet code?
There is 20 amino acids. Each amino acid must have its own code of bases. 4 bases are present. Each base for 1, then only 4 amnio acids. A pair would form 16, too small. Therefore must be 3 (more then 20).
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Because the triplet forms more then 3 combinations, what does this prove?
64 possible codes, 20 amino acids. Diffrent codes for the same amino acid.
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What does DNA look like in a prokayotic cell?
Smaller, form a circle (Plasmid) and not assosiated with protien molecules (No chromosomes).
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What does DNA look like in a eukaryotic cell?
DNA molecules are larger, form a line (linear) rather then a circle and occur in assosiation with protiens (chromosomes).
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What is the structure of a chromosome?
A DNA mlecule is combined with protiens (held in posistion).This DNS-protien complex then gets coiled and forms loops. These loops then get further looped into chromosomes.
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What is the location of a gene on a chromosone called?
the locus eg: pancreatic amylase code is located on locus p21 short arm of chromosome 1.
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What does most of the DNA on eukaryotic cells do?
Not a clue- does not code for polyepetides and we have no idea what it does.
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What is the noncoding regions called? Coding regions?
introns and extrons
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How do chromosomes appear when cells are dividing? What is each thread called?
As two threads joined at a single point. Each thread is called a chromatid.
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What is the advantage of doing this?
Lots of DNA is condensed into a single chromosome.
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How many chromosomes in each human cell?
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Why does each cell need chromosomes?
Because they divide.
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Why are the number of chromosomes always even?
Because they form homologous pairs.
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What is a homologous pair?
A pair of chromosomes which determine the same genetic characteristic.
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What is an allele?
Allele's are diffrent forms of the same gene.
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What are mitosis and meiosis forms of?
Cell division.
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What is mitosis?
When two daughter nuclei with the same number of chromosomes as the parent cell and each other are produced by cell division.
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What is meiosis?
Producing 4 daughter nuclei, each with half the number of chromosomes as the parent cell.
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Why is meiosis neccessary?
Two gametes fse together and as a result, the cell formed would have 92 chromosomes. Doubling of the same number would continue each generation.We therefore need to half the chromosones at some point in the proccess, hence meiosis.
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What is the dipolid number?
Number of chromosones in a normal cell. (46)
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What is the hapolid number?
Number of chromosomes after each homologous pairs seperate (23) and enter each gamete. When fuse at fertalisation, the dipolid number restored.
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Describe the first division
Homologous chromosomes pair up- chromatids warp round each other. We then undergo "crossing over". By end of the stage, two daughter cells are produced (each with one chromosome from each pair). Independant segregation occured & crossing over.
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Second stage.
Chromatids move apart- four cells therefore produced. End result, 23 chromatids.
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So what does meiosis do? (3 things)
Half the number of chromosomes and introduce genetic variation amoung offspring by independant segragation of homologous chromosomes & recombination of H C by crossing over.
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What is independant segregation of homologous chromosomes?
Meosis 1, each pair lines up side by side (Humans- 23 pairs). When they line up, it is done so randomly. One of each pair will pass. The posistion of each chromosome of each pair is random, Which one of the pair goes into which daughter cell is based
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off its posistion, which is random. Therefore, which chromosome moves into the daughter cell is random and therefore gives variation.
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Explain the variety further.
Each member of a homologous pair codes for the same charactieristics because it has the same genes. However, they are not identical and therefore are diffrent as they have diffrent alleles. This means each daughter cells has varieties of the same G.
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Card 2


Nucleotide structure- what are the 3 componants of a mononucleotide?


deoxy-ribose, organic base, phosphate.

Card 3


What are the 4 organic bases?


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Card 4


How are the d-oxyribose, phosphate & organic base joined?


Preview of the front of card 4

Card 5


How do two mononucleotide's join?


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