AS Biology Unit 2

DNA Replication AS AQA

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  • Created by: Kaimlee
  • Created on: 12-03-11 16:28

DNA Replication

Semi Concervative - origanal double helix seperates, 1 of the origanal strands of DNA remains intact the other strand is made of new complementary nucleotides.

1.) DNA unwinds

2.) Hydrogen bonds are broken by  DNA helicase

3.) Strands seperate & free nucleotides pair with complementary base

4.) Joined by DNA polymerase forms hydrogen bonds

5.) 2 new strands of DNA are formed

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Exam Questions

Q1.) What is meant by the term semi conservative?

Q2.) Explain what would happen to DNA replication if there was the presences of an inhibitor?

Q3.) Turtles have the same percentage of 4 different bases as rats. Explain why they are still very different animals?

Q4.) The structure of DNA in this virus is not the same as DNA in other organisms why?

Q5.) Explain how DNA is replicated?

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The Cell Cycle

Cells do not divide contineously - Interphase occupies most of the cell cycle - in this stage the nucleus is NOT dividing. It is NOT a stage of mitosis or meiosis. In Interphase there are 3 phases:

G1 = proteins are synthesised to produce more organelles

S = DNA is replicated for the cell that will divide

G2 = energy stores increase (more ATP) some organelles divide + grow

In interphase the chromosomes aren't visible

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The Cell Cycle

Nuclear division = this occurs when the nucleus divides into 2 (mitosis) or 4 (meiosis)

Cell division = following division of nucleus the whole cell divides into 2 diploid cells (mitosis) or 4 haploid cells (meiosis)

Q1.) If there were 20 units of DNA in a cell during stage G2, give the number of units of DNA you would expect to find in this cell a.) at prophase of mitosis b.) in 1 of the daughter cells produced at the end of mitosis c.) during stage G1?

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Type of cell division occurs only in eukaryotic cells, contain a complete copy of the parent cell's DNA making them genetically identical.

Mitosis = nuclear division to produce 2 nuclei each with the same number of chomosomes as the parent cell. Used for growth, repair, replacement. It is divided into 4 distinct phases: prophase, metaphase, anaphase, telophase. (PMAT)

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Prophase = Chromosomes coil - becoming shorter and fatter - meaning they can be seen under a light microscope if stained

Metaphase = Spindle fibres are formed, chromosomes line at the equator of the spindle which attaches to the centromere of each chromosome

Anaphase = Spindle fibres contract & pull sister cromatids to opposite poles V shape (from centromere)

Telophase = a new nuclear envelope forms around the sets of chromosomes (cant see chromosomes now with a light microscope) not quite divides into 2 new cells.

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Importance of Mitosis

Needed for growth, repair, replacement to produce daughter cells that are genetically identical to the parent cell.

Seeing Chromosomes:

Place in acid = the acid seperates the cells

Squash the root on the slide = to produce 1 layer of cells so that the pattern of the chromosomes is visible
Stain = so can see chromosomes when under a light microscope

Why use root tip = where cells undergoing mitosis are

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Exam Questions

Q1.) In an experiment testing the stages of mitosis explain why the root tip was a.)stained? b.)squashed between to slides? c.)placed in hydrochloric acid? d.)why the root tip was used?

Q2.) What is meant by mitosis?

Q3.) Explain the stages of mitosis & its is importance?

Q5.) Suggest why Metaphase is the longest stage?

Q6.) Vincristine is a drug used to treat cancer, it stops the a spindle forming explain how this stops cancer cells dividing?

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Cancer Cells

uncontrolable growth and replication of abnormal cells forming a tumor.Treatment often involves stopping part of the cell cycle, preventing DNA replication, inhibiting metaphase stage of mitosis where the spindle is formed.

The drugs are most effective against rapidily dividing cells - such as cancer cells, although also against normal cells that divide rapidily such as hair cells and red blood cells.

Q1.) Give a reason why during week 12 of treatment there is a lower number of cancer cells?

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Exam Question

Q2.) Describe 2 differences between the effects of the drug on the cancer cells compared to healthy cells throughout treatment?

Q3.) People who are given vincristine to treat cancer have a reduced number of red blood cells, suggest why?

Q4.) Explain why doctors may test several amounts of a dose of a cancer treating drug before giving it to patients?

Q5.) Explain why one side effect to treating cancer results in a loss of hair?

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Meiosis = nuclear division in which the chromosome number is halfed to produce 4 haploid neulei (every gamete has a homogulous pair)

Why is it important:

1.) Halfs the number of chromosomes in gamete cells so that during sexual reproduction, chromosome number doesn't double every generation.

2.) Helps produce variation in offspring (natural section) Through crossing over and independent assortment.

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Meiosis Stages

Prophase 1

  • Chromosomes condense (coil become shorter & fatter) - become visiable when strained
  • Homologous chromosomes pair up (find sister chromosomes)
  • Crossing over - between sections of homologous chromosomes resulting in allele combinations where DNA is exchanged
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Meiosis 1

Metaphase 1

  • Chromtid pairs line on the equator of the spindle independently of one other pair = independent assortment.
  • Independent assortment = results in random assortments of maternal and parental chromosomes in gametes - produce variation

Anaphase 2

  • Spindle fibres contract, pulling cromitids apart
  • Sister cromitids are seperated, (centremere not sep)
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Telophase 1

  • Nucleus splits into 2

Meiosis 2

Prophase 2 - chromatids condense (coil & shorten, fatter) are visible when stained under light microscope.

Metaphase 2 - Chromatids line on equator of spindle

Anaphase 2 - Spindle fibres contract, centremeres seperated in V shape, cromatids pulled apart

Telophase 2 - forming 4 hypoloid cells.

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Exam Question

Q1.) Give two ways in which meiosis causes genetic variation?

Q2.) Explain what crossing over is? and when does it occur?

Q3.) Why is it important the number of chromosomes half in meiosis?

Q4.) In gamete production what is an advantage of changing dipole cells into haploid cells?

Q5.) In what stage and 1st or 2nd do homologous chromosomes line on the equator of the spindle?

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Meiosis and Mitosis differences

Meiosis: + only gametes

  • number of chromosomes is halved
  • 2 divisions & results in 4 daughter nuclei
  • during prophase 1 the chromosomes are paired into homologous pairs
  • crossing over - genetic variation

Mitosis: + all over body

  • number of chromosomes remain the same
  • 1 division and only 2 daughter nuclei are formed
  • don't form homologous pairs, No crossing over
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