Biology Unit 2 DNA, Cell division and Variation

Model answers you should know for the unit 2 exam relating to DNA and cell division 

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  • Created by: Katie
  • Created on: 07-04-12 10:56

What is the difference between a nucleotide and a

A nucleotide is a monomer(1) a nucleic acid is a polymer.

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How does DNA replicate (4)

Double Helix unzips

Catalysed by helicase

Hydrogen bond break

Free DNA nucleotides in nucleus pair with complementry base pairs on both exposed strands

DNA polymerase joins nucleotides together on new strands (catalyses formation of ester bonds)


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What does semi-conservative replication mean? (3)

Two new molecules are formed which are exact copies of the original(1); one of the chains in each moleculewas present in the original molecule (1); the other chain is newly synthesised fromfree nucleotides (1)

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What is an allele? (1)

One alternative form of a gene

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Key features of DNA

Stable so can be passed from feneration to generation

Hydrogen bonds between strands can be broken easily for DNA replication

Large so can carry lots of genetic information

Base pairs are contained ithin the sugar-phosphate backbone to prevent corruption

complimentry base pairing to allow synthesis of identical daughter cells

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Important features of genetic code

It is non overlapping (specific start/stop codons)

Degenerate (ammino acids are coded for by more than one codon)

It is read in one direction

Universal- same code applies to all organisms

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Definition of a gene

A section of double stranded DNA coding for a single amino acid

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Definition of a locus

The position of a particular allele on a chromosome

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Definition of the triplet code

Three bases in a particular sequence that code for one amino acid

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Relationship between DNA and protiens

DNA has sequence of basses arrangedinto specific codons

each codon codes for one amino acid

The ribosome reads the code and reads the code and arranges the amino acids into the correct order (determined by the DNA sequence)

Peptide bonds form between amino acids to make a polypeptide chain which then fold to form the protiens specific structure

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DNA and Non functional enzymes

Change in sequence of amino acids

Changes the shape of the active site

Substrate and enzyme no longer a complimentry shape

E-S complexes no longer form

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DNA and phenotype

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Stages of cell division (Pass Me A Tenner)

Interphase-Cell grows more cytoplasm-DNA replication-organelle replication-build up of ATP (Growth 1, synthesis and Growth 2)

Prophase- chromosomes condense nuclear envelope dissapears and spindle fibres form

Metaphase - Spindle fibres attach to chromosomes- which line up on equator of the cell

anaphase- Spindle fibres contract and pull sister chromatids apart

Telophase- New nuclear envelope forms, chromoses uncoil

Cytokinesis - Cytoplasm divides and new cell membrane forms

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Purpose of mitosis

Produces IDENTICAL body cells for growth and repair

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Preparing mitotic squash

Cut off tip of growing roots add aceto-orcein stain and squash

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Why do you follow the steps in card 15

Root tip has fastest growing cells which divide by mitosis

Stain to make chromosomes visible

Squash to get a thin tissue layer to let enough light through for use under optical microscope

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Mitosis vs Meiosis

Meiosis produces gametes- mitsosis produces other body cells

meiosis produces cells with haploid number of chromosome- mitosis produces diploid  of chromosomes

meiosis produces cells that are genetically different- mitosis produces genetically identical

meiosis involves the seperation of homologous pairs- mitosis does not 

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Chromosome movement during meiosis

In first division homologous chromosomes line up and dseperate with one chromosome from each pair going into each daughter cell (this happens randomly and is called independent segregation)

Chromatids move apart during the anaphase of the second meiotic division

After the second division four haploid cells are formed with half the original amount of DNA as the original parent cell

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Meiosis and genetic variation

Recombination of sister chromatids after crossing over during prophase of meiosis 1

Produces new combinations of alleles

Independent segregation of homologous chromosomes during anaphas of meiosis 1

This produces gametes with randomly assorted chromosome combinations

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Definition of Cancer

Rapid and UNCONTROLLABLE cell division by mitosis

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How do cancer treatments work

Some prevent DNA replication

Others work by preventing spindle fibres from attatching

BOTH cause inhibition of cell division causing cells to divide less rapidly

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Tissue vs organ

Tissue is a group of similar cells organised to carry out a specific function

Organ is a group of tissues co-ordinated to perform a variety of functions

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What is cell differentiation

The process of a cell becoming specialised in structure to carry out a specific function

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its really good but what exam board are you using?



Henna wrote:

its really good but what exam board are you using?

Hey Henna

I'm on AQA but glad I can Help



Ok thats all folks for DNA more coming soon for other topics

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