Genes, Chromosomes, Cell division and Variation Flashcards
Where is DNA found ?
In the nucleus of a living cell
What does DNA control ?
Cell functions and the instructions for building all proteins within an organism
What is DNA’s structure ?
- double helix shape
- it has a sugar phosphate backbone
- has bases Adenine, thymine, guanine and cytosine
Which bases complement each other ?
Adenine = thymine Guanine = cytosine
What do the order of bases code for ?
Order of bases in the gene codes for the sequence of amino acid and protein. Three bases code for one amino acid-this is called a triplet
What is a gene ?
A gene is a length of DNA which codes for protein
What do the proteins made from a gene control ?
Chemical reactions, growth, colouration and all inheritable characteristics
What are the structures called that DNA is wrapped up into ?
Chromosomes
How many chromosomes does each person have ?
Two of each chromosome
Where do people get their two chromosomes from ?
One of each pair comes from the father the other from the mother
Where are genes for a specific characteristic situated on each homologous chromosome ? And what does this mean ?
They are situated at the same point on each homologous chromosome, therefore there is a pair of genes for each characteristic in every nucleus
How many chromosomes does a human have ?
46 chromosomes, 23 pairs
What does diploid mean ?
Two copies of each of their 23 chromosomes
What does haploid mean ?
Only contain one copy of each chromosome
What chromosomes are the sex chromosomes ?
The 23rd chromosomes
What gender are you if you contain two X chromosomes ?
Female
What gender are you if you have one X and one Y-chromosome ?
Male
Given example of a human cell that is haploid ?
Egg and sperm cells. This is so when they fuse they form a diploid offspring
What is DNA coiled around in chromosomes ?
Proteins called histones
DNA is a macromolecule what is it made up of ?
Small units called nucleotide
What does growth and reproduction require ?
Cell Division
What are the two types of cell division ?
Mitosis and Meiosis
What happens before a cell divides ?
The chromosomes replicate making an exact copy of themselves. This doubles the amount of DNA in the cell
What is mitosis ?
- produces genetically identical daughter cells
- These cells are diploid
- Mitosis is used for growth, repair and asexual reproduction
What is meiosis ?
- this produces gametes
- they are genetically varied
- meiosis is ONLY used to produce gametes for sexual reproduction
Describe mitosis
- The two pairs of chromosomes each replicated itself to form chromatids
- fibres appear in the cytoplasm and form a spindle
- The chromosomes move to the equator of the spindle and spindle fibres shortening and appeared to pull the chromatids apart by centromeres
- The chromatids are now chromosomes and have moved to opposite ends of the cell
- The cell begins to divide
- nuclear membrane forms again
- after the cell divides it creates two identical daughter cells
Describe meiosis
- chromosomes make identical copies of themselves
- similar chromosomes pair up
- sections of DNA get swapped
- pairs of chromosomes divide
- chromosomes divide
- four haploid, genetically varied cells are produced
How is variation in offspring produced ?
- genetic variation in gametes produced by meiosis
* Random fertilisation of ova by male gametes
What Reproduction is meiosis used for ?
Sexual reproduction
What reproduction is mitosis used for ?
Mitosis
How many chromosomes are in the haploid cell after meiosis ?
23 chromosomes
Why is variation important?
For evolution to occur
How can variation occur ?
can be genetic, environmental or a combination of both
Describe genetic variation
Following sexual reproduction or mutation, organisms will have different versions of genes to another individual
Describe environmental variation
The environment can also affect the characteristics of an organism
E.g. More sun = grow taller
Less sun = shorter plant
What is the mutation ?
Mutation is a change in the DNA
How can a mutation occur ?
- often a mutation will cause a base in DNA to change. So when it is read by the RNA it is incorrect this is called substitution mutation
- a cell can gain an extra chromosome or one part of the chromosome is stuck to another chromosome, this is called chromosomal mutation
What three affects can mutations have ?
- neutral - like most sequence mutations
- negative - most chromosomal mutations
- positive - tiny minority of mutations could improve an organism survival chances
What is a gene mutation ?
A single chromosome is changed as a result, a defective protein is produced. This can lead to a considerable change in characteristics, this can then be inherited.
Why do mutations occur ?
They occur spontaneously
What are mutagens ?
Factors that increase the rate of mutation
What two mutagens can increase the rate of mutation ?
- Radiation - can damage DNA and cause mutation
* chemicals - Tobacco smoke and other chemicals can cause mutations
What is a mutagen that causes uncontrolled cell division called ?
A caricinogen
