Genes, mutations and meiosis

What is an allele?

When a gene can exist in more then one form

How many chromosomes does a human have and what is the pair called?

46 chromosomes which are stored in matching pairs called homologous pairs

What is an intron

Sections of code that don't work such as multiple repeats which are non-functional

What is a proteome?

The full range of proteins that the cell is able to produce

What does a base triplet code for?

An amino acid

-base sequence of a gene will determine the sequence of amino acids

What is an allele?

When a gene can exist in more then one form

What is a gene?

Sections of DNA that contain the coded information for making polypeptides

• they occupy a fixed position (locus) on a DNA strand

• the coded information is in the form of a specific sequence of bases

What is a genome?

A complete set of genes in a cell

what is a singlet code

Where one nucleotide codes for one amino acid could only provide four different nucleotides

How mant combinations of nucleotides can be had from a triplet codes

64

What is a triplet code

The smallest coding unit that can accommodate 20 amino acids that make up a variety of proteins

What is the genetic code

DNA and RNA molecules carry genetic information in cells

It is non-overlapping

It is universal

It is degenerate

What does degenerative mean

More possible combinations of triplet codons than there are amino acids eg there are 64 codons but 20 amino acids

What is a silent mutation

Occurs when the substitution of a base still codes for the same amino acid as the original base as the genetic code is degenerate

What is a nonsense mutation

Occurs when a substitution of a base occurs leading to a premature stop codon being coded for

This would lead to the premature end to the synthesis of a polypeptide

How manu stop codons are there

3

What can nonsense mutations lead to

Thalassemia

It is a autosomal recessive blood disease

This results in a reduced rate or so synthesis of one of globin chains that make up haemoglobin

This causes anaemia

What is mis-sense mutation

A mis-sense mutation occurs when a change in base leads to a different amino acid being coded for

The polypeptide will have a single amino acid that is different

What is an effect of mis-sense mutation

If the role of the amino acid was involved in the formation of bonds, the enzyme will no longer have a specific structure and wont be able to catalyse a reaction

What disease can cause mis-sense mutation and describe the disease

Sickle cell disease

Mutation in the haemoglobin gene

Describe the deletion of bases

When a nucleotide is lost from the DNA sequence

This leads to codons not being read properly

And the sequence being shortened

What disease is caused by deletion

Tay-sachs disease

Recessive disease

Describe addition of bases

When a nucleotide is added to the DNA sequence

Causes of mutations

Can occur by mistake during DNA replication

What is a mutagenic agent

A physical or chemical agent that increases the frequency of mutations

What is asexual reproduction

New organism is genetically identical to the parent

What is sexual reproduction

Meosis produced haploid gametes

Gamtes’s fuse at fertilisation to form a diploid zygote

Genetic variation in offspring

What is the difference between haploid and diploid

Haploid (n) = half number of chromosomes

Diploid (2n) =full set of chromosomes

what is meiosis

Meiosis is a reduction division

• results in daughter cells having half the original number of chromosomes

• These daughter cells are called sexual reproduction

• Can be used by a sexual reproduction

What are the 2 divisions in meiosis

Meoisis l

Meiosis ll

What happens at early prophase l

Chromosomes condense and become visible as threads

Homologous chromosomes join to pairs called bivalent

A chromatid of one chromosome in a bivalent can cross a chromatid of the other to form a chiasma

What happens at late prophase l

Chromosomes have condensed enough to become visible

What happens at metaphase l

Nuclear envelope disappears

Spindle fibers are formed and attatch to centromeres

What happens at anaphase l

The homologous chromosomes of each bivalent separate

They are pulled to opposite poles by micro tubules

THIS STEP IS WHERE CHROMOSOMES HALF

What happens at telophase l

Chromosomes reach opposite poles and may decondnse to form 2 nuclei

What happens at cytokenisis l

The plasma membrane folds inwards to form 2 cells

The centrioles divide

What happens at prophase ll

Chromosomes condense

Spindle fibers develop

What happens at metaphase ll

Nuclear envelope disappears

Chromosomes are pulled to the equator

Difference to metaphase l as they are lined up sing,e file rather than pairs

What happens at anaphase ll

Centromeres divide so each chromatid is now a chromosome. These chromosomes move to opposite poles

What happens at telophase ll

Chromosomes reach poles

They nuclear envelope reforms around haploid nuclei

Cytokinesis ll

Original cell has produced four cells

Each cell has one chrimatid

What is a locus

Position of a gene on a chromosome

How does genetic variation happen

Through sexual reproduction which occurs by production of haploid gametes

What processes are responsible for genetic variation

Crossing over

Random assortment of chromatids

Random fertillisation

Independent assortment of chromosomes

Chromosome mutations

Why are organisms different from one another

Mutations

Random fusion of gametes

Independent segregation

Crossing over

What can meoisis lead to

Genetic variation among offspring which may lead to genetic variation

What do all members of the same species have

The same genes

What is genetic diversity

The total number of different alleles in a population or species

How does genetic diversity enable natural selection

Increased genetic diversity increases the probability that some individual will possess a characteristic that suits it to the new environmental conditions

What two things will populations with high genetic diversity have

Lots of different alleles which leads to different phenotypes

why is it good for a population to have a high genetic diversity

They can adapt to change in the environment

How can new alleles be brought into a population

Through migration

Descirbe how migration brings new alleles

A new population of the same species may enter an area and interbreed with original population

This will bring new and existing alleles into population

This is called gene flow

What is a genetic bottleneck

When the size of a population is dramatically reduced

What happens to the gene pool from genetic bottlenecks

Gene pool gets smaller

What is the founder effect

when a few members of a population leave a habitual and start a colony elsewhere

What happens after genetic diversity

Reduction in diversity

What do mutations form

New alleles

What are the stages of natural selection

Individuals with an allele that increases Their chance of survival are more likely reproduce than those without the allele

As such there is a greater frequency of this allele in the next generation of individuals

Over several generations the allele becomes much more common in the population

What are the three categories of adaptations

Behavioural, physiological or anatomical

What are behavioural adaptations

Acts made by an organism that increases its chance of survival

What are of physiological adaptation

Processes that occur inside an organism that increase its chance or survival’s

What is anatomical adaptations

Physical features of an organism that increase its chance of survival

What are the three types of selection

Disruptive

Stabilising

Directional

What is directional selection and what does it cause

When 1 allele is favoured over the other alleles

This causes an increase in the frequency of the allele which gives the organisms a survival advantage

What is stabilising selection

Occurs when alleles towards the middle of the range are favoured

Eg human birth weights