3 Inheritance Flashcards
What is mitosis?
Diploid cell copies itself and divides into two identical diploid daughter cells.
The daughter cells are clones of each other.
Every base pair of their DNA is identical
How many pairs of chromosomes are in a diploid human body cell?
23 pairs
46 chromosomes
How does a cell undergo mitosis?
cell grows
DNA replicates to form two copies of each chromosome. Copies are joined together at a single point
Chromosomes line up across centre of cell, attached to special spindle fibres
The chromatids (copies) are pulled apart and move to opposite ends of cell
The cytoplasm and cell membrane divides to form two identical cells
What is mitosis used for?
growth, repair and asexual reproduction
What is meiosis?
cell devision that produces four genetically different haploid cells
chromosome number is halved
How does a cell undergo meiosis?
parent cell grows
DNA replicates to form two copies of each chromosome
The homologous pairs of chromosomes join together and exchange sections of DNA
Then attach to spindle fibres in pairs at centre
Homologous pairs are pulled apart and move to opposite ends of the cell
Chromosomes line at centre of cell, attached to spindle fibres, chromatids separate and move to opposite ends of cell
Each cell divides again
Mitosis produces…
genetically identical daughter cells
Meioisis produces…
haploid gametes (sperm, eggs, pollen) which aren’t genetically identical
only used to produce gametes for sexual reproduction
How many chromosomes does a human haploid cell have?
23 chromosomes (unpaired)
What are the differences between meiosis and mitosis?
Mitosis:
2 genetically identical daughter cell produced
Involves one cell division
Produces diploid cells in humans
Keeps same chromosome number
Meiosis:
4 genetically non-identical daughter cells
Involves two cell divisions
Produces haploid cells in humans
Halves number of chromosomes
What are the differences in the roles of Mitosis and Meiosis?
Mitosis:
generates all adult cells except gametes (growth and repair)
occurs throughout human body
occurs throughout a plant
used for asexual reproduction
Meiosis:
only used to produce gametes
only occurs in ovary and testis
only occurs in ovary and anther
used for sexual reproduction
What is fertilisation?
when haploid sperm and egg gametes fuse - will have full number of chromosomes
How does random fertilisation produce genetic variation in offspring?
it’s random which sperm/ pollen grain fertilises an egg - therefore variation in offspring
genetic variation in gamete cells produced by meiosis (sperm cell and egg cell)
random fertilisation of ova (egg cells) by male gametes
What is a gene?
a section of a molecule of DNA which does for a specific protein
provides information on sequence of amino acids in a protein (e.g. an enzyme) which then control chemical reactions, growth, colouration and all inheritable characteristics
Gene codes for a characteristic
What is a genome?
One copy of all an organism’s DNA
(the entire DNA of an organism)
In humans this is all the DNA that makes up the 23 pairs of chromosomes found in all diploid body cells (all cells except sex cells or gametes, which only have half a person’s genome)
Where is there a pair of genes for each characteristic?
in every diploid nucleus
nucleus contains chromosomes on which genes are located
PAPER 2 Describe a DNA molecule
double helix shape (two strands wrapping around each other)
PAPER 2 What is a nucleotide?
consist of a sugar (deoxyribose), a phosphate and a nitrogenous base
PAPER 2 What are the 4 types of nitrogenous base?
Adenine - Thymine
Guanine - Cytosine
PAPER 2 What are nucleotides joined together by?
covalent bonds between the sugar of one nucleotide and the phosphate group of the next nucleotide
FORMS: sugar phosphate backbone
PAPER 2 Describe an RNA molecule
ribonucleic acid
RNA nucleotides are joined to form a single strand
PAPER 2 What is the sugar in RNA called?
ribose
PAPER 2 What is the base in an RNA strand?
Uracil - Adenine
PAPER 2 What are the two types of RNA?
mRNA - messenger RNA.
Formed in nucleus of cell
tRNA - transfer RNA.
Found in cytoplasm
PAPER 2 What is transcription?
mRNA copies the sequence of bases of a section of DNA:
Two strands of DNA helix are unzipped - caused by enzyme helicase
The enzyme RNA polymerase attaches to DNA just before gene
RNA polymerase moves along the DNA strand
RNA polymerase catalyses the formation of covalent bonds between RNA nucleotides to form a strand of mRNA
strand of mRNA is opposite copy of DNA strand (except U replaces T) - called complementary copy
mRNA leaves nucleus to go to ribosome
PAPER 2 What is translation?
for every 3 mRNA bases (codon) the ribosome lines up one complementary molecule of tRNA (complementary 3 bases of tRNA called anticodon)
tRNA picks up specific amino acids from cytoplasm and transports to ribosome
ribosome allows two tRNA molecules to sit next to each other
ribosome catalyses the formation of a peptide bond between 2 amino acids
chain of several hundred amino acids forms in specific order polypeptide folded into correct shape and becomes a specific protein
PAPER 2 What are the stages of protein synthesis?
Transcription
Translation
PAPER 2 Where are proteins synthesised?
ribosomes in cytoplasm
What is an allele?
alternative form of a gene (version of a gene) - controls characteristics
e.g. allele for tall plant. allele for dwarf plant
What is the height of a pea plant controlled by?
specific gene, found on chromosomes
the genes have two different forms called alleles
What does it mean if the parents are homozygous?
both chromosomes have the same allele (tt, TT)
What must each offspring inherit from each parent?
one allele
What is the meaning of the term dominant?
A dominant allele only needs one allele present to be expressed
What is the meaning of the term recessive?
a recessive allele needs two alleles present to be expressed
What does it mean if the parents are heterozygous?
both contain one of each allele (Tt, Tt)
What is a phenotype?
what is expressed
e.g. tall or dwarf
What is a genotype?
combination of alleles
e.g. TT or Tt
What is polygenic inheritance?
characteristics controlled by several genes which influence each other
What are most phenotypic features the result of?
polygenic inheritance
PAPER 2 What is codominance?
Three phenotypes possible even though only two alleles:
two alleles equally expressed
Paper 2 Codominance example Pᴿ allele and Pᵂ allele in petal colours
phenotype: red
genotype: PᴿPᴿ
phenotype: white
genotype: PᵂPᵂ
phenotype: pink
genotype: PᴿPᵂ
What do you need to include in a genetic cross?
genotypes of parents and offspring
phenotypes of parents and offspring
alleles found in each gamete
probability of each phenotype in offspring (fraction of ratio)
How can some diseases be inherited?
Heterozygous parents: faulty gene = recessive, do not have disease but carry an allele therefore carriers
1st child = no disease
2nd and 3rd = carriers
4th child = has disease
25% chance of child with homozygous recessive so suffers from disease
Using Pedigree diagrams:
Is the disease recessive or dominant if both parents weren’t affected but daughter was? What is the genotype of the Parents?
Disease is recessive
each parent had one copy of faulty allele so daughter would have inherited one allele from each parent
genotype = heterozygous
What is the female pair of their 23rd chromosome?
XX
What is the male pair of their 23rd chromosome?
XY
What is the likelihood of having a boy or girl?
50:50
Parents phenotypes: female, male
parents genotypes: XX, XY
gametes genotypes: X,X,X,Y
Offsprings genotypes: XX,XX,XY,XY
Offsprings phenotypes: female, female, male, male
How can variation in a particular characteristic be genetic?
Following sexual reproduction, or mutation, organisms have different versions of genes to another individual e.g. pea plant might have alleles of height gene to be tall or dwarf
How can variation in a particular characteristic be environmental?
environment can also affect characteristics of an organism
e.g. pea plant in sun will gain more sunlight, able to photosynthesise more than pea plant in shade, therefore grow more and taller
How can variation in a particular characteristic be a combination of both genetic and environmental?
usually both genetic and environmental factors contribute to the variation found in individuals of species
What are the two main types of variation?
continuous:
e.g. height, eye colour, skin colour
no set categories, is overlap
characteristic/phenotype must come from few different genes (polygenic)
discontinuous:
e.g. blood type, earlobe/ tongue rolling
distinct categories with no overlap
characteristics must be coded for by 1 gene
Which type of variation is often more heavily influenced by environmental factors?
continuous variation
What is a mutation?
a rare, random change in sequence of bases in DNA - can be inherited
PAPER 2 What does a change in DNA sequence also change?
sequence of amino acids in the protein - will normally alter shape and function of the protein
PAPER 2 How are mutations caused?
replacement of a single base pair with a different pair, or an addition or deletion of one or more base pairs
PAPER 2 What is a chromosomal mutation?
cell gains an extra chromosome (downs syndrome) or one part of chromosome stuck onto another
PAPER 2 Changes in the DNA sequence are often…
neutral (have no effect), but can less often be either positive (increase variation, may be beneficial if environment changes) or negative (protein fails to work properly)
PAPER 2 What happens if the mutation occurs during meiosis in gamete formation?
will affect all cells in individual which develops from gamete. means mutation is inherited
PAPER 2 What happens if the mutation occurs in a body cell?
will affect only those cells produced by mitosis from the affected cell
this type of mutation not inherited
PAPER 2 What can mutations in body cells cause?
cancer
PAPER 2 What are mutagens?
chemicals which cause more frequent mutations
e.g. chemicals in tabacco - carcinogens
PAPER 2 incidence of mutations can be increased by…
exposure to ionising radiation
e.g. gamma rays, x-rays and ultraviolet light
and some chemical mutagens
PAPER 2 What do mutation increase?
genetic variation in population
e.g. blue eyes are a mutation
What is natural selection?
Mutations cause variation
Rare and random mutation give an organism an advantage
E.g. camouflage, able to get more food, escape predators
This individual survives longer and reproduces more
passed on advantageous allele to offspring
happens over many generations
How do some bacteria become resistant to an antibiotic?
rare and random mutation cause variation
variation makes bacteria resistant, causing it to survive and reproduce more, passing advantageous allele to offspring. Happens by natural selection and over many generations
Increase bacterial population
can lead to infections being difficult to control
What type of mutation is severe?
insertion of base or deletion: every codon after mutation is different: frameshift
