Classic ( Mendelian ) genetics Flashcards
What is our genetic information stored in
Chromosomes which are composed of DNA
How does DNA exist
As two complimentary chains of nucleotides
In the nucleus in the form of a double helix
What does a nucleotide contain?
Identical pentose sugar ( deoxyribose or ribose )
Phosphate group
Base- C,G,A,T ( 4 different bases in DNA )
How are the chains joined together?
Adjacent/ complementary base pairs are joined by hydrogen bonds.
Pentose sugar and phosphate make up of backbone of DNA
2 chains of DNA are mirror images
Bases stick out of backbone
What is a gene
- A piece of DNA that codes for the production of a particular protein
- Series of nucleotides
How do bases code for amino acids
Each triplet codon codes for one amino acid
Coded for by a triplet codon consisting of 3 adjacent nucleotides.
What is DNA in humans split into
46 chromosomes occuring as 23 homologous pairs
Each chromosome is a chain of genes
-There are 22 pairs of autosomes and one pair of sex chromosomes - during mitosis 1 member of homologous pairs from mother and 1 from father
What happens in protein synthesis?
Transcription- transfer the triplet codons (copied) on DNA to mRNA
mRNA - leaves nucleus
mRNA binds to ribosomes and combines with complimentary tRNA molecules (translation)/
The amino acids on adjacent tRNA molecules join to form a protein- with peptide bonds.
Makes growing chain of amino acids
Protein synthesises in cytoplasm on ribosome
Where is DNA - what is its structure
DNA lies within the nucleus and is closely associated with histone proteins which are important for regulating expression of particular genes
Normally it is in fairly loose agregations and strands in the nucleus. But during mitosis it condenses
What is the format of most of DNA
Most of DNA does not code for proteins - this junk DNA is known as introns
97% of our DNA is introns
What is coding region of DNA
Exons (3% codes proteins)
How many genes do we have in total?
- 30,000
* ( same as cabbage - difference is the number of introns )
Why do we need introns
- Help in expression of certain genes
- They make you, you
- During evolution, sequence of bases in introns is conserved = over millions of years it doesnt change
- If it was truly junk DNA with no purpose, random mutations would accumulate and introns would change over time - they dont - which means they are positively selected for
How many genes do we have for most traits
2
Where are the genes for traits located?
At the same locus on homologous chromosomes
What are 2 copies of the same gene?
Homozygous alleles
e.g. both genes code for blue eyes
What are 2 copies of different genes?
Heterozygous alleles
e.g. one codes for the blue eyes and one codes for brown eyes
What are the 2 genes
Alleles
2 genes for each trait
What is genotype
A persons genetic makeup
Full complementary genes - identity of all genes
What is phenotype
The expression of this genotype - physical us
What is life span of most cells and what does that mean
Limited life span
e.g. red blood cells = 120 days
That means they need to be replaced by cell divison
Wha are the cell types which do not get replaced?
CNS neurons
Cardiac muscle - in heart
Corneal endothelial cells
How do most cells replicate?
By division
What happens when a parent cell divides?
It must pass on all its DNA to the 2 daughter cells
This is achieved by the process of MITOSIS
What is mitosis?
Division of nucleus through cell division - makes sure each cell gets full complement of DNA - all the genes - 46 chromosomes get passed onto offspring
What does cell cycle show
That life of a cell can be divided into various phases
What are the 2 phases of the cell cycle
Interphase - 4 stages
Mitotic phase - mitossi and cytokinesis
What happens in G0
Normal cell metabolism occurs - duration is variable
When most cells get ready for division they go from G0 to G1
What is G1?
- Organelles are duplicated e.g. number of mitochondria increases, ribosomes, centrioles double/replicate
- Prepares cell for replication by the multiplication of organelles
- Cell growth
- Protein synthesis
- Cell gets ready to divide
What are the 4 phases of interphase 1
- G0
- G1
- S phase
- G2
How many centrioles does cell normally have
1 but in preparing for cell division in G1, number of centrioles doubles
What happens during the S phase
- DNA replicates = to make 2 identical strands of DNA
-DNA polymerase binds onto DNA and unzips it into 2 complementary chains - 2 strands of DNA unwind
-DNA polymerase moves along the exposed chains adding complimentary nucleotides from a pool within the nuceloplasm. e.g. if its A, it adds T = 2nd complementary chain of DNA
Forms 2 identical strands of DNA (4 strands)
What happens during G2
Proteins needed for cell division are synthesised = protein synthesis
The cell is now ready for division.
Replication of the cytoplasm = cytokinesis
Division of the nucleus= MITOSIS
How does DNA change after S phase
DNA originally consists of 2 strands of nucleotides - now consists of 4 strands of nucleotides - 2 identical copes of DNA -
What is the M phase
Division of the cell/nucleus
What is interphase
Cell gets ready and everything replicates
What is early Prophase
- DNA condenses to form bar like chromosomes
-Each chromosome have 2 copies of DNA - Each copy of DNA/ chain of DNA of chromosome is known as a chromatid
- 2 chromatids of each chromosome are joined by a centrosome
-Nuclear membrane disintegrate
-Nucleolus breaks down
-2 centrioles move to opposite ends/poles of cells
-Secrete microtubules- mitotic spindle
= PROPHASE IS BIGGEST PHASE OF MITOSIS
What is the 1st phase of the division of the nucleus
Prophase
What happens during late prophase
Chromosome attach to mitotic spindle
What happens in metaphase?
- Spindle fibres form
- Chromosomes arrange themselves along the equator/ centre of the cell
What happens during anaphase
- The 2 chromatids of each chromosome split apart
- Centrosome divides/breaks
- Spindle fibre shorten
- Mitotic spindle contracts and pulls each chromatid to opposite ends of the cell.
- Ends of cell receives identical DNA
What happens during telophase
-The nuclear membrane reforms
-Chromsome elongate as chromatid uncoils
-DNA is loose and cleavage
-Nucleolus forms
-Cell division occurs
- Opposite sides of cell start to grow together and bud off via cytokinesis forming
- Prophase in reverse
2 IDENTICAL DAUGHTER CELLS
What does DNA replication allow
- Each chromatid identical
- Each chromosome is 2 copies of DNA
What do most cells contain
The full genotype
How do cells differ
As only a few genes are ‘ turned on ‘ in each cell
What are most of our cells known as
Diploid - they have 2 copies of each chromosome
What are gametes/sex cells
Haploid- contain one copy of each chromosome - sperm and egg
Haploid come together to make a gamete - diploid
What do the 2 gametes produce
A diploid ZYGOTE
How are male gametes formed
Spermatogenesis
How are eggs formed
By oogenesis
Difference between males and females in terms of sperm and egg
An average male, at puberty start producing sperm through process of spermatogenesis - continue producing sperm until you die. Many millions of sperm every day BUT females born with all eggs work hard when fetus, after that, not very much
How many eggs at 7 weeks old
7 million
What happens at peak when fetus is 12 weeks old
Start losing eggs at increasing rate, by the time you’re born - only have 2 million eggs left.After puberty - release them 1 or 2 a month, until 40/50 = 400/500 eggs released in fertile lifespan
How are haploid gametes formed
By meiosis which consists of 2 division cycles ( meiosis I and II )
What is meiosis
2 phases of cell division, instead of making 2 daughter cells, it makes 4 daughter cells
What happens in meiosis 1- prophase 1?
- Chromosome condense
- Spindle forms
- Centrioles go to opposite poles of cell
- Mitotic spindle secreted
- Difference to mitosis- Homologous pair of chromosomes line next to each other Synapsis
- They form Tetrads
- Tetrads perform crossing over
- Centrioles move to opposite poles of the cell
- Nuclear membrane disintegrated
What is difference in prophase for mitosis and meiosis
In mitosis chromosomes are independent/single, whereas, in meiosis, homologous pairs of chromosomes line next to each other
What are tetrads
Homologous pairs of chromosomes
What is synapsis
2 homologous pairs of chromosomes swap bits of DNA
What happens in metaphase 1
- Homologous pair of chromosomes (Tetrad) line up in middle of cell/ equator
- Maternal or paternal chromosomes arranged randomly
What happens in anaphase 1
- Homologous chromsome pair (Tetrads) divide and split
- Each half of cell recieves different DNA
- Mitotic spindle contract and break down
- Pulling 2 members of homologous chromosome to opposite poles of cell
- 2 ends of cell recieve different DNA
What happens in telophase 1
Nucelar membrane reforms
Cytokinesis occurs
Cell division occurs
Due to homologous pairs- 2 daughter cells formed have different DNA- haploid- have only 1 copy of each chromosome
What is interkinesis?
Meiosis 1 and 2 are separated by this which no DNA replication occurs.
What happens during Meiosis II
Same as meiosis BUT
2 daughter cells have different DNA, each of them is haploid, as each of them only has 1 copy of each chromosome - originally from Mum or Dad
Each of the 4 daughter cells has different DNA so produces variable daughter cells = HAPLOID - come together - diploid zygote
What happens in Prophase II
- Nuclear membrane dissipates
- Spindle forms
- DNA condenses
What happens in Metaphase II
- Chromosomes align themselves on the equator - INDIVIDUALLY
What happens in Anaphase II
- Centromeres break/split and chromosomes move to opposite ends of the cell
What happens during Telophase II and cytokinesis
- Spindle breaks down and nuclear membrane reforms
- Cytokinesis = cell division
What does meiosis cause
Genetic variability
So any sperm can fertilise any egg
What causes genetic variability during meiosis
- Independent assortment during metaphase 1
- Genetic recombination during prophase 1- trade bits of DNA
- Random fertilisation
What happens during crossing over
When tetrads form - adjacent homologous pairs wrap bits of DNA
How are different number of sperm and egg produced
Its based on whether during formation of tetrads, the chromosome from mother or father goes above or below baseline
2^23 chromosomes = 8.5x10^6 different sperm/egg
Normally have 23 tetrads
How can non-disjunction occur?
During Meiosis 1 resulting in one daughter cell having an extra chromosome (and other one less)- due to a tetrad not splitting apart.
- At the end of Meiosis 2 , some daughter cells will not contain a particular chromosome while others will have an extra copy.
- One of tetrads didnt split apart - one of daughter cells has 1 copy of chromosomes too e.g. got both copies of C1 and other daughter cell has no copy of C1 = non - disjunction
What is trisomy
Fertilisation using a gamete with an extra chromosome will result in a zygote with 3 copies of that chromosoem
What is the most common trisomy?What is the most common trisomy?
Chromosome 21 - has 3 chromosomes (trisomy) which results in Down syndrome
What are other trisomys
Trisomy 18 (chromosome 18)- Edwards disease Trisomy 13- pataus disease
What are chromosomes 1 -22 known as
Autosomes
What is the 23rd pair of chromosome known as
Sex chromosomes - 2 versions….
Male: XY
Female: XX
Which chromosomes are also subject to non disjunction
Sex chromosomes
What are some of the examples of Syndromes/non-disjunction for sex chromosomes
- Females may have 1 X chromsome- suffer from Turners syndrome
- 3 X chromosomes- metafemale syndrome
- XXY chromosome- males- Kleinfelters syndrome
Who is the father of genetics?
Gregor mendel (1822- 1884) Austrian monk Explains human characteristics - laws of inheritance By growing peas - tall/short.....
What are the simple assumptions
- A trait is determined by a single gene
- A chromosomes are paired, each gene is present as 2 alleles
- Alleles can either be dominant (D) or recessive (d)
- When both alleles are the same (DD or dd)- the individual is homozygous for that trait
- When the 2 alleles are different (Dd) the individual is heterozygous.
How are diseases and normal traits be inherited?
By a simple dominant manner
What is an example to explain the Hunington disease?
Punett square
- Male is heterozygous- Hh (half dominant gene for the disease and recessive gene not for the disease)
- Female is homozygous recessive (hh)
1/2 of offspring will inherit the disease
How does that gene cause disease?
The gene undergoes a mutation then will have that negative consequence- causing the disease.
How can genes which can cause disease be beneficial?
A gene that causes sickle cell anaemia also protects against malaria
What are most diseases?
Recessive and hence are only expressed in the homozygous condition. (e.g hh)
What are example for the diseases which are recessive?
Albinism
Cystic fibrosis
E.g if you assume that both parents are heterozygous for albinism. - they do not express the disease however are carriers.
1/4 of offspiring will be albinos (hh) (aa)
1/2 will be carriers (Aa, aA)
1/4 will not even be carriers (AA)
Not all traits are?
simple dominant or recessive.
-Many show intermediate inheritance.
What is intermediate inheritance?
What is an example of a disease inherited this way?
When both alleles are expressed
Sickle cell anaemia
How is Sickle cell anaemia intermediate inheritance?
note: all autosomal inheritance
A single aa in one of the haemoglobin chains is abnormal.- Makes it much less effective in carrying oxygen
SS- Homozygous dominant are completely normal
ss- Homozygous recessive have severe respiratory problems as none of their haemoglobin can carry enough oxygen
Ss- heterozygous individuals produce both normal and sickled haemoglobin. They are normally symptomless, but in times of increased respiratory demand they have problems.- If excess demand- have respiratory problems- inherited in intermediate ways as both alleles are expressed.
What are the 2 inheritances?
Autosomal inheritance
Sex-linked inheritance.
What is sex-linked inheritance?
Females are XX
Males are XY - something on Y chromosome therefore makes them male
-The sex of the child is determined by the father
-Theoretically 50% of all children should be male
What is the male female ratio?
106 :100 by age of 18: 100:100 by age of 50 85:100 by age of 85: 50:100 100 years: 20:100
What are the basics of the sex-linked inheritance?V
Y chromosome is smaller than X. There is a region on the X chromosome where there is the X-linked trait (allele) whereby there is no corresponding allele on Y chromosome.
Any trait which is coded for in the X chromosome for which there is no corresponding part on the Y chromosome- It is SEX-LINKED
What is a trait that is inherited via sex-linked inheritance?
- Possession of a green sensitive visual pigment.
- If the allele on the X chromosome is dominant (G) we have the pigment- Colour vision is normal
- If it is a recessive (g) we do not and are red/green colour blind (deuteranopia)
How are females and men express the genes for colour vision?
Female has - is heterozygous Gg- will be a carrier for denteranopia however will have normal colour vision as she has one copy of the dominant gene- ensuring she has the green pigment
Male- however he will express the trait even if he has only one copy of the recessive allele as he has no corresponding allele on the shorter Y chromosome.- Thus is more common in men - deuteranopia.
How is a female colour blind?
If both her alleles are recessive- Which is rare.
What do many traits depend on?
Polygenic inheritance
What determines phenotype?
some genes are only expressed in environmental conditions such as (e,g the genes for freckles expressed more in sunlight).
Thus both nature (genotype) and nurture ( environment) determine phenotype.
Just having a gene doesn’t mean it will be expressed, that depends on environmental factors aswell.
