8.2 DNA and chromosomes Flashcards
What is the structure of DNA in prokaryotic organisms
. DNA molecules are short and form a circle
. They are not associated with protein molecules
So prokaryotic do not have chromosomes
What is the structure of DNA in eukaryotic organisms
What are histones
. DNA molecules are longer
. Linear DNA so form lines not circles
. DNA associate with proteins called histones to form chromosomes
DNA in chromosomes is held by histones
DNA is in mitochondria and chloroplasts, and is short and circular and not associated with histones
When are chromosomes visible as distinct structures
When a cell is dividing
Usually they are widely dispersed in the nucleus
What is a chromatid
When the chromosomes become visible, they appear as two threads joined by a central centromere
Each thread is called a chromatid because the DNA has already replicated to give two identical DNA molecules
Describe structure of a chromosome
2m of DNA in every human cell which is tightly coiled and folded into chromosomes
Double helix structure
This DNA helix is wound around histones to fix it in position
So DNA-Histone complex is coiled
Coils fold to form loops
Loops coil and pack together to form the chromosome
How many molecules of DNA is in each chromosome
1 but its very long and tightly coiled
Can different species have different numbers of chromosomes in them
How many chromosomes in a human
Yes
There are 46 chromosomes in humans so 23 pairs
In dogs there are 78 chromosomes
What are homologous chromosomes and where are they found
What is the total number of chromosomes in an organism called
Found in sexually produced organisms eg humans
. They are formed by the fusion of sperm cell and egg cell, and each of which contribute to one complete set of chromosomes to the offspring
So one chromosome in each pair is from each parent, so one from mother in egg and one from father in sperm
These are homologous pairs
The diploid number is the total number of chromosomes in an organism. Eg 46 in humans
A homologous pair (two chromosomes, each from mother or father)
Do they carry the same genes?
All chromosomes carry the same genes
But not necessarily the same allele of genes
Eg both mother an fathers chromosomes contain gene for blood type A, but one may carry the allele for blood type B whilst the other carries allele for blood type A
What happens during meiosis
The number of chromosomes are halved in a way that each daughter cell receives one chromosome from each homologous pair
So each cell receives one gene for each characteristic of an organism , and when these haploid cells combine they make the diploid state with paired homologous chromosomes
What is an allele and how do they link to genes
An allele is one of a number of alternative forms of a gene.
Genes are sections of DNA that contain coded info in the form of sequences of bases.
Each gene can exist in two or more forms (usually 2) and each of these forms is an allele.
Each individual will inherit one allele from each parent, and these two alleles may be the same, or different.
When they are different, each allele has a different sequence of bases so different amino acid sequence so produces a different polypeptide.
What happens when each allele inherited from each parent is different
Each individual will inherit one allele from each parent, and these two alleles may be the same, or different.
When they are different, each allele has a different sequence of bases so different amino acid sequence so produces a different polypeptide.
What is a mutation, how does this lead to enzymes not functioning
Any changes in the base sequence of a gene produce a new allele of that gene, which is a mutation and results in a different sequence of amino acids being coded for.
This will lead to a different polypeptide being produced so a different protein.
This protein may not function properly so eg if it is an enzyme, the active site may change shape so won’t fit substrate and won’t work
