A level biology help

I'm really struggling to understand alleles, DNA and genes found on chromosomes, even though i have read through the topic many times.

I know that alleles are different versions for a gene, so a gene can code for different characteristics, but how are genes found on chromosomes and how do chromosomes have genes which code for a polypeptide when the DNA has 2 polynucleotide strands, which have triplets coding for amino acids which in turn code for a polypeptide.

How are chromosomes even linked to DNA?

Overall i dont know how chromosomes, DNA and alleles/genes are linked, and I do not understand how a cell undergoes meiosis to produce 4 genetically different offspring, because wouldn't that need a change in DNA, not chromosomes? I would appreciate any help.

Let's start with a molecule of DNA. This is the double helix - the two DNA strands wound together.

A chromosome (the X-shaped thing) is made of two chromatids. Each chromatid is one molecule of DNA. So one chromosome is two molecules of DNA, and thus contains 4 strands of DNA in total, separated into two double helices.

A gene is a section of DNA - a group of the nucleotides which code for a protein.

An allele is a version of a gene.
For example, there may be a gene that codes for a protein which determines eye colour. All people will have this gene present in their DNA - we all have the same basic genome, so the sections are in the same places. However, the particular nitrogenous bases within that gene (sectio of DNA) could be different in different people. The gene will always code for eye colour, but the different alleles (different versions) will code for 'blue' eyes or 'green' eyes, etc.

Genes are found in chromosomes/DNA as just part of the sequence. So if DNA was -----, the gene might be ----- a little section of it. I don't know if that helps in any way.

Transcription: a strand of mRNA made using DNA as a template. Each DNA nitrogenous base was responsible for one mRNA nitrogenous base.
Translation: a polypeptide chain made using mRNA as a template. The mRNA nucleotides are in codons (triplets of bases). These temporarily bind to tRNA anticodons (triplets of bases which are complementary to the mRNA triplets of bases). Each tRNA molecule (with three bases on it) holds a specific amino acid. The amino acid is specific to the anticodon bases. Thus, when two adjacent mRNA codons temporarily bind to their two tRNA anticodons (with the ribosome helping), the two amino acids (one on each tRNA) are held next to each other long enough for a peptide bond to form between them. This continues until a polypeptide (chain of amino acids) is formed.
Thus, the sequence of bases in DNA determine the sequence of codons in mRNA, the anticodon of the tRNA, and then the order of amino acids in the polypeptide, and eventually it determines the protein which is produced.

During transcription, not all of the DNA genome is copied to mRNA - only one gene (to produce one protein at the end of the translation). So each mRNA strand is only one gene long. That is why one gene codes for one protein.

Meiosis: Because chromosomes are DNA, a change to chromosomes results in a change to DNA. The genetic variation occurs because:
(1) In prophase 1, when the homologous chromosomes line up next to one another (X-shaped remember, so they line up like XX), the 'tails' sometimes wrap around each other and swap. This is called crossing over. This means some of the genes from one X will then be on the other X. This means neither chromosome is the same as it was at the beginning. Then, during anaphase 1, the homologous chromosomes are separated and one goes to each pole of the cell. Since in metaphase 1, they lined up randomly, this means at each pole, you will have a random mix of maternal and paternal chromosomes. This is 'independent assortment'. Then, during anaphase 2, the chromatids are again separated randomly, so the genetic material is mixed up even more. This is independent assortment again. Thus, the gametes produced are genetically different and contain a mix of the maternal and paternal alleles. These then undergo random fusion with another gamete to produce genetically different offspring.

I haven't written any of this in a 'good' way, so don't quote me in exams, but does this help you understand it more?

If you're still confused, I'd recommend asking your teacher for help. YouTube videos are also really good as they show clear diagrams.

Thank you soooo much. I tried reading some of the foundations of genetics but you explained it a LOT better. Now i understand why things are happening. I just have 1 question. During meiosis 1, the cell divides to produce 2 genetically different offspring with 23 chromosomes each, But both of the genetically different offspring divide again to produce 4 genetically different daughter cells. How do these daughter cells have 23 chromosomes if the cells halved again so should it not be 23/2. Once again thanks so much for explaining it, i understand it a lot better now.

I haven't written any of this in a 'good' way, so don't quote me in exams, but does this help you understand it more?

In humans, the normal amount of DNA for a cell to have is 46 chromosomes (in 23 pairs). This is diploid.
Gametes, such as sperm and egg cells, have 23 chromosomes. This is haploid.

Before meiosis takes place, each chromosome is duplicated so it contains double the amount of DNA. So the cell enters meiosis with double the normal amount of DNA (although still only 46 chromosomes as the 'extra' DNA is still part of the same chromosome).
During meiosis 1, the amount of DNA is halved so the 2 cells produced are diploid (containing the normal amount of DNA).
During meiosis 2, the amount of DNA is halved again so the 4 cells produced are haploid (containing half the normal about of DNA).

During fertilisation, two gametes (the sperm and the egg) fuse to produce a diploid zygote.