Chapter 3: Eukaryotes (C5381789)

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1 Eukaryotic chromosome organization

A chromosome is created from a single double stranded DNA molecule [quite long, at around 1.5m long], wrapped around the histones, which are structural proteins. In humans, DNA is found only in the nucleus and mitochondria. Human cells have 22 pairs of non-sex chromosomes, and one pair of sex[-linked] chromosomes. Females have XX as their sex chromosomes, and males have XY. In total, humans thus have 23 pairs of homologous (meaning similar, but not identical) chromosomes, totaling 46 chromosomes in total. Cells with homologous pairs of chromosomes are known as diploid cells. In contrast, cells with no homologs are known as haploid cells. Chromosomes can either be drawn as a full butterfly, or half a butterfly: the half butterfly represents the chromosome before replication, and the full butterfly represents the chromosome after replication.

Frequently asked questions
What is a chromosome?
It is the combination of a full [double] strand of DNA, wrapped around structural proteins.

Where are they found?
Where DNA is found, so again, that's the nucleus and mitochondria.

How many chromosomes do humans have?
We have 22+1 pairs of chromosomes. We say this because men and women share 22 pairs of chromosomes. And the final pair of chromosome depends on gender - males have XY. and females have XX.

You say "pairs" of chromosomes. Is this like twins?
The pairs are not perfectly identical. They're similar, but not identical. One is taken from the father, and one is taken from the mother. The exception is the sex chromosome for males. In order to get a combination, their mother can only offer the X chromosome, and so the father definetely offers the Y chromosome.

What's up with the full and half butterfly?
Half butterfly is the chromosome before replication, and the full butterfly is the chromosome after replication. So in the full butterfly, there is twice the genetic material.

2 Eukaryotic gene expression

Gene expression is the process by which genetic information is used to create a protein. This involves the process of transcription and post-transcriptional modification already discussed , followed by translation.

Step 3) Translation is the process of synthesizing polypeptides from RNA. Having converted DNA into RNA [as DNA is not small enough to pass from the nucleus to the cytoplasm, through the nuclear pores; but RNA is] through transcription and post-transcriptional modification, the mRNA leaves the cell nucleus and moves into the cytoplasm.

The mRNA then binds to a ribosome, which are found free floating in the cytosol. The ribosome/mRNA complex then binds to the cytosol side of the rough ER, such that as the polypeptides are formed, they are released inside the rough ER lumen. Lumen is the inside space of a tubular structure. Ribosomes consist of two major subunits, the small subunit which reads mRNA, and the large subunit which joins amino acid to form a polypeptide chain. The large subunit has a P site, the polypeptide site; and A site, the amino acid site. The mRNA is lined on the large subunit. The tRNA [carrying an amino acid] moves toward, and sits in the large subunit’s P site. The tRNA contains the three bases complementary to the bases of the mRNA codon, known as the anticodon. For example, at the start, the tRNA carries the complementary of the RNA start codon AUG, as well as the amino acid the original codon coded for, which is methionine. The anticodon of AUG is [thought to be], since A binds with T [of which T is replaced with U in RNA], and G binds with C: UAC. However, because the codon and anticodon face opposite directions, the direct converse of the codon [in the example , UAC] is the anticodon being read backward [upstream, from 3’ to 5’ direction][which is not how anticodons are read]. Unlike replication and transcription, [codons and] anticodons are read downstream [in 5’ to 3’ direction]. Therefore, the anticodon of AUG is actually CAU (opposite direction).

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Then, the next [incoming] tRNA [also carrying an amino acid, and also with an anticodon complementary to the next mRNA codon] moves, and sits in the large subunit’s A site. Once the A site is filled, the mRNA shifts [in the image , moves left], releasing the first tRNA [detached from its previously carried amino acid], and shifting the second tRNA [from the A site] into the P site. Also, the amino acid [of the first tRNA] attaches to the amino acid [of the second tRNA], thereby creating a growing peptide chain.

Frequently asked questions
What is gene expression?
Where genetic information is used to create a protein. It involves [the previously discussed process of] transcription and post-transcriptional modification, followed by translation.

What is translation?
RNA -> Protein

Why did transcription (DNA -> RNA) need to occur? Why not go straight from DNA -> Protein?
DNA is found in the nucleus. Protein is synthesized in the cytoplasm (specifically, the ribosome). DNA is not small enough to pass from the nucleus to the cytoplasm, but RNA is. Hence, DNA must first be converted into RNA, and then protein.




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Chapter 3: Eukaryotes - Biology - Pre-med science - MR. SHUM'S CLASSROOM