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G1 and G1/S Cyclins- Budding Yeast (S. cerevisiae)
These are two of the four groups that cyclins can be classified into in budding yeast.

G1 Cyclins
These are cyclins that bind and activate cyclin-dependent kinases that stimulate entry in a new cell cycle at Start; their concentration depends on the rate of cell growth on growth-promoting signals rather than on the phase of the cell cycle. The G1 cyclins contribute to the control of cell-cycle entry in response to extracellular factors. The G1 cyclins consist of Cln3 in budding yeast. They help coordinate cell growth with entry into a new cell cycle and are required in many cell types to stimulate entry into a new cell cycle at the Start checkpoint; The G1 cyclins are unusual among cyclins in that their levels do not oscillate in a set pattern during the cell cycle, but increase gradually throughout the cycle in response to cell growth and external growth-regulatory signals.

Cln3
Cln3 has a role in regulating transcription of other G1 cyclins, Cln1 and Cln2. It is a G1 cyclin involved in cell cycle progression. Regulated by phosphorylation and proteolysis. Cln3 was found to be an unstable activator of Cdc28. Cln3 is negatively regulated by Whi3, which has an RNA-recognition motif that binds specifically to Cln3 mRNA. Cln3 transcription is not regulated by the cell cycle.

G1/S Cyclins
The cyclins of this phase are essential for controlling the cell cycle at the G1/S transition. G1/S cyclins activate Cdks that stimulate progression through Start; their concentration peaks in late G1 phase. The G1/S cyclins in budding yeast consist of Cln1 and Cln2. These cyclins oscillate during the cell cycle- rise in late G1 and fall in early S phase. The primary function of G1/S cyclin-Cdk complexes is to trigger progression through Start and initiate the processes leading to DNA replication, principally by shutting down the various braking systems that suppress S-phase Cdk activity in G1. G1/S cyclins also initiate other early cell-cycles events such as duplication of the spindle pole body in yeast.

The rise of G1/S cyclins is accompanied by the appearance of the S cyclins (Clb5 and Clb6 in budding yeast), which form S cyclin-Cdk complexes that are directly responsible for stimulating DNA replication.

Cln1 and Cln2
They are involved in regulation of the cell cycle. Cln1 is closely related to Cln2 and has overlapping functions with Cln2. For instance, Cln1 and Cln2 repress the mating factor response pathway at Start. Additionally, both Cln1 and Cln2 are expressed in late G1 phase when they associate with Cdc28p to activate its kinase activity. Lastly, late G1 specific expression for both of them depends on transcription factor complexes, MBF and SBF.

History
Leland H. Hartwell, R. Timothy Hunt, and Paul M. Nurse won the 2001 Nobel Prize in Physiology or Medicine for their discovery of cyclin and cyclin-dependent kinase.